Download - ch3-lect 3
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Chapter 3 Lecture 3
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• Chromaticity diagram
• Luminance signal, colour difference signals
• Chrominance signal
• Why G-Y signal is not transmitted?
• How Compatibility is achieved.
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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.
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• 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.
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• 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).
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• 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.
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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.
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• 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
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• 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.
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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
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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.
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• 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.
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• 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.
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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.
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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.
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• 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)
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• 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
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• (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:
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• 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
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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:
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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.