Study of Colour Television


Introduction: A colour picture is actually monochrome picture on a white raster but with colours added for the main parts of the scene. The required colour information is in the chrominance(C) signal broadcast with the monochrome signal in the standard 7 MHz TV broadcast channel. Practically all the colours can be produced as combinations of Red, Green and Blue – the three primary colours.

Colour Primaries: The colour TV system utilizes the red, green and blue (R, G, B) colour video signals corresponding to the colour information in the scene. Almost any colour can be produced from various combinations of these three basic colours. These three colours (R, G, B) are known as primary colours, because none of the three primary colours can be obtained by mixing the other two.

Additive Colour Mixtures: The three circles in red, green and blue overlap partially. Where the circles are superimposed, the colour shown is the mixture produced by adding the primary colours. At the centre all three colours circles overlap, resulting in white. The other three triangular lobes contain cyan, yellow and magenta produced from the mixture of blue (B) + green (G), (G +R) and (R +B) respectively in certain proportions. This process of colour mixing is known as additive colour mixing.

Complementary Colours: The colour that produces white light when it is added to a primary colour is called its compliment. For example yellow when added to blue, produces white light. Therefore, yellow is the compliment of green and red respectively.

Colour Video Voltage: In Fig. 6.2, three separate vidicon camera tubes are used for red, green and blue. These colours form the object are separated for the camera tubes by optical colour filters. A special type of mirror called ‘diachroic mirror’ the property of which is to reflect a specific colour and allow other spectral frequencies to pass through, is used in camera tube as shown in the figure. As a result, the output from camera tube1 is a red (R) video signal that contains information for only the red parts of the scene. Similarly tubes2 and 3 produce green (G) and blue (B) video signals.

Channel Bandwidth for Colour Transmission: In the channel bandwidth diagram as shown in Fig. 6.4, the frequency axis is scaled relative to the picture carrier which is marked as 0 MHz. This makes the diagram very informative, since details such as the widths of the upper and the lower sidebar is and the relative position of the sound carrier are easily read off.

The sound carrier is always positioned at the extremity of the fully radiated upper sideband and hence is 5.5 MHz away from the picture carrier. The FM sound signal frequencies have a frequency spectrum of about 75 KHz around the sound carrier. However a band of 0.25 MHz is allowed on the sound carrier side of the channel to allow for adequate inter-channel separation.

The narrow band chrominance (colour) signal is multiplexed with the wideband luminance (brightness) signal in the standard 7 MHz TV channel. This is achieved by modulating the colour signal with a carrier frequency which lies within the normal channel bandwidth. This is called the colour subcarrier frequency and is located towards the upper edge of the video frequencies to avoid interference with the monochrome signal.

Why it is called a subcarrier? Because it carries the colour information and at the same time it is carried by the channel picture carrier. So it is a carrier on a carrier.

In a PAL colour system the colour subcarrier frequency is located 4.43 MHz away from the picture carrier. The bandwidth of signals is restricted to about 1.2 MHz about the subcarrier Fig. 6.4. gives necessary details of the location of the monochrome (picture) colour and sound signal spectrums, all within the same channel bandwidth of 7 MHz.

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