Abbreviation for automatic gain control. On a TV or VCR, AGC is a circuit that automatically adjusts the incoming signal to the proper levels for display or recording. On a video camera, AGC is a circuit that automatically adjusts the sensitivity of the pickup tube to render the most pleasing image.
Automatic gain can be applied ahead of almost any circuit that requires specific levels to operate. As an example, the color decoder needs to see a particular level of color subcarrier at its input in order to display the correct color at the output. There is a reference burst of color subcarrier at the beginning of each horizontal line of known original amplitude. The AGC circuit looks at the incoming signal level and applies what ever gain correction necessary to make that burst correct in amplitude. In the process of doing that, it also corrects the amplitude of the color information in the active picture area. There are certain limits in the accuracy of this circuit. If the incoming burst amplitude is really low, the AGC circuit might not function at all or amplify the signal more than it should. This is of particular concern when attenuation in the cable delivering the video signal to the set reduces the color information much more than it should. The AGC circuit will actually cause the color to look high, if it has over amplified the weak signal, or the set may only display a black and white picture if it doesn’t amplify the signal at all. Several S-Video cable manufacturers have been know to play this game, claiming their cable delivers richer color quality.