~ .. ,

RESEARCH DEPARTMENT

Visit to U.S.A.-sth November -19th December 1954

Repo rt No. T~05q

[1955/3]

THE BRITiSH BROADCASTING CORPORATION

ENGINEERING DIVISION

RESEARCH' DEPARTMENT

V 1'5 I T TO U 050 A o~· 8 .. th NOVEM,BER- 19th DECEMBER 1951J

Report No. T=!0511

. [1955/sJ

G. G. Gou ri et, A.M.I 0 E. E. (G. G. Gou ri et)

This Report is the propsrty ot the British Broadcastin! CorporatIon and .ay not be reproduoed or disclosed to a third party in any form without the written permission of the Corporation.

Section

1

2

3

4

5

6

Report No. T-054

VISIT TO U •. S. A •. - 8th NOVEMBER-19th DECEMBER

Title

PURPOSE OF VISIT •

ITINERARY

BLACK-AND-WHITE TELEVISION • 3.1. 3.2. 3.3. 3.4.

General Camera Tubes • Film Transmission Telerecording

COLOUR TELEVISION 4.1. 4.2. 4.3.

4.4.

4.5.

4.6.

General. Suitability of N.T.S~C. Standards Cameras 4.3.1. Three-tube Image Orthicon 4.3.2. Single-tube Vidicon 4.3.3. Chromacoder Colour Telecine 4.4.1. Three-Vidicon Intermittent Motion Telecine • 4.4.2. 4.4.3. 4.4.4.

Continuous Motion Rotating Polygon • Continuous Motion Twin-Mirror Telecine • Colour Masking •

Telerecording of Colour Programmes • 4.5.1. Direct Scanning of Eastman Negative 4.5.2. Colour Photography by "Color Vision" • 4.5.3. Video Recording on Magnetic Tape. Picture Tubes 4.6.1. Shadow-mask Tube. 4.6'-2. 4.6.3.

Single-Gun Lawrence Tube • Philco 21 in. "Beam-Index Tube •

4.6.4. Three-Tube Projection System • 4.6.5. G.E. Single-Gun Tube.

4.7. Maintenance of Studio Equipment 4.8. Colour Film 4.9. Long Distance Links

MISCELLANEOUS ITEMS OF INTEREST 5.1. 5.2. 5.3. 5.4. 5.5.

"Phone Vision" • U.H.F. Broadcasting Optical Lens Testing • Information Theory Applied to Television • Network Synthesis by Delay Combinations

CONCLUSIONS

APPENDICES.

1954

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, February 1955 Report No. T-054

b955/S]

CONFIDENTIAL

VISIT TO Uo:So:Ao:~8th NOVEMBER~19th DECEMBER 19,54'

1. PURPOSE OF VISIT.

During the period 8th November to 19th December, 1954, the writer visited the U.S.A. with the object of obtaining a first-hand impression of current and future trends'in television engineering, with particular reference to colour television.

Throughout the visit, the aim was to obtain views and information concerning American practice and experience,which might be of value to the Corporation in planning future investigations in colour television, but interest was naturally also taken in matters relating toblack-and-white television.

This report summarises the information and impressions gained during the visit, under separate headings. Subjects which are considered to warrant a more detailed account are dealt with separately in appendices, reference to 'which is made in the main text. Mention is also made of some items of more general interest which were encountered during the visit.

2. ITINERARY.

Left Southampton Srd November.

Arrived New York 8th November.

J)ay

November 9th

10th

11th

12th

13th (Sat.)

14th (Sun.)

15th

PLace Visited

N.B.C. Studios, Radio City, New York.

Telechrome Inc., Amytiville, L.I.

Du Mont Studios, New York.

Chromatic Labopatories, New York.

N.B.C. Transmitters, New York.

N.B.C. Studios, Brooklyn.

R.C.A. Research Laboratories, Princeton, N.J.

2

Day

November 16th

Place Visited

Bell Laboratories, Murray Hill.

17th Discussion with Mr. Vernon Duke, N.B.C.

18th C.B.S. Studio 72, New York.

A.B.C. Studios, New York.

19th Hazeltine Corporation, Littleneck, L.r.

20th (Sat.) C.B.S. Studio 72, New York (Colour rehearsal).

21st (Sun.) Viewed compatible picture of colour transmission.

22nd Philco Corporation, Philadelphia.

23rd General Electric, Syracuse.

24th Eastman Kodak, Rochester.

25th (Thanksgiving Day) Travelled to Chicago.

26th Motorola Radio, Chicago.

27th (Sat.)

28th (Sun.)

29th

3)th

December 1st

2nd

3rd

4th (Sat.)

5th (Sun.)

Zenith Radio, Chicago.

Travelled to San Francisco.

Ampex Corporation, Redwood City, Cal.

Chromatic Laboratories, Emeryville, and on to Los Angeles.

C.B.S. Studios·.(Television City), Hollywood, L.A.

NeB.C. Studios, Hollywood, L.A.

Bing Crosby Enterprises, Hollywood, L.A.

N.B. C. Studios, Burbank, L.A. "

Technicolor Motion Picture Corporation, Hollywood, L.A.

Travelled to Cleveland, Ohio.

3

nay PLace Visited December

6th Brush Laboratories, Cleveland.

7th F.C.C. Washington, D.C.

8th Compared B.B.C. and N.B. C. telerecordings, New York Office.

9th A.T. and T., West Street, New York.

10th R.C.A. Plant, Lancaster, Pas

11th (Sat. )

12th (Sun. ) New York.

13th ~.C.A. Plant, Harrison, N.J. and on to Boston, Mass.

14th M.I.T., Cambridge, Mass. and return to New York.

15th C.B.S. Studios, Madison Av., New York.

16th N.B.C., New York (Film Transmission).

17th N.B.C., New York (Telerecording).

18th (Sat. ) New York Office.

19th (Sun. ) Left New York by air for London.

3. BLACK-~WHITE TELEVISION.

3. 1. Gener al.

Before discussing colour television, ,which was of main interest during the visit, it is felt that a few-brief comments on matters relating to black-and-white television will not be inappropriate.

In spite of all fears to the contrary, black~and-white receivers have continued to sell at the normal rate. The fall-off in orders placed on manufacturers earlier in the year was due, not to a decrease in demand, but to a lack of confidence by dealers in maintaining stocks.

The most popular tube size is now 21, in.-, but the trend is towards 24 in., and it is expected that this will become the most popular size in the near ~uture. It is agreed by many that a 24 in. tube cannot be viewed at the ideal viewing distance in most homes; nevertheless people seem content to sit at a distance of three or four times the picture height and discount the defects which are visible at this close range.

4

The statement made by H.R.D. in 1950, to the effect that pictures in control rooms were invariably excellent, whilst those in the field were consistently poor, is as true today as it was then. Receiver manufacturers in America certainly do not in general achieve the same standard of performance as do British manufacturers.

Theoretically, an American receiver should resolve approximately 350 lines in a horizontal distance equal to the picture height as opposed to our 400 lines, but in fact, seldom does a receiver resolve 300 lines. The attitude of receiver manu­facturers is probably influenced by three factors which prevail in America:

a. In large cities, multipath effects are usually severe and set a limit to the quality of the received picture,

b. In provinci al areas the quality of the signal transmitted by "local" stations is often not up to standard. This is particularly true when the programme is of local origin, but even with programmes originating from the main networks the quality often suffers because of bad transmitter line-up.

c, The receiving installation is usually a compromise to give the best result wi th as many as eight channels.

It would seem, therefore, that there is no great incentive for the manu­facturer to produce a high quality receiver on a competitive basis-··--the overall improvement for the average viewer would probably not warrant the increase in cost.

The price of black-and-white receivers at present ranges from about $150 to $350, the average cost of a 21 in. model being about $200. The cost of a 21 in. picture tube is about $25. It is difficult to make any direct comparison with prices in this country, but taking into account wages and cost of living a figure of $5 to the pound is perhaps not unreasonable.

The four main networks, N.B.C., C.B.S., Du Mont, and A.B.C., all produce a black-and-whi te picture of excellent techniclil,l quality on live pick-up, and good quality with film. There is little doubt that the performance obtained with the R.C,A. image-orthicon camera tube in America is better than is generally obtained with the E.E.V. image-orthicons used by the Corporation. This is particularly true for studio programmes, the most noteworthy features of ·the picture being, low noise level, good grey scale, and absence of halo. There is some slight over-accentuation of edges, characteristic of the image-orthicon, but to a lesser extent than is usually experienced here. Indeed, on occasions the WI'i ter judged the picture as being com­parable to that obtained with a C.P.S. camera operating under ideal conditions, but with noticeably less noise. The possible reasons for these differences are dealt wi th in Section 3~ 2 below.

A further point of interest is the relatively consistent quality of tele-recordings, as compared with our own. Section 3.4.

This also is discussed in greater detail in

5

3.2. CalIlera 'lUbes.

The image-orthicon continues to be used exclusively for the purpose of live pick~up, with the Vi'dicoIi as the only possible future alternative, if and when sufficiently developed. The Vidicon is already replacing the simple iconoscope in film projection equipment, but is not considered to have been developed sufficiently to replace the imag~orthicon for studio use. The main objection is lack of sensi­tivity which is manifest as "smearing" or "trailing", when movement occurs in conditions of inadequate lighting. Dr. Rose of, R.C. A., who is working at the Princeton labora­tories on this ppoject, does not expect rapid progress to be made. Theoretically, the performance could be improved by a factor of one million, but in practice an improvement by a factor of two is difficult to obtain on account of impurities. The photo-conductive target which may either be antimony sulphide, or alIlorphous seleni'!;un, would require impurities to be reduced to one part in one hundred million for the maximum performance to be obtained, and this is-well beyond the scope of any known measuring technique. The ability to measure and control the degree of impurity in the photo-conducting surface, is in fact proving to be the limiting factor to further progress. Details concerning the operation of the tube are given in Appendix 1.

For live pick-up, the standard 3 in. image~orthicon (R.C. A. type 5280) is used universally. As operated by the four large networks in the U. S.A., this tube is producing a consistently good picture with a signal-noise ratio noticeably better than is obtained using the E.E. V. image-orthiccm in this country. The following reasons have been put forward by Mr. F. S. Vei th in charge of production at the R. C.A. plant at Lancaster, and others, as likely reasons for the difference:

i. The rate of tube production by R.C.A. is relatively very high, so that it is possible to select only first~rate tubes for use by the main networks.

ii. The design of the E.E.V. tube differs from the' R.e.A. in two main respects; in particular, an additional mesh is included on the scanning side of the target which, R.C.A. believe, causes additional noise. Further details

are given in Appendix 2.

iii. R.C.A. may be obtaining a higher depth of modulation of the scanning bealll as a result of continued long-term research on the gun and catb:ode surface, which are vital factors in the design.

