Control for varying the amplification of an amplifier, used to compensate for variations in input signal strength. (a) Automatic gain control uses a feedback loop whereby the output level controls the gain so as to keep the output level within certain limits, but then information contained in the variable gain is apt to be lost. (b) Ganged automatic gain control has the gain of all channels be the same although automatically determined, the basis being one single channel or the average energy level of a number of the channels. (c) Preset or programmed gain control has the gain as a function of record time determined beforehand. (d) Binary gain control (or quaternary gain control) has the gain vary only by factors of two (or four), but the times at which the gain changes occur are determined automatically; the times at which the changes occur are recorded so that the gain effects can be removed (see Figure B-3). (e) With instantaneous-floating-point control the gain is determined for each sample based on the amplitude of that channel without prejudice by earlier samples or the amplitude of other channels.