- Automatic gain control
Automatic gain control (AGC) is an adaptive
system found in many electronic devices. The average output signal level is fed back to adjust thegain to an appropriate level for a range of input signal levels. For example, without AGC the sound emitted from an AMradio receiver would vary to an extreme extent from a weak to a strong signal; the AGC effectively reduces the volume if the signal is strong and raises it when it is weaker. AGC algorithms often use aPID controller where the P term is driven by the error between expected and actual output amplitude.AM radio
A very common and typical example is the AGC used in AM radio. Such a receiver is essentially
linear —the sound volume is proportional to the radio signal strength, because the information content of the signal is carried by the changes ofamplitude of the carrier wave. If the circuit were not linear, themodulation could not be recovered with reasonablefidelity . However, the strength of the signal received will vary widely, depending on the power and distance of thetransmitter , and signal pathattenuation . The AGC circuit keeps the receiver in its linear operating range by detecting the overall strength of the signal and automatically adjusting the gain of the receiver to maintain an approximately constant average output level. For a very weak signal the AGC has no effect; as the signal increases, the AGC reduces the gain.It is usually disadvantageous to reduce the gain of the
front end of the receiver on weaker signals as this can worsensignal-to-noise ratio andblocking rejection . Many designs reduce the gain of the first stage only for stronger signals, known as a delayed AGC circuit.Radar
A related application of AGC is in
radar systems, as a method of overcoming unwantedclutter 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.Audio/video
An
audio tape generates a certain amount ofnoise . If the level of thesignal on the tape is low, the noise is more prominent, i.e., thesignal-to-noise ratio is lower than it could be. To produce the least noisy recording, the recording volume should be set as high as possible without being so high as to clip or seriously distort the signal. In professionalhigh-fidelity recording the level is set manually using apeak-reading meter.If high fidelity is not a requirement, a suitable recording level can be set by an AGC circuit which reduces the gain as the average signal level increases. This allows a usable recording to be made even for speech some distance from the
microphone of an audio recorder. Similar considerations apply withVCRs .The disadvantage of AGC is that when recording, say, music with quiet and loud passages, the AGC will tend to make the quiet passages louder and the loud passages quieter, reducing the
dynamic range and losing musical quality.Most
reel-to-reel tape recorders andcassette deck s have AGC circuits. Those used for high-fidelity allow it to be overridden manually.Most VCR circuits use the amplitude of the vertical blanking pulse to operate the AGC. Video copy control schemes such as
Macrovision exploit this, inserting spikes in the pulse which will be ignored by mosttelevision sets, but cause a VCR's AGC to overcorrect and corrupt the recording.Telephone recording
Devices to record both sides of a
telephone conversation must record both the relatively large signal from the local user and the much smaller signal from the remote user at comparable loudnesses. Some telephone recording devices incorporate automatic gain control to produce acceptable-quality recordings.See also
*
Squelch
*Companding
*Audio level compression
*Gain compression
* Voice-operated gain-adjusting device
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