Subject :- Television Engineering

Topic :- Color TV

Difficulty :- Low

• Compatibility implies that:

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

  (ii) Colour 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 colour signal must meet the following requirements:

  (i) It should occupy the same bandwidth as the corresponding monochrome signal.

  (ii) The location and spacing of picture and sound carrier frequencies should remain the same.

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

  (iv) The composite colour signal should contain colour information together with the ancillary signals needed to allow this to be decoded.

  (v) The colour information should be carried in such a way that it does not affect the picture reproduced on the screen of a monochrome receiver.

  (vi) 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 colour 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 colour signal back in its original form for feeding it to the tri-colour 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 colours to produce different hues on the picture screen together with limitations of the human eye to perceive them.