In the interest of picture quality, R.C.A. tend to favour operation of the image-orthicon towards the linear range of its characteristic. It was with this in mind that, some years ago, they produced the 5286 tube, the main feature of which was an extended linear range at the expense of some reduction in sensitivity. This tube used with a gamma correcting circuit should have a performance similar to that of the C.P.S. Emitron. It is interesting to note that this tube never gained favour with operators as it was found to be more difficult· to handle. There is the further point that the accentuation of edges due to operating above the "knee" of the characteristic

6

has proved to produce a type of picture which the public seem to prefer. This preference has been confirmed by recent experience with colour transmissions, for which operation below the "knee" is essential (see Section 4.2). The best results, however, app_ear to be obtained by adopting a compromise in which moderately high incident lighting is used, but the contrast range is reduced by toning down "whites" and in general avoiding surfaces of high reflectance. This practice, which really amounts to "gamma correcting-" the scene, as opposed to the signal, is of course adopted with the C.P.S. Emitron at Lime Grove, in addition to signal gamma-correction.

On the question of the general consistency of the pictures seen in studio control rooms, it is relevant that all studio programmes are lit and produced with telerecording in mind. This means that -not only is the contrast ratio of the scene restricted, but every endeavour is made to keep relevant detail in the mid-grey range of the brightness scale. It can be argued that this practice produces a rather monotonous "sameness" in picture quality, but on -the other hand it does result in a picture which is consistently acceptable. Artistic effects, using low key lighting, tend to be avoided, the argument being that even when such effects produce a pleasing picture on the producer's monito:r>, the result maybe lost on the majority of receivers, few of which will be correctly adjusted to give the contrast range required,

3.3. Film Transmission,

It is estimated that over 60% of the prog-ramme material in America originates from film. In the interest of good quality the main networks use 35 m.m. film whenever possible. Many programmes including some of the main features are filmed exclusively for television showing, and this practice is likely to increase. From the American viewpoint there are two advantages which would not apply at the present time in this country: first, the programme is automatically available, with good technical quality, for a delayed trans-continental- repeat, and secondly, it permits of editing to avoid mistakes which can prove very expensive should a sponsor be dis­satisfied.

At the J;U'esen-t time, the simple iconoscop.e -telecine with the 3/2 intermi ttent pull~down, and pulsed arc light source, is still in general use for black-and~white film transmission. Considerable progress has been ,made in the last few years in adapting the iconoscope for this purpose; in particular, edge illumination of the target has largely overcome shading, and the resulting picture is very satisfactory, although not of the standard produced by our own continuous motion scanners.

The Vidicon telecine produced by R.C.A. works on an identical principle but uses a Vidicon type 6326 in place of the iconoscope. This arrangement is giving a really excellent performance both for 16 and 35 'm.m. film, mainly on account of entire absence of shading, a true black-level, and an exceptionally high signal-noise ratio. The performance of the 35 m.m. version is considered to compare favourably with .that of a continuous motion flying-spot scanner. Vidicon telecines are already used by N.B.C., C.B.S., and A.B.C., and-it is likely that they will ultimately replace the existing iconoscope machines everywhere. The changeover is not expected to be rapid, however, because of the large amount of capital invested in the present installations. The application of the Vidicon to colour film scanning is dealt with in Section 4.4.1.

7

It is ,of some interest to note that the introduction of colour ~as already influenced black-and-white film transmission in two minor respects: first, in pre­filming expensive productions, the large networks sometimes use Eastman colour negative, from which a black-and-white positive print is made. The cost of this is not unduly greater than for a black-and-white negative, and it affords the opportunity of a repeat transmission in colour at a ,later date, involving the cost of colour printing only. Secondly, C.B.S. are using the green channel of their 35 m,m. Philco colour scanner for the transmission of black~and~white film, when high quality is particularly desirable. The result is very superior to that obtained with the iconoscope telecine.

3.4. Telerecording.

The technique of telerecording is now well established, and vast quantities of programme material are now telerecorded as a routine operation. The large networks use telerecording equipment for as much as fifty hours' a week, mainly on account of the requirement for delayed transmission. The quality obtained with 35 m.m. film is consistently satisfactory, and certainly better than the average result obtained here. This is particularly'true in respect of the contrast range and signal-noise ratio of recordings made of studio programmes.

Apart from the differen,ces' in the method of film traction necessitated by the difference in field-rate, the general techniques used are very similar to those adopted by the Corporation. It is probable, however, that the large networks have more control than we have over the processing laboratories, for the reason that they are the main customers. In this connection it is interesting to note that anyone of the large networks uses more film stock than the whole of the motion picture industry put together.

A further point is'that to improve the consistency of processing, it has become standard practice in the U. S. A. to record on the film leader a neutral grey­scale as a check on the overall process.

Notwithstanding these differences, the writer is of the firm opinion that the superiority of the average American telerecordingis very largely, if not entirely, due to the consistency of the picture arriving at the recording monitor. The fact that studio p_rogrammes are produced and lit with the limitations of telerecording in mind is regarded as being most significant. This was borne out by C.B,S. tele­recording staff who stated that: "The lesson had been learned only as the result of long and bitter experience".

The following extract from a report by G. Edward Hamilton of A. B. C. is not irrelevant, for although the recommendations are concerned mainly with the production of films for television, they will apply even more so to telerecordings, which inevi tably suffer a twofold degradation due to the limitations of television. The rules referred to include restricting the scene contrast range to 20:1 and avoiding large dark areas.

" ••• At times, adherence to the above rules may seem to have the effect of destroying desired moods and patterns in the picture as immediately viewed on the

8

control room monitors. However, we must remember that the broadcast viewer would never receive the intended mood or pattern. In effect, the attempt to achieve such dramatic results may cause a degradation of the picture making a worse impression on the home viewer than if the scene were lighted conservatively. A further disquieting factor regarding changing lighting background, is the fact that for the past few years it has been the custom of the set manufacturers to omit the DC restorer. This allows the "black" level to shift, which of course is undesirable. This shift is at times so severe as to show the retrace lines. The best way to compensate for the above described condition is ·to maintain relative constant background illumination."

4. COLOUR TELEVISION.

4.1. General.

In spite of the fact that colour programmes have been radiated in the U.S.A. for approximately one year, it is true to say that as far as the public is concerned colour is virtually at a standstilL Plans for the future expansion of colour broadcasting are going ahead on a fairly large scale, but the fact nevertheless remains that at the present time it is exceedingly difficult to arrange to see a colour broadcast on a domestic receiver.

Of the f-ew thousand colour receivers so far manufactured, it seems that only a very small fraction have been purchased by the public. At the present < time there is literally no sale---not even for the R.C.A. 15 in. receiver originally $1000, and now offered at $350.

The only manufacturer continuing to market a colour receiver is Motorola. Their latest set uses· a 19 ·in. shadow-mask tube (C.B. S. Hytron), employs 28 valves, and costs $875. The performance of this set is not considered to be good, the main fault being the inter-dependence of controls.

tube. $895.

R.C.A. will shortly be marketing a new receiver using their latest 21 in. The price of this receiver, which also employs 28 valves, is understood to be The writer did not have the opportunity of seeing this receiver in operation

but unbiased reports of its performance are favourable, particularly in regard to the independence of controls and general stability.

The two factors which appear to be hindering progress most at the moment are (a) - the high cost of a colour receiver, and (b) -the fact that less than 2~% of the programmes broadcast by anyone of the networks are<in colour. The present situation is compared by many with the "dead-lock" which existed in the U.S.A. when black..,-and­white television was first introduced: sponsors will not finance programmes because there are i~sufficient sets in the hands of th~ public, and the public·will not buy sets because there are insufficient programmes. If the comparison is valid there is likely to be very rapid expansion once the "threshold" has been passed.

Of the four major networks, three arenadiating colour programmes at the present time, although Du Mont are using colour film only. The colour broadcasts which took place during two separate' weeks arbitrarily chosen were as follows:

13th November to 19th

N.B.C.

C.B.S.

Du Mont

11th December to 17th

N.B.C.

C.B. S.

Du Mont

November incLusive

Full length play

Programme for Women

Short feature

"You Are There"

Short feature

Two films (consecutive)

December incLusive

Full length play

Short feature

Play

Two films (consecutive)

(live)

(live)

(film)

(live)

(film)

Total for week

(live)

(film)

(live)

Total for week

H

1

&

~ .1 2j:

1

4it

H .1 :2

1

1

4

9

hours

hour

hour

hour

hour

hour

hours

hours

hour

hour

hour

hours

These examples seem to be fairly typical, the total period of colour broadcasting by the three networks usually amounting to 4 or 5 hours a week.

4.2. Suitability of NeT.S.C. Standards.

In general, engineers employed by the large networks are in favour of the N.T.S.C. colour standards, and regard the decision to go ahead with colour as being Correct. This view, however, is not shared by all members of research organisations or industry, many of whom think the decision to introduce colour was premature.

