Compatibility

COMPATIBILITY

Regular color TV broadcast could not be started till 1954 because of the stringent requirement of making color TV compatible with the existing monochrome system. Compatibility implies that

(i) The color television signal must produce a normal black and white picture on a monochrome receiver without any modification of the receiver circuitry

(ii) A color receiver must be able to produce a black and white picture from a normal monochrome signal. This is referred to as reverse compatibility.

To achieve this, that is to make the system fully compatible the composite color signal must meet the following requirements:

1.     It should occupy the same bandwidth as the corresponding monochrome signal.

2.     The location and spacing of picture and sound carrier frequencies should remain the same.

3.     The color signal should have the same luminance (brightness) information as would a monochrome signal, transmitting the same scene.

4.     The composite color signal should contain color information together with the ancillary signals needed to allow this to be decoded.

5.     The color information should be carried in such a way that it does not affect the picture reproduced on the screen of a monochrome receiver.

6.     The system must employ the same deflection frequencies and sync signals as used for monochrome transmission and reception.

In order to meet the above requirements it becomes necessary to encode the color information of the scene in such a way that it can be transmitted within the same channel bandwidth of 7 MHz and without disturbing the brightness signal. Similarly at the receiving end a decoder must be used to recover the color signal back in its original form for feeding it to the tri-color picture tube. Before going into details of encoding and decoding the picture signal, it is essential to gain a good understanding of the fundamental properties of light. It is also necessary to understand mixing of colors to produce different hues on the picture screen together with limitations of the human eye to perceive them. Furthermore a knowledge of the techniques employed to determine different colors in a scene and to generate corresponding signal voltages by the color television camera is equally essential.