Automatic Gain Control
Automatic Gain Control (AGC) is a system used in many electronic devices. Its purpose is basically to adjust the gain to an appropriate level based on the input signal level. It applies to video and audio systems in many ways. An example of its necessity is an AM Radio receiver. The signal strength varies and this would have an affect on the output. In this case, AGC alters the volume to maintain a form of balance.
AGC is used in many other cases, such as with a color decoder. Macrovision attempts to block attempts to make illegal recordings using a VCR by inserting spikes in the pulse that wont affect most television sets but will cause the AGC operation in the VCR to distort the recording.