Differential gain

Differential gain is a kind of linearity distortion which affects the color saturation in TV broadcasting.

Composite color video signal (CCVS) consists of three terms:

The first two terms are usually called composite video signal (CVS)

The modulation technique of the color subcarrier is quadrature amplitude modulation (QUAM or QAM) both in PAL and NTSC systems. The amplitude of the color signal represents the saturation (purity) in both systems. On the other hand, the level of the CVS represents the brightness. So in order to reproduce the original vision in the receiver the ratio between these two pieces of information should be kept constant in the receiver.

The main steps of visual signal from the scene to receiver screen (for terrestrial broadcasting) are as follows:

In cable broadcasting and satellite broadcasting some of the above maybe replaced by other equipment.

All of the above circuitry include active circuit devices. These devices are only approximately linear devices. In particular, amplification factor is not constant for all levels. Usually the amplification factor decreases as the input level increases. This is known as gain nonlinearity. In system specifications, the nonlinearity in percentage is almost always specified. It must be under a tolerable level depending on the required sensitivity of the system.

Differential gain is a special case of gain nonlinearity. Let CCVS is given by

The output of an ideal amplifier with the amplification factor of A is;

However, in practice this goal is almost never attained. The amplification factor of color signal is always slightly different than that of CVS. (The color signal is superimposed on CVS).

Any color can be totally described by luminance, saturation and hue. When the gain of the reproduced color signal is lower than that of luminance, the perceived colors are paler than their originals. Conversely, when the gain of the reproduced color signal is higher than the luminance, the perceived colors are too loud.

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