The following examples of opinions expressed may be of interest:

i. BeLL Laboratories (television group under Dr. Axel Jensen) are not entirely happy about the choice of standards. They consider the degradation of the compatible picture is serious both in terms of "dot-structure" and reduced resolution. On the other hand, they agree that if compatibility is regarded as being essential there is no obvious alternative.

ii. HazeLtine Corporation regard the N.T.S.C. standards as satisfactory but do not attach much importance to the "orange-cyan" theory and doubt if receivers will ever be designed to take advantage of it.

iii. PhiLco think that at times the black-and~white compatible picture suffers noticeable degradation and revealed that some complaints had already been received from viewers. The complaints were that the dot-structure was

10

annoying, and that the picture suffered a loss of definition when a "live" programme was r adi ated in colour. The loss of definition (according to Philco) results from the fact that the three-tube colour cameras seldom achieve perfect registration of the three .images. Also operating the tubes below the "knee" gives a "softer" picture since edges are no longer accentuated. They also think that multipath effects may have been under­estimated during trials.

iv. GeneraL E7,ectric are not happy about the.compatible picture because of general loss of quality and dotr-structuI1e. They tin fact went so far as to suggest that this country would do better to use a field-sequential system in one of the higher frequency bands, and if necessary convert to a compatible signal for Band 1. (Their view is no doubt biased by their own interest in the Chromacoder.)

v. MotoroZa seem to be in some doubt about the wisdom of the N.T.S.C. standards. Apart from producing receivers, they are one of the largest

.manufacturers of .micro-wave link equipment. In their opinion the colour signal will always require very careful "handling", and the degree of control and maintenance required for long distance links may prove very costly.

vi. Zenith Radio consider the N.T.S.C. standards to be quite satisfactory but think that the decision to proceed with colour was premature. In two years time it may be possible to go ahead, but they regard the colour tube as being the greatest obstacle and do not see any quick solution.

vii. A.P. and :r. expressed the view that the N.'L'.S.C. system was a brilliant achievement, but it had resulted in a "fragile" signal which would always require careful handling. Long-distance links had introduced many prohlems which had now been partially solved (see Section 4.9). The coast-to-coast link was achieving excellent performance but to maintain the performance without frequent adjustment was still a problem.

viii. P.C.C. (Mr. Edward AlIen) regard theN.T.S.C. standards as satisfactory but think that the introduction of colour was very premature. They are concerned about the slow rate of development and think it would have been better to have waited until it could be launched on a sound economic basis.

It should be mentioned that on the two occasions when the writer had the opportunity of viewing the compatible black-and-white picture on a typical domestic receiver, there was little to reveal that a colour signal was being radiated. The picture certainly lacked-crisp definition,but not to an extent noticeably greater than for a normal black-and-white transmission. The performance of the receiver seemed to be quite typical, but there is no doubt that the colour sub-carrier was being severely attenuated since dot-structure was barely visible, except on close inspection. It seems probable that where complaints concerning the dot-structure have ari sen, it has been due to the receiver having an abnormally high response at the sub-carrier frequency.

II

A further ~oint in this connection concerns the feature of variable tuning, common to nearly all American receivers. In addition to the channel switch, it is normal for the receiver to include a tuning control which ~rovides adjustment over a relatively narrow frequency band. Since the F.M. sound is usually received by the inter-carrier method, the sound volume gives no indication of correct tuning and the a~~earance of the ~icture is the only guide. The viewer will normally tune a black­and~white receiver to give the best definition, consistent with the absence of ~attern interference caused by the sound carrier. To the unskilled, however, the inter­ference caused by the colour sub-carrier, may easily be confusedwi th sound inter­ference so that if the normal ~ractice is ado~ted, the receiver will be mistuned to a ~oint where this colour sub-carrier is barely visible and the definition corres­~ondingly .P90r. This might account for com~laints of ~oor definition on existing receivers when a colour signal is radiated, as o~posed to the explanation given by Philco, since in the writer's ~iew, the com~atible pictures seen in control rooms were not sufficiently degraded to warrant comment from a viewer.

The ~roblem would not ari se to the same extent here because of the use of A.M. sound which would serve as a guide to the correct tuning adjustment.

The following comments are also relevant to this section:

a. No receivers are being designed to take full advantage of the wide-band I signal. It is generally held that the chea~er form of receiver which detects the (R - Y) and (B - Y) signals gives an equally good result, and it is extremely doubtful if the more elaborate matrix circuits will ever be used. In view of this, consideration should be given to the question of whether the I and Q signals are in fact the best signals to transmit.

b. It has now become standard ~ractice to limit the luminance band in the colour receiver to 3 Mc/s, thus giving horizontal definition equivalent to only 260 lines. This fact, combine.dwi th (a), means that band-sharing has virtually been abandoned as far as the colour receiver is concerned.

c. The o~timum choice of com~atible colour standards might ultimately be determined by the most ~racticable design of receiving tube. Hazeltine have shown that for a singl~gun tube the N.T.S.C. signals do not ~rovide for the sim~lest design of decoding circuits in the receiver. (See Section·4.6.)

All that has been said in this Section serves to em~hasise the need for careful consideration, and ~rolonged field trials, before any decision regarding actual standards is taken.

4.3. Camer as.

4.3.1. Thre~tube Image Orthicon.

Colour cameras were seen in o~eration at N.B.C.' s new studio at Brooklyn, Studio 3-H, New York, and also the C.B. S. Studio 72. The R. C •. A. three-tube camera using im"age orthicons type 1854 is still the only available colour camera and, bearing

12

in mind the problem of matching tubes and registering the images, the results were considered to be exceptionally good. There were no obvious signs of camera tube mis-registration except on one occasion during a rehearsal when a camera developed a fault; the degree of mis-registration gradually increased during the half-hour period of the rehearsal.

The colour rendering was in general good~-at least as good as average colour film, and at times more natural. The greatest difficulty seems to be that of keeping the contrast range of the scene within strict limits. The penalty of allowing the contrast range to exceed capabilities of the channel is much greater for colour than for black-and-white; in particular the flesh tones deteriorate rapidly if the light reflected is either inadequate, or excessive. In the former instance the difficulty is due to the failure to obtain identical overall gamma characteristics for the three colour signals. As with photography, at low levels of illumination even small differences in contrast law result in large errors in colour balance. The result is even more disastrous if the light reflected from any part of the scene is sufficient to cause one or more of the camera tubes to operate above the "knee". The colour balance in the offending area is, of course, completely upset because of the high degree of non-linearity introduced.

Because of these limitations, every endeavour is made to keep the contrast ratio of the scene lo~less than 20;1 is desirable.

In spite of the high sensitivity of the image orthicon, the amount of light required for successful operation of the cameras, as at present designed, is excessive­ly high. Both N .B. C. and C.B. S. are using 400 ft-candles although 350 ft-candles (eight times that required for black-and~white) is stated to be sufficient. Tungsten lighting is used exclusively so that artists are once more facing the problem of excessive heat, as in the early days of television. The necessity for so much light is partly due to limitations imposed by the use of a relay lens, which is essential if a lens turret is used.

At the present time both N.B.C. and C.B.S. adopt the practice of lining up the camera to give three equal signal outputs when presented with a "white" test-card illuminated with tungsten light of 2900° K. Assuming that the viewer 9 s receiver is correctly lined up to give Illuminant C (6500° K) when three equal signals are applied to the tube, all "whites" in the scene will thereafter be reproduced as Illuminant C. This is considered to be a mistake, since in typical viewing conditions, with moderate tungsten lighting, Illuminant C appears noticeably "bl1ie". Comments on the choice of Illuminant C as the reference white are given in APpendix 3.

So far, it has not been found possible to treat the camera as an objective device which will reproduce accurately the colours in the scene. This is not so much due to shortcomings in the colour analysis process, as to the need to work with a restricted contrast range. A good example of this was revealed during the rehearsal of a play at the N.B.C. studios, Brooklyn. The aim was to achieve oak panelling in a scene depicting the library of a large residence. The natural colour of oak would have had inadequate reflectance, so that a lighter colour approaching fawn had to be chosen. Considerable difficulty had been experienced in getting the right compromise, but the final result as seen on the colour-tube was quite realistic. Coupled to this

13

is the further difficulty that "sets" must be designed to give satisfactory reproduc­tion in black-and-white. The fact that all coloured surfaces, fabrics, etc., have to be chosen with these problems in mind is at present adding materially to the cost of colour productions, but "rules of thumb" are emerging as experience is gained.

Three specially selected matched tubes are used in each camera, and in general if one tube fails all three are changed. The average life of the tube is about 400 hours but on occasions failure occurs after a few tens of hours. Careful line up of all cameras is required before each rehearsal or transmission, a procedure which normally takes at least one hour. Care must also be taken to ensure that all cameras are lined uP . .with respect to each other, since a change of colour balance on "cutting" from one shot to another is particularly objectionable.

4.3.2. Single-tubeVidicon.

The single-tube Vidicon colour camera was seen at Princeton, but not operating. R.C.A. are still not prepared to divulge any details of the method of operation, but from discussion it was concluded that they do not expect rapid progress to be made.

4.3.3. Chromacoder.

Chromacoder installations were seen at both the G.E. laboratories, Syracuse, and C.B.S. Studio 72. Neither installations were fully operative at the time of the visit, but G.E. were able to show the "green" channel on a black~and-white monitor. C.B.S. are completing the studio installation with the object of carrying out full­scale comparison tests against a three-tube camera in about two months' time.

In principle, the Chromacoder is a most attractive proposal in that all problems of signal matching and image registration are located in a single piece of fixed apparatus. Apart from the addition of a small rotatory colour disk, the studio camera is similar in design to a conventional black-and-white camera, and no more difficul t to line-ul? _and operate. The method does, however, fundamentally involve the process of standards conversion with the consequent loss of picture quality. For this reason, technical opinion in America is in general opposed to the method, the main objection being the degradation of the compatible black~and-White picture.

Both the G.E. and C.B.S. installation now use the C.P.S. Emitron camera tube for the conversion. The quality· of the picture obtained from the "green" channel alone, as demonstrated by G.E., was very poor in terms of both resolution and contrast law and definitely inferior to the result recently demonstrated by E.M.I.

If an entirely new method of standa.rds conversion, giving adequate re­solution, were to be evolved the principle would warrant serious consideration. There is, however, one additional defect which some people regard as objectionable~ i.e., the colour break-up which occurs with fast moving objects due to sequential analysis.

4.4. Colour Telecine.

Three types of colour telecine were seen in operation, two of which are in current use. Each of these has its own advantages and disadvantages, but all were considered to have good performance.

14

4.4.1. Three~Vidicon Intermittent Motion Telecine.

This telecine has been developed by R.C.A. for both 16 m.m. and 35 m.m. colour film. It is now in use at the N.B.C. studios in New York, and is already installed at the new Burbank studios, Los Angeles, ready for use when these studios come into operation next month. It uses the standard 3-2 intermittent motion mechanism, and is in fact a direct application to colour of the principle which is normally used in America for black~and~White film transmission.

An important feature of the method is the large output signal which is achieved with the use of a powerful light source. A 1000 watt tungsten projection lamp is used, which provides 150 ft~candles incident on to the photo surface of each Vidicon. With such high illumination, the characteristic defects of this camera tube are entirely absent (see Appendix 1), and a signal-noise ratio of 50 dB is obtained.

