Differential gain

Differential gain is a  term used in TV broadcasting to describe the amount of color saturation change that occurs when the luma changes.

Composite color video signal

The composite color video signal (CCVS) consists of three terms:

• Luminance (monochrome) signal
• Auxiliary signals (sync pulse and blanking level signals )
• Chrominance, which is actually a subcarrier modulated by chroma information

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

The modulation technique of the color subcarrier is quadrature amplitude modulation (QUAM) 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.

Nonlinearity in the broadcast system

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

• Cameras and associated buffer stages
• Recording circuits and storage medium (Compact disks, tapes, etc.)
• Playback units and studio equipment
• Cable, microwave, or TVRO equipment
• Transmitters (and sometimes also transposers)
• Receivers (and sometimes also antenna amplifiers and distribution units)

In cable broadcasting and satellite broadcasting, some of the above may be replaced by other equipment.

All of the above circuitry include active circuit devices. These devices are only approximately linear devices. In particular, the amplification factor is not constant for all levels. The amplification factor may decrease or increase 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 at a tolerable level depending on the required sensitivity of the system.