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CHAPTER 26

Colour Signal Transmission and

Reception

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Three different systems of colour television (CTV) emerged after prolonged research

and experimentation. These are:

(i) The American NTSC (National Television Systems Committee) system

(ii) The German PAL (Phase Alteration by Line) systems(iii) The French SECAM (Sequential Couleures a memoire) system

When quality of the reproduced picture and cost of equipment are both taken into

account, it becomes difficult to establish the superiority of one system over the other

Therefore, all the three CTV systems have found acceptance in different countries and

the choice has been mostly influenced by the monochrome system already in use inthe country

Since India adopted the 625 line CCIR (B standards) monochrome system it has

chosen to introduce the PAL system (B & G standards) because of compatibility

between the two, and also due to its somewhat superior performance over the other

two systems

In many respects transmission and reception techniques employed in the NTSC andPAL systems are similar

These are, therefore, treated together before going into encoding and decoding details

of each system

The SECAM system, being much different from the other two, is described separately

in the later part of this chapter

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26.1 COLOUR SIGNAL TRANSMISSION

The colour video signal contains two independent informations, that of hue and

saturation

It is a difficult matter to modulate them to one and the same carrier in such a way that

these can be easily recovered at the receiver without affecting each other

The problem is accentuated by the need to fit this colour signal into a standard TV

channel which is almost fully occupied by the Ysignal

However, to satisfy compatibility requirements the problem has been ingeniously

solved by combining the colour information into a single variable and By employing what is known as frequency interleaving

Frequency Interleaving

Frequency interleaving in television transmission is possible because of the

relationship of the video signal to the scanning frequencies

It has been determined that the energy content of the video signal is contained inindividual energy bundles occuring at harmonics of the line frequency (15.625,

31.250 ... KHz)

The components of each bundle being separated by a multiplier of the

field frequency (50, 100, ... Hz)

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The shape of each energy bundle shows a peak at the exact harmonics of the

horizontal scanning frequency

This is illustrated in Fig. 26.1

As shown here, the lower amplitude excursions that occur on either side of the

peaks are spaced at 50 Hz intervals Also represent harmonics of the vertical scanning rate

The vertical sidebands contain less energy than the horizontal because of the

lower rate of vertical scanning

Note that the energy content progressively decreases with increase in the

order of hormonics and is very small beyond 3.5 MHz from the picture carrier

Ref: Monochrome n Colour Television by R.R. Gulati (2nd Edition Revised Version)

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It can also be shown that when the actual video signal is introduced between the line

sync pedestals, the overall spectra still remains bundled around the harmonics of the

line frequency and

the spectrum of individual bundles become a mixture of continuous portion due to the

video signal are discrete frequencies due to the field sync as explained earlier

Therefore, a part of the bandwidth in the monochrome television signal goes unused

because of spacing between the bundles

This suggests that the available space could be occupied by another signal

It is here where the colour information is located by modulating the colour difference

signals with a carrier frequency called coloursubcarrier The carrier frequency is so chosen that its sideband frequencies fall exactly mid-way

between the harmonics of the line frequency

This requires that the frequency of the subcarrier must be an odd multiple of half the

line frequency

The resultant energy clusters that contain colour information are shown in Fig. 26.2by dotted chain lines along with the Y signal energy bands.

In order to avoid crosstalk with the picture signal, the frequency of the subcarrier is

chosen rather on the high side of the channel bandwidth

It is 567 times one-half the line frequency in the PAL system

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This comes to: (2 283 + 1) 15625/2 = 4.43 MHz

In the American 525 line system, owing to smaller bandwidth of the channel, the

subcarrier employed is 455 times one-half the line frequency i.e., (2 227 + 1) 15750/2

and is approximately equal to 3.58 MHz

Ref: Monochrome n Colour Television by R.R. Gulati (2nd Edition Revised Version)

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26.2 BANDWIDTH FOR COLOUR SIGNAL

TRANSMISSIONThe Y signal is transmitted with full frequency bandwidth of 5 MHz for maximum

horizontal details in monochrome

However, such a large frequency spectrum is not necessary for colour video signals

The reason is, for very small details, the eye can perceive only the brightness but not

the colour

Detailed studies have shown that perception of colours by the human eye, produced

by combinations of the three primary colours is limited to objects which have

relatively large coloured areas ( 1/25th of the screen width or more) On scanning they generate video frequencies which do not exceed 0.5 MHz.