The period for which the image is applied to the Vidicon is not restricted to the field~suppression period, as with the simple iconoscope, but is extended to 30% of the total field period. Operating in this manner it is not necessary to syn­chronise the film frame period with the television scanning period, an advantage which applies also to the single Vidicon black-and~white telecinec

Beam splitting the single image into three colour~separation images is achieved in a conventional manner using dichroic mirrors.

Undoubtedly, the greatest advantage of this approach to film projection is in the· ease with which a high level. of illumination can be obtained. One of the main difficulties with typical colour film is the large variation of average density. The variations in a single' reel are sometimes greater than the total available contrast range ,of the television system. Since the illumination available is greater than is required for satisfactory operation of the Vidicon, such variations may be compensated by means of an adjustable neutral density wedge inserted in the light path.

Perhaps the only disadvantage is the need for accurate registration of three images. The picture demonstrated by N.B.C. staff showed no obvious signs of mis­registration, but trouble might be experienced with less skilled operators.

R.C.A. make the point that one Vidicon assembly may be used with two or three picture sources; 16 m.m. film, 35 m.m. film, and slides is a typical "multiplex" arrangement. Such an arrangement does not, however, permit of preview when changing from one source to another.

The picture obtained from both the 16 m.m. and 35 m.m. versions of this telecine was excellent in respect of colour rendering and absence of noise, but in the writer's opinion would have been improved by the use of more aperture correction.

4.4.2. Continuous Motion Rotating Polygon.

A 35 m.m. version of this telecine was first introduced by Philco; later Du Mont introduced a 16 m.m. version, and now both Philco and Du Mont are marketing "multiplex" assemblies which operate with 16 m.m. film, 35 m.m. film, and slides.

15

This type of scanner was seen in operation both at the Du Mont and C.B.S. studios. At Studio 72, C.B.S. are using the Philco scanner for 35 m.m. film, and a Du Mont multiplex for 16 m.m. film, and slides. C.B.S. do not favour the multiplex arrange­ment, and at Television City, Los Angeles, separate facilities are provided for slide scanning.

The mode of operation is continuous motion, the image being immobilised with respect to each successive film frame by means of a rotating prism in the form of a glass polygon. A 24r-sided polygon which rotates once per second is used in the Philco 35 m.m. version, and also the Du Mont 16 m.m. model; the Philco 16 m.m. model uses an 18-sided polygon to obtain greater light transmission.

Compensation for film shrinkage is achieved in an ingenious manner. The rotating prism is hollow and the effective optical diameter is adjusted by means of a glass wedge inserted in the light path through the prism. The position of the glass wedge, and hence' the total amount of glass in the optical path, is controlled auto­matically by a mechanical arrangement which measures the shrinkage factor.

The principal disadvantage of this type of telecine lies in the relative inefficiency of the optics. The equivalent aperture of the 35 m.m. version is about f/7'5 whilst for the 16 m.m. it is about f/5. In spite of this the result obtained with both versions is excellent although the 16 m.m. is perceptibly more "noisy".

The Philco scanners use the latest R.C.A. photo~multipliers which are specially selected for l~w noise, whilst Du Mont use their own photo-multipliers, of similar performance. Both Philco and Du Mont are using the red-enhanced zinc oxide phosphor, R.C~A. type SAU-P24, a Levy and West (of the U.K.) product developed specially for colour.

The fact that a satisfactory signal-noise ratio is being achieved with relatively inefficient optics is indicative of the high performance of the projection tube and photo-multipliers.

Some special features incorporated in the Du Mont scanner are worthy of mention~

i. A high level of brightness is obtained from the projection tube by driving it with an oil immersed 45 kV E.H.T. supply.

ii. A neutral density face-plate with 60~ transmission is used for the projection tube with the object of reducing halation effects. Du Mont say that in spite of the loss of light the overall signal~noise ratio is actually improved, since for the same resolution less electrical equalisa­tion is required.

iii0 They have recently developed a dichroic mirror which reflects green light and transmits red and blue. This they consider to be an achievement since it increases the efficiency of beam-splitting in respect of si~al~ noise ratio.

16

A useful feature of the rotating polygon telecine is its ability to operate at any speed; like the Mechau, it will operate on a stationary frame, and can be run up to speed during transmission. In the multiplex arrangement the three photo­multipliers must be repeated for each facility, but this has the advantage of allowing pre--view.

4.4.3. Continuous Motion Twin-Mirror Telecine.

Eastman Kodak have successfully developed a 16 m.m. continuous motion flying-spot telecine for colour film, in which the image is immobilised by means of a pair of cam-operated rotating mirrors. TheY' are now working on the production of a 35 mom. version.

General Electric intend to market this scanner, and the 16 m.m. version which they have recently purchased was seen in operation during a visit to their laboratories at Syracuse. The performance details were also discussed with the Eastman Kodak staff at Rochester.

Eastman Kodak have developed this type of scanner, because they do not approve of the inefficient optics inherent with the rotating polygon, nor of the image registration problem in the three-Vidicon approach. Disregarding the small loss introduced by the mirrors, the effective aperture is f/1°5, and for 16 m.m. film they were certainly achieving a remarkably good signal~noise ratio.

The "dissolve" worked very well and there was no sign of any flicker component; there was, however, a slight interlace "weave" which is now known to be due to a fault in the design of the optical system, which can be remedied. In all other respects the result was considered to be excellent.

The app.aratus had the appearance of being very well engineered, and, in view of the promising performance, it is considered worth bearing this development in mind when considering 35 m.m. colour telecine.

4.4.4. Colour Masking.

Du Mont have developed a unit called a "colour masking" unit, which· is now in general use with colour telecine. It is a direct development of the proposal made by Brewer, Ladd and Pinney* of Eastman Kodak for overcoming certain deficiencies in colour film, in particular, errors of hue and desaturation of colours resulting from unwanted "coupling" of dyes in the film. The Kodak proposal aimed at correcting these errors electrically, by operating on the primary signals produced by the tele­cine, and in addition applying compression to the contrast range so that it would not exceed the contrast range of the television system. The Du Mont unit contrives only to correct for the colour deficiencies and does nothing in the nature of compression. It is called a "masking unit" since it achieves electrically the result which, in colour photography, is achieved by photographic masking.

In principle, it is simply a linear matrix unit which allows quantities of any two of the primary signals to be subtracted from the third. In the interest of reducing noise the bandwidth of the signals subtracted is reduced to 1"5 Mc/s, which

*W.L. Brewer, J.B. Ladd, J.E. Pinney, "Brightness MOdIfication Proposals for Televising Color Film', Proc. I.R.E., vol. 42, pp. 174-191, January 1954

17

is permissible since, in the N.T.S.C. system, the chrominance signal is similarly restri cted.

Very impressive results can be obtained by the judicious use of such a unit, but experience has shown that it also invites mis-use. For example, settings which are found on a trial-and-error basis to give the best subjective result for a given scene may be entirely unsuitable for a different scene. The reason for this is that in making the adjustments subjectively there is no way of differentiating between linear and non-linear errors, and the best subjective result will probably include compensation for both.

The use of colour masking was discussed at length with a number of organisa­tions including Eastman Kodak and the Technicolor Corporation, Hollywood. The considered opinion of those who had studied the problem was that a masking unit could be used with definite advantage, but as a pre-set de"dce. The optimum settings for the various colour film processes should first be arrived at by careful experiment, and thereafter the appropriate setting should be selected by a switch according to the type of colour film being used. Recommendations in this respect are given in a published paper by J.H. Haines of Du Mont Laboratories, Inc.*

This is a matter that is well worth further investigation, for there is no doubt that the quality of colour film can be improved by masking technique to the point where the television reproduction is better than that obtained by direct optical projection.

4.5. Telerecording of Colour Programmes.

At the present time colour transmissions are being telerecorded only on rare occasions for experimental purposes. The fact that a satisfactory method of recording colour signals has not yet been established means that colour programmes intended for evening viewing are being transmitted on the west coast in the late afternoon. Three possible methods of recording colour signals are being considered, mainly with the object of delay in mind. It is, of course, possible to make a colour telerecording by the conventional method, but using colour film. R.C.A. have already demonstrated an excellent example of a 35 m.m. print obtained from an Eastman colour negative, using a three-tube (triniscope) display. Apart from the time required to process a colour print, there is also the question of cost. For a black-and-white tele­recording (positive print) the overall cost is approximately $1000 per hour; a colour print from an Eastman negative would cost $4000 per hour. Bearing in mind the extent to which telerecording is used in America, there is every incentive to find a method which is both quick and cheap.

4.5.1. Direct Scanning of Eastman Negative.

One possibility is to telerecord with Eastman negative and to use the negative itself in the colour film scanner for the delayed transmission. In theory, phase reversal and gamma correction of the scanner output signals would produce the required positive primary signals. In practice there are two difficulties to be overcome. First, the Eastman negative when developed is predominantly yellow in colour due to a yellow pigment used as a "mask". This is corrected when a positive

*WColor Characteristics of a Television Film SeannerwQ InRoEo Convention Record, Part 7 - Broadcasting and Television. 1954

18

print is made, and would, therefore, need to be corrected "electrically" at the output of the film scanner i£ the negative were used for transmission. Secondly, accurate gamma correction is vital for colour and this is more difficult to achieve with negative film.

Eastman Kodak have already worked on this problem and think it can be done, but they are not satisfied with the result so far obtained. N.B.C. have also conducted experiments and say that with the Vidicon scanner the result obtained was good enough to use, without additional gamma correction. It is worth noting that it is common practice in America to use the negative of black~and-white telerecordings for delayed transmission, and with the iconoscope and Vidicon scanners no gamma correction is found to be necessary.

4.5.2. Colour Photography by "Color Vision".

Color Vision, Inco, Hollywood, have been working for some considerable time on a method of making colour films which involves photographing the three colour­separation images, separately on a single frame of 35m.m. film. A little over a year ago, H.R.D. andA.H.R.D. discussed the method with Mr. V. Lee, President of the company, during his visit to England. At the time the method was still in the experimental stage of development. The writer had the opportunity of meeting Mr. Lee and two of his assistants, in Hollywood, and of discussing recent developments.

