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

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Grassmans law

Tristimulus values of Spectral Colours

25.5 Luminance, hue and saturation

Luminance/brightness

Hue

Saturation

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Colour Plate 3: Variation in colour on the surface of the chromaticity diagram

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25.6 Colour Television Camera


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Fig 25.5 shows a simple block schematic of a colour TV camera

It essentially consists ofthree camera tubes

Each tube receives selectively filtered primary colours

Each tube develops a signal voltage proportional to the respective colourintensity received by it

Light from the scene is processed by the objective lens system

The image formed by the lens is split into three images by means of glassprisms

These prisms are designed as diachroic mirrors A diachroic mirror passes one wavelength and rejects other wavelengths

(colours of light)

Thus red, green, and blue colour images are formed

The rays from each of the light splitters also pass through colour filterscalled trimming filters

These filters provide highly precise primary colour images converted intovideo signals by image-orthicon or vidicon camera tubes

Thus the three colour signals Red (R), Green (G) and Blue (B) signal aregenerated

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Simultaneous scanning of the three camera tubes is accomplished by a masterdeflection oscillator and sync generator which drives all the three tubes

The three video signals produced by the camera represent three primaries of thecolour diagram

By selective use of these signals, all colours in the visible spectrum can bereproduced on the screen of a special (colour) picture tube

Colour Signal Generation

At any instant during the scanning process the transmitted signal must

indicate the proportions of red, green and blue lights present in the elementbeing scanned

To fulfil the requirements of compatibility, the luminance signal whichrepresents the brightness of the elements being scanned must also begenerated and transmitted along with the colour signals

Figure 25.5 illustrates the method of generating these signals The camera output voltages are labelled as VR, VG and VBbut generally

the prefix V is omitted and only the symbols R, G, and B are used torepresent these voltages.

With the specified source of white light the three cameras are adjusted togive equal output voltage

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Gamma Correction

To compensate for the non-linearity of the system including TV camera andpicture tubes, a correction is applied to the voltages produced by the three

camera tubes

The output voltages are then referred to as R, Gand B

However, in our discussion we will ignore such a distinction and use thesame symbols i.e., R, G and B to represent gamma corrected outputvoltages

The camera outputs corresponding to maximum intensity (100%) ofstandard white light to be handled are assumed adjusted at an arbitraryvalue of one volt

Then on grey shades, i.e., on white of lesser brightness, R, G and B voltagewill remain equal but at amplitude less than one volt

25.7 The luminance signal To generate the monochrome or brightness signal that represents the

luminance of the scene, the three camera outputs are added through aresistance matrix (see Fig. 25.5) in the proportion of 0.3, 0.59 and 0.11 ofR, G and Brespectively

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This is because with white light which contains the three primary colours inthe above ratio, the camera outputs were adjusted to give equal voltages

The signal voltage developed across the common resistance RC represents

thebrightness of the scene and is referred to asYsignal

Therefore, Y = 0.3 R + 0.59 G + 0.11 BColour Voltage Ampli tudes

Figure 25.6 (a) illustrates the nature of output from the three cameras whena horizontal line across a picture having vertical bars of red, green and blue

colours is scanned At any one instant only, one camera delivers output voltage corresponding

to the colour being scanned

In Fig. 25.6 (b) dif ferent valuesof red colour voltage are illustrated.

Here the red pinkand pale pink which are different shades of red havedecreasing values of colour intensity

Therefore the corresponding output voltages have decreasing amplitudes

Thus R, G or B voltageindicates information of the specific colour whiletheir relative amplitudes depend on the level of saturation of that colour

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Next, scanning a picture that has yellow and white bars besides the three

pure colour bars

The voltages of the three camera outputs are drawn below the colour bar

pattern in Fig. 25.7

The values shown for yellow is an example of a complementary colour

Since yellow includes red and green, video voltage is produced for both

these primary colours Since there is no blue in yellow, the blue camera output voltage is at zero

for the yellow bar

The white bar at the right includes all the three primary colours

So all the three cameras develop output voltage when this bar is scanned

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Y Signal Amplitude

As already stated, the Y signal contains brightness variations of the

picture information

Formed by adding the three camera outputs in the ratio, Y = 0.3 R + 0.59 G+ 0.11 B

These percentages correspond to the relative brightness of the three primary

colours

So, a scene reproduced in black and white by the Ysignallooks the same

as when it is televised in monochrome Figure 25.8 illustrates how the Y signal voltage is formed from the

specified proportions of R, G, and B voltagesfor the colour bar pattern

The addition, as already explained is carried out (see Fig. 25.5) by the

resistance matrix

Note that, the Y signal forwhite has the maximum amplitude (1.0 or100%) because it includes R, G and B

For the other bars the magnitude of Y decreases or changes in accordance

with the colour or colours that form the bars (colour plate 5)

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All the voltage values can be calculated as illustrated by the writeup below

Fig. 25.8

If only this Ysignalis used to reproduce thepattern

It will appear as monochrome bars shading-off from white at the left togrey in the centre and black at the right

These values correspond to the staircase pattern ofY voltage shown in the

figure

Note the progressive decrease in voltage for the relative brightness of the

various colour bars

Production of Colour Diff erence Voltages

The Y signal is modulated and transmitted as is done in a monochrome

television system

Instead of trasmitting all the three colour signals separately , the red and

blue camera outputs are combined with the Y signal to obtain the colour

dif ference signals

Colour difference voltages are derived by subtracting the luminance

voltage from the colour voltages

Only (R

Y) and (B

Y)

are produced

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It is only necessary to transmittwo of the three colour difference signals

since the third may be derived from the other two

The reason for not choosing (G Y) for transmission and how the green

signal is recovered are explainedin a later section of this chapter

The circuit of Fig. 25.5 is reproduced in Fig. 25.9 to explain the generationof (BY) and (RY) voltages

The voltage VYas obtained from the resistance matrix is low because RCis

chosen to be small to avoid crosstalk

Hence it is amplified before itleaves the camera subchassis

Also the amplified Y signal is invertedto obtainYas the output

This is passed on to the two adder circuits

One adder circuit adds the red camera output to Y to obtain the (RY)

signal

Similarly, the second addercombinesthe blue camera output to

Y and

delivers(BY) Signal. This is illustrated in F ig. 25.9

The differencesignals thus obtained bear information both about the hue

and saturation of different colours

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Compatibi l i ty Considerations

The colour difference signals equal zero when white or grey shades arebeing transmitted

This is illustrated by two examples:

(a) On peak whites let R = G = B = 1 volt

Then, Y = 0.59G + 0.3R + 0.11B = 0.59 + 0.3 + 0.11 = 1 (volt)

(R Y) = 1 1 = 0 volt and (B Y) = 1 1 = 0 volt

(b) On any grey shade let R = G = B = v volts (v < 1)

Then Y = 0.59v + 0.3v + 0.11v = v

(R Y) = v v = 0 volt and (B Y) = v v = 0 volt

Thus the colour difference signals during the white or grey content of acolour scene during the monochrome transmission completely disappear

This is an aid to compatibility in colour TV systems

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