Further, for medium size objects or areas which produce a video frequency spectrum

between 0.5 and 1.5 MHz, only two primary colours are needed This is so, because for finer details the eye fails to distinguish purple (magenta) andgreen-yellow hues from greys

As the coloured areas become very small in size (width), the red and cyan hues alsobecome indistinguishable from greys

Thus for very fine colour details produced by frequencies from 1.5 MHz to 5 MHz, all

persons with normal vision are colour blind and see only changes in brightness even

for coloured areas

Therefore, maximum bandwidth necessary for colour signal transmission is around 3

MHz ( 1.5 MHz)

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26.3 MODULATION OF COLOUR DIFFERENCE SIGNALS

The problem of transmitting (B-Y) and (R-Y) video signals simultaneously with onecarrier

frequency is solved by creating two carrier frequencies from the same coloursubcarrier without

any change in its numerical value. Two separate modulators are used, one for the(B-Y) andthe other for the (R-Y) signal. However, the carrier frequency fed to onemadulator is given a

relative phase shift of 90 with respect to the other before applying it to the

modulator. Thus, the two equal subcarrier frequencies which are obtained from a common

generator are said to be in quadrature and the method of modulation is known asquadrature modulation.

After

modulation the two outputs are combined to yield C, the resultant subcarrier

phasor. Since

the amplitude ofC, the chrominance signal, corresponds to the magnitudes ofcolour difference

signals, its instantaneous value represents colour saturation at that instant.

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Ref: Monochrome n Colour Television by R.R. Gulati (2nd Edition Revised Version)

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Maximum

amplitude corresponds to greatest saturation and zero amplitude to no saturation

i.e., white.

Similarly, the instantaneous value of the C phasorangle () which may vary from 0

to 360

represents hue of the colour at that moment. Thus the chrominance signal contains

full

information about saturation and hue of various colours. This being a crucial point in

colour

signal transmission, is illustrated by a few examples. However, it would be necessary

to first

express (R-Y) and (B-Y) in terms of the three camera output voltages. This is done by

substituting

Y = 0.59G + 0.3R + 0.11B in these expressions. Thus (R-Y) becomes R 0.59G 0.3R

0.11B = 0.7R 0.59G 0.11B. Similarly, (B-Y) becomes B 0.59G 0.3R 0.11B = 0.89B

0.59G

0.3R.

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Now suppose that only pure red colour is being scanned by the colour camera.

This

would result in an output from the red camera only, while the green and blue

outputs will be zero. Therefore, (R-Y) signal will become simply + 0.7R and (B-Y) signal will be

reduced to

0.3R. The resultant location of the subcarrier phasor after modulation is

illustrated in Fig.

26.3. Note that the resultant phasor is counter clockwise to the position of + (R-Y)

phasor.

Next consider that the colour camera scans a pure blue colour scene. This yields

(R-Y)

= 0.11B and (B-Y) = 0.89 B. The resultant phasor for this colour lags + (B-Y) vector

by a small

angle. Similarly the location and magnitude for any colour can be found out. This isillustrated

in Fig. 26.3 for the primary and complementary colours.

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Another point that needs attention is the effect of desaturation on the colour

phasors.

Since desaturation results in reduction of the amplitudes of both (B-Y) and (R-Y)

phasors, the resultant chrominance phasor accordingly changes its magnitude depending on

the degree of

desaturation. Thus any change in the purity of a colour is indicated by a change in

the magnitude

of the resultant subcarrier phasor.

Colour Burst Signal

Suppressed carrier double sideband working is the normal practice for modulating

colourdifference

signals with the colour subcarrier frequency. This is achieved by employing

balanced

modulators. The carrier is suppressed to minimize interference produced by the

chrominance

signals both on monochrome receivers when they are receiving colour

transmissions and in

the luminance channel of colour receivers themselves

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As explained in an earlier chapter the

ratio of the sideband power to carrier power increases with the depth ofmodulation. However,

even at 100% modulation two-thirds of the total power is in the carrier and only

one-third is the useful sideband power. Thus suppressing the carrier clearly eliminates the

main potential

source of interference. In addition of this, the colour-difference signals whichconstitute the

modulating information are zero when the picture detail is non-coloured (i.e., grey,

black or

white shades) and so at such times the sidebands also disappear leaving nochrominance

component in the video signal.

As explained above the transmitted does not contain the subcarrier frequency butit is

necessary to generate it in the receiver with correct frequency and phaserelationship for proper

detection of the colour sidebands. To ensure this, a short sample of the subcarrieroscillator, (8 to 11 cycles) called the colour burst is sent to the receiver along withsync signals.