They are now of the opinion that all difficulties have been overcome and make the following claims:

i. Colour may be "shot" on standard 35 m.m. black~and~white stock, using a standard Newall or Mitchell motion picture camera, to which their optical device may be attached in a few minutes.

ii. The three images produced by the optical attachment will photograph to have resolution 40% better than for 16m.mo film used conventionally. (The area is actually 40% greater.)

iii. The film can be developed and printed using standard processing as for black-and-white. For colour prints, they have produced a contact printer which will produce a positive colour film.

iv. For television a colour print is not necessary. A similar optical device is incorporated in the scanner so that the three images are scanned simultaneously with a single projection tube, which may be monochromatic, as for a black-and-white scanner.

v. Recent improvements· in the optical system have brought the efficiency to the point where colour film can be "shot" in 12) ft~candles, comfortably, whilst 60-70 ft-candles is sufficient for a satisfactory result at lens aperture T. 2.

vi. Used for colour telerecording the cost would be $180 per hour increase on the cost of black-and-white, this being their charge for hire of the device.

,,-------------------------------------------------------------

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This development seems well worth bearing in mind, as it might well prove to be the best and cheapest way of making colour film for television, quite apart from the application to telerecording. The most important feature of the method applied to television is that it avoids completely the limitations of colour film.

They are about to carry out trials with N.B.C., who seem interested, but rather doubtful of success. They have already enlisted some licensees amongst whom are the Swiss Broadcasting Administration.

4.5.3, Video Recording on Magnetic Tape.

This method of recording is equally applicable to black~and~whi te television, but is dealt with in this section since colour is probably the main incentive to further development, For the large networks it would certainly afford a quick and cheap means of achieving delayed transmission of a colour programme.

Four commercial organisations who are actively engaged on the problem, either directly or indirectly, were visited. These were, R.C.A. (Princeton), Ampex Corporation (San Francisco), Bing Crosby Enterprises (Hollywood), Brush Laboratories (Cleveland) ID The present state of progress in this field is still somewhat obscure, but some detailed technical information was obtained which is given in Appendix 4.

General information may be summarised as follows:

RoCoAo have not given any demonstrations recently and had nothing to show when the writer visited the Princeton laboratories. The equipment is shortly being installed in the N,B.C. studios, New York, with the object of putting it to use, but N.B.C. do not expect quick results.

Ampex Corporation manufacture large numbers of portable tape recorders for industrial and commercial use. They have also developed multi~track recorders for specialised applications, and are studying the problem pf recording video frequencies, from the point of view of getting maximum information on tape. They, are not concerned specifically with the development of a television tape recorder but collaborate closely with Bing Crosby Enterprises, particularly in the matter of head design.

Bing Crosby Enterprises have abandoned time multiplex approach to video recording on tape, and are now using the same method as R.C.A., i.e., a single track for each video signal, and an additional track for synchronising.

A recorder which they have developed for Westinghouse, for a special application, was demonstrated whilst undergoing final tests before delivery. As with the R.C.A. recorder, five tracks were recorded on standard ~ in. tape running at 360 in. per second. It was designed to record a bandwidth of 2 Mc/s with a signal­noise ratio of 30 dB, and the recording and replay of a 2}.Lsec test pulse with a rise-­time of i}.Lsec was demonstrated to show that this requirement had been met.

Because of irregularities in the oxide layer the signal occasionally fails, a condition known as a "drop-out". To overcome this they have developed a diversity system in which a signal is recorded simultaneously on two tracks. Two signals are

20

therefore available on replay, the best of which is selected automatically, from instant to instant, as with diversity reception. The five tracks recorded thus comprised two pairs of diversity tracks, for recording two signals, and a synchronising track.

They also claim to have successfully recorded colour in a manner similar to that used by R.C.A. The experimental apparatus was seen, but unfortunately not operating as it was undergoing further modification. Addi tional technical details are given in Appendix 4.

Brush Laboratori es are interested in telerecording indirectly, in that they are primarily concerned with the· more general problem of storing information at a maximum rate. They are actively engaged in the development and design of the recording head, which, they contend, is the limiting factor in the present state of the art. It was revealed that they are making heads for video recording, under contract, and for that reason were unable to divulge details of design. They have, however, been pressing for permission to' release information, and hope to be able to do so early in the year.

Mr. Otto Kornei, who is directing the work, said that experiments had shown that standard tap,e could resolve 10000 ,cycles per in., which, at a tape speed of 360 in./sec would rep,resent 3'6 Mc/s. He believed that 5'0 Mc/s was not an im­practicable limit, and this indeed was their aim. The technique of recording high frequencies had now in fact advanced to the point where the low frequency end of the television spectrum was proving to offer a greater problem.

The opinion generally held is that, whilst it is possible' at the present time to record video signals on magnetic tape, considerable development will be required for it to become a routine operation. The practical difficulties include, short life of the head due to severe'wear caused by contact with the tape, the serious effect of even minute particles of oxide dust in the gap of the recording or repro­ducing head, and the fact that a change of tape speed of as little as one part in a million is sufficient to cause a displacement of -1 in. on a 15 in. picture.

4.6. Picture Tubes.

The picture tube is still proving to be a major obstacle to rapid expansion of colour television in America and many hold the view that it is the only obstacle. The difficulty is not technical performance, but cost, which is still considered to be prohi bi the.

4.6.1. Shadow-mask Tube.

The only type of tube in general use is the three-gun shadow-mask (phosphor­dot) tube. R.C.A. introduced this originally as a 15 in. tube in which the phosphor dots were arranged on a flat screen mounted within the main envelope. A later development was the C.B.S. "Hytron" version, a 19 in. tube in which the phosphor dots were dep,osited directly on the face-plate of the envelope, and the latest R.C.A. 21 in. tube is virtually a larger version of the "Hytron". Du Mont are also manu-

21

f acturing a 19 in. tube similar to the "Hytron". operation and all have excellent performance.

All three tubes were seen in

The various processes involved in the manufacture of the 21 in. tube were seen at the R.C.A. plant, Lancaster. Briefly, the method is this: one of the phosphors, say the "red", is first set.tledon the face-plate by conventional technique. The phosphor is then covered with a film of photo-sensitive material, known as a "photo-resist ", which has the property of hardening when exposed to light. The shado~mask (a perforated metal disk which has been pressed to have the same curvature as the tube face) is then mounted immediately behind the face-plate and a powerful mercury light source is used to expose the "photo-resist" through the mask. Hardening takes place according to the regular pattern of the mask and subsequent "washing" therefore removes the phosphor from all unexposed areas, leaving a regular array of "red" dots. The p.rocess is repeated for the "green" and "blue" phosphors with the mask slightly displaced in each case by the amount necessary to produce the desired triangular groups of "red", "green" and "blue" dots. removed by baking.

Finally the "photo-resist" is

In order to avoid unwanted exposure of the "photo--resist", a blue sensitive emulsion is chosen and the whole of the production area is lit with yellow light from sodium sources.

When exposing the prepared face-plate, a special lens is used which com­pensates optically for the fact that in practice the effective point at which the scanning beam is deflected is not fixed, but varies with the angle of the beam. This correction substantially eases the problem of registration.

Apart from the larger screen the new tube has the advantages of:

i. improved brightness and contrast resulting from better optical efficiency;

ii. better registration, mainly due to the curved screen,

iii. better control over beam-convergence, is provi ded.

separate external magnetic control

The number of phosphor dots per unit area is approximately the same as for the original 15 in. tube so that the total number of phosphor dots is about one million---a 50% increase,

When correctly aligned this tube gives an excellent colour picture having good definition and adequate brightness. The accuracy of registration is very much better than is generally obtained with the earlier tube, and there is very little evidence of moire pattern, due to interference between the dot pattern and the scanning raster,

Normally operated, the high-light brightness is about 16 ft-lamberts which, for a colour picture, seems more than adequate with moderate ambient lighting.

The cost of this tube is $175---about $25 less than the average cost of a complete 21 in. black-and-white receiver.

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4.6.2. Single-Gun Lawrence Tube.

The tube developed by Chromatic Laboratories was demonstrated at their New York office, and was also seen working at the San Francisco plant. Westinghouse have manufactured this type of tube, under licence, and their version was seen in operation during a visit to Hazeltine Corporation.

At the present time Chromatic are producing tubes only for experimental purposes. They have stopped manufacture because they feel that R. C. A. have captured what little there is of the existing market. Their policy is to concentrate on improving the performance and reducing the cost of manufacture. They are still fully confident that a ·single-gun tube is the u1 timate approach and they are expanding their facilities to carry out more experimental work.

They have improved the method of manufacturing in the last year, and they claim that all the processes are now suitable for mass~production, although this has not yet been planned.

The tubes whichthey·are making are 19 in. rectangular metal, and 21 in. glass. There are appr.oximately 1000 phosphor strips in the present design, 500 of which are green, 250 red and 250 blue. They agree that this is not enough, but it is not a fundamental limitation, and the numbers can be increased later without undue difficulty.

The writer saw all of the manufactuDing processes, none of which seems to present a major problem. They do, however, need to "tidy up" some of the processes in order to reduce the reject rate which is at present very high.

The phosphor strips are horizontally disposed and are stencilled on to flat glass plate by a "silk-screen" process. (The "silk" screen is in fact a fine mesh of steel wire.) They have eliminated the need for the auxiliary grid of vertical wires, which was originally required to maintain uniform focus.

The problem of radiation from the deflection grid, due to the relatively 1 arge amount of r. f. power applied to the element, has now been overcome. The two sets of wires which form the grid are electrically balanced and wired in such a manner that the fields cancel.

The colour p.icture produced by this tube is quite acceptable but is not considered to be as good as that obtained with the latest shado~mask tubes. There is a. tendency for colour contamination to occur at the picture extremities where the scanning angle is large, and also slight general desaturation which is believed to be due to secondary emission from the grid. On the other hand, the "reverse-compatible" performance for a black-and-white picture is in some respects better than that of th e shado~mask tube. It gives a uniform neutral colour over the entire picture area, and does not suffer from colour fringes due to misregistration.

An interesting feature of this type of tube was recently revealed by Loughlin of Hazel tine CorJ2.0rat·ion. Loughlin * has shown that by using the sub-carrier fre­quency for beam deflection, the tube itself will perform the major part of the de-

*B.D. Loughlin, 'Processing o~ the •• T.S.C. Color Signal ror One-Gun Sequential Color

Displays'.," Proc. I.R.E oo vol. 42, No. 1, pp. 299-308. January 1954

23

coding operation, thereby simplifying the circuitry in the receiver. The general principle of using the tube in this manner will apply equally to any single-gun tube which is sequentially operated. The N.T.S.C. signal specification is not, however, ideal for the purpose, and if such a tube were in general use, a different specifi~ cation of the luminance and chrominance components would lead to even simpler receiver design. The fact that the mode of operation of the receiving tube can influence the most suitable choice of colour signal standards is regarded as an important develop­ment which deserves careful study.

