Automatic gain control

The signal to be gain controlled (the detector output in a radio) goes to a diode & capacitor, which produce a peak-following DC voltage.[5] Design of the AVC system has a great effect on the usability of the receiver, tuning characteristics, audio fidelity, and behavior on overload and strong signals.[6] FM receivers, even though they incorporate limiter stages and detectors that are relatively insensitive to amplitude variations, still benefit from AGC to prevent overload on strong signals.Video copy control schemes such as Macrovision exploit this, inserting spikes in the pulse which will be ignored by most television sets, but cause a VCR's AGC to overcorrect and corrupt the recording.It is usually used in radio transmitters to prevent overmodulation and to reduce the dynamic range of the signal which allows increasing average transmitted power.While clipping limiters are often used as a form of last-ditch protection against overmodulation, a properly designed vogad circuit actively controls the amount of gain to optimise the modulation depth in real time.A good vogad circuit must have a very fast attack time, so that an initial loud voice signal does not cause a sudden burst of excessive modulation.Too short a decay time leads to the phenomenon of "breathing" where the background noise level gets boosted at each gap in the speech.Further on in the visual system, cells in V1 are thought to mutually inhibit, causing normalization of responses to contrast, a form of automatic gain control.
Schematic of an AGC used in the analog telephone network; the feedback from output level to gain is effected via a Vactrol resistive opto-isolator.
Vactrolfeedbackamplifierradio receiversloudnesssignal strengthfadingdetectorcapacitorHarold Alden WheelerKarl Küpfmüllerreceiversamplitudecarrier wavefidelitytransmitterattenuationRF front endsignal-to-noise ratioblockingCommunications receiversclutteraudio tapesignaldistorthigh-fidelitypeak-readingmicrophonedynamic rangecompandingreel-to-reeltape recorderscassette decksvertical blanking pulseMacrovisiontelevisioncompressorovermodulationtelephonyclamp diodesclipping limitersradiotelephonesattack timebreathingnoise cancellingtelephonevertebratevisual systemretinalphotoreceptorsauditory systemfull break-inClipping (audio)Dynamic range compressionGain compressionHigh dynamic rangeSquelchGlossary of video termsRadiotron Designer's Handbook