Chapter 3 Lecture 3

• Chromaticity diagram

• Luminance signal, colour difference signals

• Chrominance signal

• Why G-Y signal is not transmitted?

• How Compatibility is achieved.

Chromaticity Diagram

• Chromaticity diagram is a convenient space coordinate representation of all the spectral colours and their mixtures based on the tristimulus values of the primary colours contained by them.

• Fig.below is a two dimensional representation of hue and saturation in the X-Y plane .

• If a three dimensional representation is drawn, the ‘Z ’ axis will show relative brightness of the colour.

• As seen in the figure the chromaticity diagram is formed by all the rainbow colours arranged along a horseshoe-shaped triangular curve.

• The various saturated pure spectral colours are represented along the perimeter of the curve, the corners representing the three primary colours—red, green and blue.

• As the central area of the triangular curve is approached, the colours become desaturated representing mixing of colours or a white light.

• The white lies on the central point ‘C’ with coordinates x = 0.31 and y = 0.32.

• Actually there is no specific white light—sunlight, skylight, daylight are all forms of white light.

• The illuminant ‘C’ marked represents a particular white light formed by combining hues having wavelength:

700 nm (red) 546.1 nm (green) and 438.8 nm (blue).

• A practical advantage of the chromaticity diagram is that, it is possible to determine the result of additive mixing of any two or more colour lights by simple geometric construction.

• The colour diagram contains all colours of equal brightness. Since brightness is represented by the ‘Z ’ axis, as brightness increase, the colour diagram becomes larger.

COLOUR TELEVISION CAMERA

• Figure below shows a simple block schematic of a colour TV camera.

• It essentially consists of three camera tubes in which each tube receives selectively filtered primary colours.

• Each camera tube develops a signal voltage proportional to the respective colour intensity 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 glass prisms. These prisms are designed as diachroic mirrors.

• A dichroic 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 filters called trimming filters.

• These filters provide highly precise primary colour images which are converted into video signals .

• Thus the three colour signals are generated, R, G, B

• Simultaneous scanning of the three camera tubes is accomplished by a master deflection oscillator and sync generator which drives all the three tubes.

• The three video signals produced by the camera represent three primaries of the colour diagram.

• By selective use of these signals, all colours in the visible spectrum can be reproduced on the screen of a picture tube.

THE LUMINANCE SIGNAL

• To generate the monochrome or brightness signal that represents the luminance of the scene, the three camera outputs are added through a resistance matrix in the proportion of 0.3, 0.59 and 0.11 of R, G and B respectively.

• Therefore, Y = 0.3 R + 0.59 G + 0.11 B

Colour Voltage Amplitudes • Figure below illustrates the nature of output

from the three cameras when a horizontal line across a picture having vertical bars of red, green and blue colours is scanned.

• Note that at any one instant only one camera delivers output voltage corresponding to the colour being scanned.

• In Fig. different values of red colour voltage are illustrated.

• Here the red pink and pale pink which are different shades of red have decreasing values of colour intensity.

• Therefore the corresponding output voltages have decreasing amplitudes.

• Thus we can say that R, G or B voltage indicates information of the specific colour while their relative amplitudes depend on the level of saturation of that colour.

• Next consider the scanning across 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.

Y signal amplitude

• A scene reproduced in black and white by the ‘Y ’ signal looks the same as when it is televised in monochrome.

• Figure illustrates how the ‘Y ’ signal voltage is formed from the specified proportions of R, G, and B voltages for the colour bar pattern.

• The addition, as already explained is carried out by the resistance matrix.

• Note that the ‘Y ’ signal for white has the maximum amplitude (1.0 or 100%) because it includes R, G and B.

Production of Colour Difference Voltages

• The ‘Y ’ signal is modulated and transmitted as is done in a monochrome television system.

• However, instead of trasmitting all the three colour signals separately the red and blue camera outputs are combined with the Y signal to obtain what is known as colour difference signals.

• Colour difference voltages are derived by subtracting the luminance voltage from the colour voltages.

• Only (R – Y) and (B – Y) are produced. It is only necessary to transmit two of the three colour difference signals since the third may be derived from the other two.

• As an illustration let R = 0.7, G = 0.2 and B = 0.6 volts

• (i) The luminance signal

• Y = 0.3 R + 0.59 G + 0.11 B.

• Substituting the values of R, G, and B we get Y = 0.3 (0.7) + 0.59 (0.2) + 0.11(0.6) = 0.394 (volts).

• (ii) The colour difference signals are:

• (R – Y) = 0.7 – 0.394 = + 0.306 (volts)

• (B – Y) = 0.6 – 0.394 = + 0.206 (volts)

• Reception at the colour receiver—At the receiver after demodulation, the signals,

Y, (B – Y) and (R – Y), become available. Then by a process of matrixing the voltages B and R are

• obtained as:

• R = (R – Y) + Y = 0.306 + 0.394 = 0.7 V

• B = (B – Y) + Y = 0.206 + 0.394 = 0.6 V

• (G – Y) matrix—The missing signal (G – Y) that is not transmitted can be recovered

• by using a suitable matrix based on the explanation given below:

• Y = 0.3 R + 0.59G + 0.11B

• also (0.3 + 0.59 + 0.11)Y = 0.3R + 0.59G + 0.11B

• Rearranging the above expression we get:

• Also since the proportion of G in Y is relatively large in most cases, the amplitude of (G – Y) is small.

• It is the smallest of the three colour difference signals.

• The smaller amplitude of this signal would cause high S/N problems

Chrominance signal

• The colour difference signals are modulated on a sub carrier (QAM) and this process is called colour encoding. ( to differentiate it from AMVSB).

• This signal is called the chrominance signal or the chroma signal.

• Three types of encoders were developed:

SUMMARIZE

• Chromaticity diagram

• Colour camera, voltage magnitudes, luminance signal, colour difference signals

• Chrominance signal

• Why G-Y signal is not transmitted?

• How Compatibility is achieved.