4.6.3. Philco 21 in. "Bea;m-Index" Tube.

Technical details of this tube have not yet been released, but Philco hope to be able to demonstrate its performance in a few months time.

According to Philco this tube will overcome most of the difficulties; it can be mass-produced with relative ease, and will sell at about $37, i.e., 50% increase on the price of a 21 in. black-and-white tube.

The term "bea;m-index" is used for a class of tube for which the position of the scanning beam is "indicated" on arrival at the phosphor, and corrected to fall on the appropriate colour strip by means of a servo system. The feed-back loop for the servo control utilises a secondary emission signal derived from strips of metallic powder embedded in the phosphor. Only one of the three colour strips need be located, since the position of the other two is then determined.

The main advantage of this principle is the total utilisation of the scanning beam (as with the Lawrence tube), but without the need for high-power beam deflection at a point close to the phosphor.

Although a tube of this form may be cheap to produce, it will require fairly elaborate external circuitry which will increase the initial cost considerably. Philco admit this, but argue that in time the circuitry will be simplified, and meanWhile, the tube replacement cost is low.

This is regarded as being a most important development, further details of which are awaited with great interest.

4.6.4. Three-Tube Projection System.

Hazeltine have spent two years in developing a three-tube projection display using the Philips type projection units. They recently gave a private demonstration to their licensees and from all accounts the picture was very satisfactory. The trade, however, show little interest in this approach and it is doubtful if much more will be heard of it. The price of the complete unit would be $250, Which is not very different from the price of the R.C.A. 21 in. tube together with the scanning and focusing components,

4.6.5. G.E. Single-Gun Tube.

A single-gun tube developed by G.E. was described by Lafferty in last October's issue of the Proc. I.R.E.

24

G.E. say that this was merely an account of experimental work which had been conducted, and attach little importance to it. The beam-utilisation efficiency is no better than for the shado~mask tube, and this is regarded as a major disadvantage.

4.7. Maintenance of Studio Equipment.

N.B.C. state that maintenance effort for colour is about four times that for black-and-white. At Studio 72, however, C.B •. S. have a maintenance team of twelve engineers, as opposed to two for black-and-white. The equipment at this studio uses approximately 10000 valves.

A particular difficulty which all the networks appear to be experiencing, is drift of the balanced-modulator in the colour coding equipment. One result of this dr·ift is to cause excessive interference with the black-and-white picture, !'lince the sub-carrier no longer disappears on "white". In the equipment built by Research Department, the balance is primarily determined by a transformer, in place of valves, and this arrangement is believed to be more stable.

Details of test equipment have been given in an earlier report* and will therefore 'not be included here.

4~8. Colour Film.

Eastman Kodak (Rochester), and the Technicolor Corporation (Hollywood), are both stUdying the requirements of colour film for television viewing. Experience has shown that about 80% of colour film produced for direct projection is quite unsuitable for television, the main difficulty being the excessive contrast range and variation of average density.

Both these organisations have installed television scanning apparatus and have an electronics group working on the problem,

At Eastman Kodak, a convincing demonstration was seen, showing the effect of changing the ratio of key-light to fill-in-light used for the colour scene. The motion picture industry normally use a ratio of 4:1, but, for colour television, the best compromise was achieved with a ratio of 2:1 and this is their recommendation.

The Technicolor Corporation have studied the question from the point of view of print density. They recommend that the density range of the positive print should be restricted to 0-3 to 2-0---a contrast range of 50:1. This results in a print considerably "lighter" than for normal projection, for which the density range may extend from 0"3 to 3-0, corresponding with a contrast ratio of 500:1. They have produced 16 m.m. and 35 m.m. versions of a test film having approximately the re­commended density range, and this is in general use by networks for test purposes. A 16 m.m. pyint of this film was procured for use with our own 16 m.m. colour telecine.

Both of these organisations are very alive to the impact of television on the film industry and eaoh are producing ~elevision pictures of exceptionally high quality for the purpose of experimental work.

*Report OD Visit to U.S.A. by B.W. Baker, R.S.B. Bowell, R.B. Bammans and S.N. Watson

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4.9. Long Distance Links.

A.T. and T. were visited in New York and the problems arising in trans­mitting colour signals over long links were discussed. The problem was also discussed with Motorola (Chicago) who have developed micro-wave F.M. equipment for this purpose.

Although the coast-to-coast link is working exceptionally well, long term stability is still a problem. The tolerances for the N.T.S.C. signal are very small, and the overall transmission characteristic is liable to drift with time and tempera­ture. A further difficulty arises in the process of signal amplification in the link "repeaters", variations in the amplitude of the F.M. signal are liable to be converted into variations of phase, a problem which they believe to be common to both "klystron"

. and "travelling-wave" equipment, although they are only using the former.

Initially, the most serious difficulty was the variation of phase with modulation level, introduced in the receiver. This has been remedied, partly by including phase--equalisation in the i.f. circuit of the re.ceiver, and partly by "pre-­emphasising" the video signal. The phase equalisation is effected by inserting two bridged "T" equalising sections immediately following the frequency changer. These sections had to be specially designed to work at the i.f. frequency of 76 Mc/s. The pre--emphasis is actually a lo~frequency .cut of 12 dB which is introduced in the video' signal at the sending end and restored at the receiving point. By removing the low frequencies in this manner the total excursion of the modulating signal, and hence the carrier deviation, is reduced.

For low frequencies, the signal-noise ratio of the links is sufficiently high to stand for this attenuation, but extra care must be taken with "clamp" circuits since any errors will be magnified when the gain is restored.

The technique used by Motorola for permanent links is interesting, in that the parabolic reflector is placed horizontally at ground level, and is directed up towards a 45° reflector mounted on the tower. The reflector is not "plain" but has some curvature which further increases the effective gain. construction and maintenance.

5. MISCELLANEOUS ITEMS OF INTEREST.

5.1. ''Phone Vision".

This arrangement eases

This is the main project being handled by the research group of Zenith Radio. It offers a means of financing a television se.rvic·e without recourse to either licensing or sponsoring, and could prove to be of major importance.

Zenith have spent large sums of money on the development of proto-type equipment, and say that they have now overcome all practical difficulties. The Australian Broadcasting Corporation is now a licensee, and the approval of the F.C.C. is now being sought for use of the system in the U.S.A.

In principle, the idea is simply to "code" the picture in such a manner that it can be decoded only by viewers who pay a subscription. The actual method proposed

26

for achieving this, however, is very elaborate, and has undergone extensive tests to prove its secrecy and reliability. A convincing demonstration was given to the writer, in which both sound and vision signals were observed, coded and decoded. In fact, sound had proved the most difficult problem and the solution had only recently been found.

So much thought has gone into the design and development of this scheme, that it is considered worth while including the more detailed· description which will be found in Appendix 5.

5.2. U.H.F. Broadcasting.

General Electric have developed a high-power u.h.f. transmitter, which employs two pairs of di-plexed klystrons, and delivers a power of 45 kW into the aerial. From calculations based on experimental work, they have produced a chart showing the relative powers required for u.h.f. and v.h.f. broadcasting, in order to have the same coverage over relatively flat ground. Assuming transmission from Washington, using a 500 ft tower, the figures quoted are:

UHF VHF Grade A

ERP ERP Ratio Coverage KW KW UHF/VHF (Miles) Channels 14-83 Channels 7-13

32 1000 200 5:1

24 250 60 4;1

10 5 1'6 3:1

Mr. Edward AlIen of the F.C.C. gave a somewhat more pessimistic view of the capabilities of u.h.f. as compared with v.h.f., saying that over typical terrain as much as fifty times the power may be needed to provide the same coverage as v.h.f. This was on the basis of a u.h.f. receiver with a noise factor of 12 dB, and a v.h.f. receiver noise factor of 5 dB. In his view, the difference in the powers required for similar coverage was likely to increase, since the noise factor of v,h.f. receivers was steadily improving, which is not the case for u.h.f. receivers.

The reason that u.hef. is not proving a success, however, is not primarily due to technical limitations, but to the difficulty of competing with an established v.h.f. service. If a v.h.f. transmission is available a sponsor will naturally favour it in preference to a newly started u.h.f. service for which there may be no viewers.

5.3. Optical Lens Testing.

An interesting d€velopment in optical lens testing was demonstrated by Mr. W. Herriot, who is investigating optical problems at Bell Laboratories. In principle, the method is similar to that used by Research Department, but instead of using a square wave, or a sinusoidal grating, as the test pattern, a sinusoidal

variabLe-area pattern, similar to a film sound track, is employed. The pattern~ recorded on film stock of very high resolution, in the manner of a sound-track, with a continuous variation of frequency commencing at' about 50 cycles/in. and increasing to 800 cycles/in. The test film is mounted round a transparent cylinder, illuminated from within, Which is rotated at a moderate speed. By synchronising the time-base of a cathode-ray oscilloscope with the rotational frequency of the test pattern, the response of the lens under test can be measured photo-electrically and displayed as a continuous curve. Whether a response curve measured in terms of a variable-area pattern can be interpreted in the same manner as a grating pattern with a sinusoidal density variation is not yet established. It is an interesting approach which deserves study.

5.4. Information Theory Applied to Television.

The application of Information Theory to television, with the object of bandwidth reduction, is being studied by a number of organisations, including Bell Laboratories, M.I.T.~ R.e.A.~ and G,E. These particular organisations were visited, but no doubt many others are similarly engaged. In all cases, as far as could be ascertained, the investigations are concerned more with a study of picture statistics than with actual methods of compression.

Following the recent work of Kretzmer* and Harrison** Bell Laboratories hold the view that little is to be gained by exploiting the "short-term" correlation which exists in single lines of a television signal, and practical methods of ex~ ploi ting the correlation between successive frames have yet to be developed. They are now turning to the question of how much information is necessary from the sub­jective-viewpoint, and are planning to study thi~ problem by constructing a picture using pin heads as picture elements. The aim is to find out what restrictions can be applied to a picture image before "information!l is perceptibly lost.

At the present time they feel that, for long distance transmission, "E" layer scatter is more promising than frequency compression.

R.e.A., at Princeton, have been studying the statistics of television signals in terms of the "difference" signal obtained by subtracting successive lines and fields. This work was not seen, however, and no mention was made of any definite approach to bandwidth compression.