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This is

located in the back porch of the horizontal blanking pedestal. The colour burst

does not interfere

with the horizontal sync because it is lower in amplitude and follows the sync

pulses. Its exact

location is shown in Fig. 26.4. The colour burst is gated out at the receiver and is

used in

conjuction with a phase comparator circuit to lock the local subcarrier oscillator

frequency and

phase with that at the transmitter. As the burst signal must maintain a constantphase

relationship with the scanning signals to ensure proper frequency interleaving, the

horizontal

and vertical sysc pulses are also derived from the subcarrier through frequency

divider circuits.

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26.4 WEIGHTING FACTORS

The resultant chrominance signal phasor (C) is added to the luminance signal (Y)

before

modulating it with the channel carrier for transmission. The amplitude, i.e., level

line of Y

signal becomes the zero line for this purpose. Such an addition is illustrated in Fig.

26.5 for a

theoretical 100 percent saturated, 100 percent amplitude colour bar signal. Thepeak-to-peak

amplitude of green signal ( 0.83) gets added to the corresponding luminance

amplitude of

0.59. For the red signal the chrominance amplitude of 0.76 adds to its brightness

of 0.3.

Similarly other colours add to their corresponding luminance values to form the

chroma signal.

However, observe that it is not practicable to transmit this chroma waveform

because the

signal peaks would exceed the limits of 100 percent modulation.

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This means that on modulation

with the picture carrier some of the colour signal amplitudes would exceed thelimits of

maximum sync tips on one side and white level on the other. For example, in thecase of

magenta signal, the chrominance value of 0.83 when added to its luminanceamplitude of

0.41 exceeds the limits of 100 percent modulation of both white and black levels.Similarly blue

signal amplitude greatly exceeds the black level and will cause a high degree ofovermodulation.

If overmodulation is permitted the reproduced colours will get objectionablydistorted.

Therefore, to avoid overmodulation on 100 percent saturation colour values, it isnecessary to

reduce the amplitude of colour difference video signal before modulating themwith the colour

subcarrier. Accordingly, both (RY) and (BY) components of the colour video signalare scaled down by multiplying them with what are known as weighting factors.

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Those used are 0.877

for the (RY) component and 0.493 for the (BY) component. The

compensated values are

obtained by using potentiometers at the outputs of (RY) and (BY) adders

(see Fig. 25.9).

Note that no reduction is made in the amplitude ofY signal. It may also be

noted that since the

transmitter radiates weighted chrominance signal values, these must beincreased to the

uncompensated values at the colour TV receiver for proper reproduction

of different hues.

This is carried out by adjusting gains of the colour difference signal

amplifiers. The unweighted and weighted values of colour difference signals are given below in Table

26.1.

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26.5 FORMATION OF THE CHROMINANCE SIGNAL

Using the information of Table 26.1, Fig. 26.6 illustrates the formation of the chromasignal for

a colour bar pattern after the colour difference signals have been scaled down inaccordance

with corresponding weighting factors. Note that new amplitudes of the chrominancesubcarrier

signals are 0.63 for red and cyan, 0.59 for green and magenta and 0.44 for blue andyellow.

These amplitudes will still cause overmodulation to about 33%. This is permitted,because in

practice, the saturation of hues in natural and staged scenes seldom exceeds 75percent. Since

the amplitude of chroma signal is proportional to the saturation of hue, maximumchroma

signal amplitudes are seldom encountered in practice.

Therefore, the weighted chroma values

result in a complete colour signal that will rarely, if ever, overmodulate the picturecarrier of

a CTV transmitter.

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Hence it is not necessary to further decrease the signal amplitudes by

employing higher weighting factors.

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Chroma Signal Phasor Diagram

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The compensation (readjustment) of chroma signal values results in a change of

chroma phase

angles. In the NTSC system it is a common practice to measure phase angles

relative to the (BY) phasor. This location has been designated 0 or the referencephase position on the

phasor diagram (see Fig. 26.7) because this is also the phase of the colour burst

that is

transmitted on the back porch of each horizontal sync pulse. Referring to Fig. 26.7

the compensated colour magenta is represented by a phasor at an angle of 119. In

the same

manner the diagram indicates phase angles and amplitudes of other colour signals.

Note that

primary colours are 120 apart and complementary colours differ in phase by 180

from their

corresponding primary colours.