Automatic gain control

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Automatic gain control (AGC) is an adaptive system found in many electronic devices. The average output signal level is fed back to adjust the gain to an appropriate level for a range of input signal levels. For example, without AGC the sound emitted from an AM radio 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 a PID controller where the P term is driven by the error between expected and actual output amplitude.

Example use cases

AM radio

In 1925, Harold Alden Wheeler invented automatic volume control (AVC) and obtained a patent. By the early 1930s essentially all broadcast receivers included automatic volume control.^[1]

A very common and typical example is the AGC used in AM radio.^[2] 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 of amplitude of the carrier wave. 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 power and distance of the transmitter, and signal path attenuation. 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 low gain can worsen signal-to-noise ratio and blocking;^{[citation needed]} therefore, many designs reduce gain only for stronger signals.

Since the AM detector diode produces a DC voltage proportional to signal strength, this voltage can be fed back to earlier stages of the receiver to reduce gain. A filter network is required so that the audio components of the signal don't appreciably influence gain; this prevents "modulation rise" which increases the effective modulation depth of the signal, distorting the sound. Communications receivers may have more complex AVC systems, including extra amplification stages, separate AGC detector diodes, different time constants for broadcast and shortwave bands, and application of different levels of AGC voltage to different stages of the receiver to prevent distortion and cross-modulation.^[3] 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.^[4]

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.

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.

Audio/video

An audio tape generates a certain amount of noise. If the level of the signal on the tape is low, the noise is more prominent, i.e., the signal-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 professional high-fidelity recording the level is set manually using a peak-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 with VCRs.

A potential disadvantage of AGC is that when recording something like music with quiet and loud passages, the AGC will tend to make the quiet passages louder and the loud passages quieter, compressing the dynamic range; the result can be a reduced musical quality if the signal is not re-expanded on playback, as in a companding system.

Most reel-to-reel tape recorders and cassette decks 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 most television 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.

Biological

As is the case with many concepts found in engineering, automatic gain control was also discovered by natural selection. For example, in human vision, calcium dynamics in the retinal photoreceptors adjust gain to suit light levels. Further on in the visual system, cells in V1 are thought to mutually inhibit to mediate both normalization of responses to contrast, and automatic gain control.

See also

• Squelch
• Companding
• Dynamic range compression
• Gain compression
• VOGAD (Voice-operated gain-adjusting device)

References

1. ^ http://www.nap.edu/openbook.php?record_id=10094&page=281 Memorial Tributes: National Academy of Engineering, Volume 9 (2001) page 281 , retrieved 2009 Oct 23
2. ^ F. Langford-Smith (ed.), Radiotron Designer's Handbook 4th ed., RCA, 1953, chapter 27 section 3
3. ^ Langford-Smith 53, page 1108
4. ^ Langford-Smith 53, chapter 25 page 1229