At M.LT. a demonstration was given of apparatus which measures the proba­bility distribution of the variations of light in any given direction across a television picture.

The method, which is relatively simple to instrument, is described in a published thesis by W.F. Schreibert. They are using picture facsimile equipment for the purpose of studying "quantization" of a picture signal, the ultimate object being to remove redundancy in both the horizontal and vertical directions. This, they believe, will be necessary for any worth while degree of bandwidth compression. This is long-term work and no great effort is being expended.

*E.R. Kretzmer, "Statistios of. Television Signals". Bell System Technical Journal, Vol. 31, pp. 751-763, July 1952

**C.W. Harrison, "Experiments with Linear Prediction In Television". Journal, Vol. 31, pp. 764-783, July 1952

tWilliam F. Schreiber, "Probability Distribution of Television Signals", liarvard University, December 1952

Bell System Technical

Cruft LaboratOrY,

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5.5. Network Synthesis by Delay Compinations.

During the visit to M.I.T., the writer attended an interesting lecture on the subject of network synthesis. The lecturer, Dr. G. Farrell, showed that any network could be synthesised by means of additions and subtractions of delayed signals. The method has much in common with "derivative equalisation", and the mathematical development proceeds on very similar lines. This work will be published in the near future.

6. CONCLUSIONS.

The aim of this report has been to present information, views, and impres­sions collected during a six-week visit to the U.S.A. in which twenty-two separate organisations were visited. Needless to say, many details have been omitted, but much that has been left unsaid will be found in the reports of previous visitors, in particular the recent report by Messrs. Baker, Howell, Hammans and Watson, which deals at length with operational aspects.

In the writer's opinion, the more important conclusions to be drawn from the visit are:

i. The introduction into the U.S.A. of colour television as a public service has been premature. Whilst it is unlikely that any serious mistakes have been made in the choice of system, there remain a number of technical problems which will need to be solved before colour can become a regular feature of television broadcasting.

ii. The present high cost of the colour receiver is undoubtedly hindering progress, and this in turn is largely influenced by the cost of the solour tube. It is regarded as si'gnificant that with a single-gun tube the receiver could be cheapened slightly by adopting standards which differ in . detail from the N.T.S.C. specification. If possible, it would seem therefore desirable to leave details of the signal specification until after the development of a satisfactory colour tube.

iii. The cost of the colour receiver would not account fully for the complete indifference of the American public towards colour, which seems to exist at the present time. The very limited hours of colour broadcasting is probably a factor of equal importance which is also primarily governed by economic consideration.

iv. The implications arising from the fact that colour television requires very much more light than black-and-white television do not seem to have received sufficient emphasis. A lighting level of 300-400 ft-lamberts is in excess of that which was needed for the simple iconoscope in the early days of television and represents a most stringent requirement, particu­larly for outside broadcasting. If a colour service is to compare in flexibility with the present day black-and-white service it is vital that the means are found to reduce this requirement. The ultimate sensitivity

-

of the Chromacoder compared with that of the triple-gun camera should be studied, as this could influence the extent to which it might be worth developing further the principle on which the Chromacoder operates.

v, In general commercial colour film is not suitable for television trans­mission, on account of having an excessive contrast range, and means will have to be found for obtaining adequate supplies of colour film, which is suitable in this respect.

For experimental purposes a limited quantity of satisfactory material might be foundby careful selection from available supplies, but ultimately the production of special films for television transmission may have to be faced.

vi. Telerecording, for the purpose of "repeats"', is particularly attractive, in view of the relatively high cost of colour as compared with black-and­white. For single repeats, the possibility of telerecording with Eastman negative and using the negative for subsequent transmission is worth serious consideration.

vii, It is desirable to establish if image orthicon camera tubes, produced in America, are in fact giving a better performance than are the tubes used here. A comparison should be made under a fixed set of conditions and, in p_articular, the signal-noise ratio and . the depth of beam-modulation should be compared for the two tube's.

viii. According to American e:x;perience a good telerecording of a studio programme can be achieved only by lighting the scene with the limitations of tele­recording in mind. Low scene-contrast and the avoidance of large dark areas are the important factors. If the Corporation is to adopt tele­recording, as a routine means of repeating programmes, a similar technique will no doubt have to be adopted here. As an experimental procedure it might be worth studying the form of lighting that will produce the most acceptable picture on a monitor which has been modified to reproduce the type of distortions incurred by telerecording.

29

In conclusion, the writer would like to e:x;press appreciation for the helpful co~operation extended by Mr. Basil Thornton and his staff at the B.B.C. New York Office. In particular, thanks are due to Mr. A.S.R. Toby for his invaluable help in making numerous appointments and arranging a complicated itinerary.

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. .APPENDIX 1

PERFORMANCE OF THEVIDICON CAMERA

Demenstratiens ef the Vidicen are semetimes se excellent as to' cause ene to' wender why these tubes. are net in more general use. This matter was discussed with Mr. R.G. Neuhauser ef N.B.C. whO', being aware ef certain miscenceptiens, had presented a paper at a recent Cenference* in erder to' clarify the matter.

He peints eut that the Vidi,con pessesses the fellewing features:

i. High sensit'ivity (clese to' that ef the image erthicen)

ii. High signal-neise ratiO'

iii. Excellent resolutien

iv. Flatbackgreund

v. Negligible' "smear" en meving ebjects

vi. Leng life.

All ef the claims ·are true, but net allc.an be realised at the same time.

Fer example, when eperated in a manner to' have a sensitivity cemparab1ewith the image erthicon, the "smear" becO'mes ebjectionable and the "backgreund" ne 1enger flat. If, hewever, the twO' tubes are cempared with lenses chesen to' give the same depth ef fecus and viewing angle, the Vi'dicen will give a. similar perfermance to' the image erthicen in virtually all respects, but will require ten times the il1uminatien. Thus, an image erthicen will normally require abeut 50 ft-candles ef incident scene il1uminatien with the lens aperture at f/5' 6, and a; similar resul twill be ebtained with a Vidicen with 500 ft--cand1es, but with a lens aperture ef f/2'8 en acceunt ef the smaller p.hete-cathede. The viewing angle and .depth of fecus will then be neminal1y the same.

The figure ef 500ft-cand1es, weu1d certainly prehibit the use ef the Vidicen fer general use as a camera tube, and R.C.A. regard this as the minimum i11uminatien necessary fer a perfermance equal to' that ef the image erthicen. With less i1luminatien ene er ether ef its characteristics will, ef necessity, have to' be sacrificed.

These figures apply equally to' the 6198 type tube, a general purpese tube fer "live" pick-up, and to' the 6326 type tube which was designed fer film pick-up. In the app).icatiO'n to' film pick-up there is ne preblem in ebtaining sufficient illuminatiO'n, and the perfermance ebtained is excellent.

*VIII NoAoRoToBo Cont'erence, Chi e a,:!; ° , 1954

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APPENDIX 2

PERFORMANCE OF IMAGE ORTHICON

The E.E.V. image orthicon used by the Corporation appears to have a per­formance somewhat inferior to that of the R.C.A. 5820 tube used in America, mainly in respect of signal-noise ratio. A comparison of some of the design features is therefore of interest.

The E.E.·V. and R.C.A. designs differ in two main respects: first, the spacing between the target and the mesh in the image section is 0'0015 in. for the E.E.V. tube, and 0'0025 in. for the R.C.A. tube. The closer spacing would not influence the signal-noise ratio for a given level of illumination, but would have the effect of raising the position of the "knee" to a point corresponding with about twice the level of illumination. For the same illumination the E.E.V. tube would therefore suffer less from the effects of electron redistribution which show as I1lialo" and excessive contrast on edges. In fact, R.C.A. recommend that the 5820 tube should be operated with the lens aperture increased by one stop from the point where the highest highlight reaches the knee, so that for similar illumination and aperture the E.E. V. tube would be operating on a substantially linear characteristic, and suffer no redistribution effects. It would, however, require gamma correction in order to produce a satisfactory overall contrast law. With approximately twice the illumi­nation the tube should operate normally a~ound the knee, but in theory, with a 3 dB increase in signal-noise ratio, due to the increased light.

It is of interest to note that ReC.A. virtually admit the need for gamma correction when their tube is operated normally. According to their recommendations, if a dark background is to be "brought out .. , thi s should be accompli shed by lighting the area in question, and not by opening the lens aperture. Illuminating the dark areas of a scene will approximate to the effect of gamma correcting the signal, but will do so without increasing the noise level.

The R. C. A. 1854 tube, which has been designed for colour, uses. an even closer target to mesh spacing of 0'001 in., which further raises the position of the knee by a f actor of about 1" 5. For colour operation thi s tube is operated stri ctly below the knee and therefore requires full gamma correction.

The second difference concerns the deoelerating grid on the scanning side of the target. With the R.C.A. tube, this is in the form of a ring, but the E.E.V. tube uses a mesh through which the scanning beam must pass in order to reach the target. Taking into account the return beam, the scanning beam in fact passes through the mesh twice, and R.C.A. maintain that this will result in partition noise, as with a pentode compared with a triode. (Mr. McConnel of the M.W.T. Co. has since confirmed this, but states that the measured increase in noise is only two or three decibels. It would howe,ver offset the advantage of the closer target to mesh spacing, in respect of noise.)

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The factor which is likely to have the greatest effect on the noise level, is the uniformity of the cathode surface in the gun. The effect of irregularities in the cathode is to introduce a wide spread in the velocities of the individual electrons forming the scanning beam. With the tube potentials correctly adjusted, the slower moving electrons will not reach the target and will therefore not contribute to the modulation of the return beam. Thus, the greater the spread of velocities, the lower will be the depth of modulation in the return beam, which means a lower signal-noise ratio.

The validity of this can easily be checked by comparing the depth of modulation obtained with each type of tube working under identical conditions.

APPENDIX 3

CHOI CE OF ILLUMINANT C

The N.T.S.C. colour system specifies Illuminant C (6500° K) as the reference white. Some confusion appears to exist, even in America, concerning the significance of this choice, and there is some doubt about the correctness of the procedure which is being used to line up studio cameras and film scanners.

According to the N.T.S.C. specification, if the light entering the camera lens corresponds with Illuminant C, then the primary signals produced by the camera should be of equal amplitude, whilst the amplitude of the chrominance signal should be zero. For this condition, a receiver correctly aligned should produce three equal signals which, applied to the colour tube, will reproduce Illuminant C.

