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.
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