Automatic gain control
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Automatic gain control (AGC) is an electronic system found in many types of devices. Its purpose is to control the gain of a system in order to maintain adequate performance over a range of input signal levels.
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[edit] AM radio
A very common and typical example is the AGC used in AM radio. Such a receiver is essentially linear — that is, the output is proportional to the input. This is a necessary requirement because the information content of the signal is carried by the changes of amplitude of the carrier frequency. If the circuit were not linear, the modulation could not be recovered with reasonable fidelity. However, the strength of the signal received will vary widely, depending on the range to the transmitter, signal path attenuation, and so on. The AGC circuit keeps the receiver in its linear operating range by measuring the overall strength of the signal and automatically adjusting the gain of the receiver to maintain a constant level of output. When the signal is strong, the gain is reduced, and when weak, the gain is increased, or allowed to reach its normal maximum.
It is usually not advantageous to reduce the gain of the front end of the receiver on weak signals, to maximise signal to noise ratio, and keep the first stage in the linear range for blocking rejection. Many designs reduce the gain of the first stage only for stronger signals, known as a delayed AGC circuit.
[edit] Radar
A related application of AGC is in radar systems, as a method of overcoming unwanted clutter echoes. This method relies on the fact that clutter returns far outnumber echoes from targets of interest. The receiver's gain is automatically adjusted to maintain a constant level of overall visible clutter. While this does not help detect targets masked by stronger surrounding clutter, it does help to distinguish strong target sources. In the past, radar AGC was electronically controlled and affected the gain of the entire radar receiver. As radars evolved, AGC became computer-software controlled, and affected the gain with greater granularity, in specific detection cells.
[edit] Audio/video
AGC is also found in many types of tape recorder, including basic cassette decks and VCRs, the object being to record to tape a good quality signal but one that does not saturate the tape and cause distortion and other problems, even if the signal to be recorded varies in level. Video copy control schemes such as Macrovision exploit this, inserting signal spikes which will be ignored by most televisions but cause the deck's AGC to overcorrect and corrupt the recording.
[edit] Telephone
Telephone recording devices must cope with a difference in signal gain between the relatively high-gain signal from the line it is in parallel with vs. the relatively low-gain signal from the remote party/parties. Some telephone recording devices (PhoneValet Message Center for instance) incorporate automatic gain control to produce acceptable-quality recordings.