Ideally, any other colour presented to the camera should also be reproduced accurately by the receiver, so that the choice of the "reference" illuminant has no bearing on the colour rendering properties of the system, but effects only the "colour" for which the sub-carrier disappears. It is obviously an advantage from the point of view of dot-interference to have the sub-carrier disappear on a colour close to "white"', but the exact colour is relatively unimportant.

The choice of the reference "white" does however affect the "reverse compatible" picture. For a black~and~white transmission, the colour sub-carrier will be absent, so that the colour receiver will reproduce the black-and-white picture in whatever "colour" is chosen for reference "white", The slightly blue tint of Illuminant C has been generally favoured for black-and-white receivers in recent years, and was chosen as the reference in the N.T.SeC. specification, solely with this in mind.

Notwithstanding this preference in black-and-white reproduction, it does not follow that Illuminant C is the correct choice for the "white" in a colour picture, and many people here and in America hold the view that it is too "blue". If, however, the colour channel is correctly adjusted, the "white" in the colour picture is quite independent of the reference white, and ideally will be determined entirely

by the colour of the lighting used in the studio. seems to be arising.

33

It is on this point that confusion

Present practice in America is to line up the camera to give equal primary signals, using a plain White card, illuminated with the normal studio lighting which is tungsten run at 2900° K. With a receiver correctly lined up, the test card, and any "white" surface subsequently introduced into the colour scene, will be reproduced as Illuminant C, which is undesirable for the reason stated. Strictly speaking the whole colour balance has been upset by the procedure, although the overall effect is mainly to produce a rather "cold" picture. By ana,logy, a similar effect is produced when film, Which ,has been processed for tungsten projection, is projected with an arc of colour temperature close to Illuminant C.

For the scene to be accurately reproduced, the correct procedure would be to line up the camera using an Illuminant C light source. Alternatively, a blue tinted test-card could be used, Which, When illuminated with tungsten would reflect Illuminant C. In either case, assuming an ideal colour system, the colours reproduced by the receiver would correspond exactly.with the colours in the scene.

It may be argued that tungsten lighting at 2900° K is too "yellow" for colour reproduction, even for "indoor" scenes, and that it is uneconomical to increase the colour temperature of studio lighting. If this is the case, a compromise could be effected by using a slightly "bluish" test-card for camera line-up, which, when illuminated with tungsten had a colour temperature somewhat less than Illuminant C. The exact colour of the test card could be determined by experiment, to provide a condition for Which a normal white surface would be reproduced by the receiver to have a colour temperature of, say, 3500° K, as recommended in Research Department Report No. T-045 (1954/8).

The same practice could be adopted when transmitting colour film which had been processed for tungsten projection. In this case a slightly "bluish" trans­parency would be used for the scanner line-up.

APPENDIX 4

MAGNETIC RECORDING OF VIDEO SIGNALS

This appendix supplements the information given in Section 4.5.3. of themam text, concerning the activities of organisations who are working on various aspects of the problem of video recording on magnetic tape.

Apart from the more general problem, the question of recording the video signal on a rotating drum was disoussed with the staff of Aropex Corporation, who offered the following comments:

Standard tape can easily record 2000.cycles/in., which corresponds to a

34

wavelength on the tape of 0'0005 in. For a drum of 30 in. circumference, this would mean 60000 cycles per revolution, so that at 3000 rev/min 3'0 Mc/s would be recorded. Assuming that the recording head is not in contact with the tape, ferrite would be the best material to use for the head. In their opinion, the smallest effective gap which could be obtained with ferrite was about 0'0005 in., on account of the granular structure of the material. If a similar clearance could be maintained between the head and the tape, it should not be difficult to record 2'5 to 3'0 Mc/s with a 12 in. drum.

It was suggested that the required clearance between the head and the tape might be maintained by using the Bernoulli effect. The idea is to spring-load the head lightly on to' the tape, and to force air through a small hole drilled in the head. This lifts the head off the tape by an amount depending on the air pressure, so that the head automatically "servos" itself to have the correct clearance, regard­less of eccentricity of the drum. Bing Crosby Enterprises have successfully used this method for applications involving a drum.

To gain the advantage of contact tvorking, a gap considerably smaller than 0'0005 in. must be used, and Ampex hold the view that ferrite is no longer practicable, so that laminated metal must be used. The problem was that of using extremely thin (less than 0·0005 in.) laminations, and of providing the necessary insulation without seriously reducing the effective working area.

Bing Crosby Enterprises have abandoned the time-multiplex a~proach, in which a large number of tracks are used, because unavoidable skew of the tape caused timing errors which resulted in a "herring-bone" pattern on the picture. In a manner similar to R.C.A., they are now recording a complete video signal on a single track using a tape speed of 360 in./sec. Wi th a recording head gap of 0'000125 in., they say they can achieve about 8000 cycles/in. on the tape, which at 360 in. /sec represents a frequency of approximately 2'9 Mc/s.

For colour they use five tracks, three of whi'ch are for the red, green, and blue signals, one for sound, and one for synchronising the tape speed. The tape is locked between two idlers which run on a servo-controlled capstan to form a loop which is isolated from the drive and take-up. Servo-control of the tape speed is effected on replay by means of a 100 kc/s sine wave recorded on the fifth track. The re­produced frequency is divided down to 1000 c/s, and compared with a reference frequency derived from a tuning fork, and the error signal thus obtained is used to vary the angular velocity of the capstan, so that, at any instant, the replay speed corresponds to the recording speed.

For the sound track they are using F.M., with a frequency deviation of 50 kc/s on a carrier frequency of 150 kc/so The frequency discriminator used for demodulation is a true frequency counter. This, they claim, gives exceptionally good sound quality with a signal-noise ratio of about 70 dB.

As a next move, they are proposing to reduce the tape-speed to 180 in./sec. To achieve this they will use two tracks for the green signal, each carrying only one­half of the video-frequency band, and will reduce the bandwidth of the red and blue

signals to approximately I' 5 Mc/s.

35

The greatest difficulty at the present time is the occasional failure of the signal, known as "drop-outs", due to impurities in the oxide deposit on the tape. Because of the high tape-speed a very small blemish (0'03 in.) can cause the loss of a complete television line. The Minnesota Mining Corporation, who are one of the biggest tape manufacturers in the V.S.A., will soon be in a position to supply a new tape which they are now developing, and which, they claim, will overcome most of this difficulty.

Brush Laboratories probably know more about the design of recording heads than any other organisation in the V.S.A. Whilst they were not in a position to divulge details of the head design, they revealed that they had succeeded in developing a ferrite core for working in contact with the tape. The size of the gap was smaller than anything so far talked about; "one wavelength of green light" was the dimension actually quoted, i.e., about 0·00002in. In their view tape was capable of recording more than 10000 cycles/in., and they were aiming at achieving an upper frequency Hmi t of 5 Mc/s.

Other points of interest were raised in a discussion with Dr. J. Young who is working in the magnetic laboratory of M.I.T. Dr. Young has been investigating a theory that a single crystal of the oxide deposit represents more than one magnetic domain, so that the smallest wavelength which can be recorded is smaller than the dimensions of the individual crystals. The form of the crystals is believed to be cylindrical, with tapering sections, rather like two telescopes joined at the largest diameter. The whole cryst.al is believed to be about 3 microns long, and 1 micron thick. The theory is that each sect:i:on represents one magnetic domain.

Methods of processing the tape were also discussed. It is common practice to deposit the oxide layer under the influence of a strong magnetic field whilst the base is still plastic. The object of this is to align the crystals along the length of the tape, but the effect of the orientation is to produce a slight roughening of the surface, which cannot be tolerated for contact working.

Dr. Young also suggested that the effective gap which could be obtained with ferri te was probably substantially larger than the physical gap on account of th e surface crystals being destroyed in the polishing process. He believed it would be possible to remove the surface layer after polishing by means of electrolytic etching.

APPENDIX 5

"PHONE VISION"- ZENITH RADIO

The method by which "Phone Vision" operates is as follows:

At the transmitter the starting time of the line time-base is "keyed" by a square-wave signal of frequency one-eighth the line frequency. The effect of this on a normal receiver is to displace laterally alternate groups of 32 lines by about one

36

inch. There are, however, six possible phases for the keying signal, and for each field one of the phases is selected at random, by means of random noise applied to an electronic switch. Information of the particular phase selected for each field is transmitted to the receiver by means of a short burst of video frequency inserted in the field-suppression period. The burst may have anyone of six possible frequencies, corresponding with the six phases.

~rthermore, the "burst" can occur in anyone of eight possible positions in a sequence of eight fields, the order of appearance being determined by the "programme item" code. Thus, for a particular item in a programme, the position sequence over eight frames might be, for example, 3, 5, 2, 1, 7, 4, 8, 6, but this would be changed for the next item. Together with the phase code there are thus 68

combinations of position and frequency, only one of which is correct for a sequence of eight frames.

The sound channel uses the same code, except that in this case the square wave is used to reverse the phase of the sound signal.

For the purpose of decoding, the subscriber is given a small box, with six knobs each with eight positions. This is connected to any normal receiver by removing certain valves and replacing them with plugs similar to the valve base but wired to the box, The box generates a keying signal similar to that at the trans­mitter, but with. the phase controlled by the position and frequency of the incoming "burst". Since there are 6 8 ways (over a million) of wiring the controls, it is possible to give each subscriber an almost unique arrangement, which is identified by a code number. (In fact, it would not be necessary to exploit all of the possibili­ties provided the chance of an immediate neighbour having a similar code was made exceedingly small.)

One method of operating the system is for each item of the programme to be published with a code number, and for the subscriber to phone the broadcaster giving the code of the desired programme and his box code, whereupon he would be told the appropri ate knob settings. Calculation of the settings from the available data would of course be p.ossib1e, but totally impracticable, and for this purpose an elaborate electronic computer has been developed. By pressing numbered buttons the relevant information is fed to the computer, which, almost instantly, displays the required settings as a six digit number.

A postal method has also been devised which greatly simplifies the problem of accountancy. Each sub~criber would receive, once a week, a special type of punched card which is prepared automatically by the computer. The card is marked with the code numbers of all the programmes for the week, and so constructed that when a peg is pushed through the code number of the desired programme the appropriate settings are revealed on a small tab. The amount to be paid by the subscriber, for the week, is determined immediately by inserting the perforated card into an accounting machine.