Editing Automatic gain control (section)

== Example use cases ==

=== AM radio receivers ===
In 1925, [[Harold Alden Wheeler]] invented automatic volume control (AVC) and obtained a patent.  [[Karl Küpfmüller]] published an analysis of AGC systems in 1928.<ref>K. Küpfmüller, "Über die Dynamik der selbsttätigen Verstärkungsregler", ''Elektrische Nachrichtentechnik'', vol. 5, no. 11, pp. 459-467, 1928. (German) [http://ict.open.ac.uk/classics/2.pdf On the dynamics of automatic gain controllers], (English translation)</ref> By the early 1930s most new commercial broadcast receivers included automatic volume control.<ref>[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</ref>

AGC is a departure from linearity in AM radio [[receiver (radio)|receiver]]s.<ref>F. Langford-Smith (ed.), ''[[Radiotron Designer's Handbook]]'' 4th ed., RCA, 1953, chapter 27 section 3</ref> Without AGC, an AM radio would have a linear relationship between the signal amplitude and the sound waveform – the sound [[amplitude]], which correlates with loudness, is proportional to the radio signal amplitude, because the information content of the signal is carried by the changes of amplitude of the [[carrier wave]]. If the circuit were not fairly linear, the modulated signal 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's output level from fluctuating too much by detecting the overall strength of the signal and automatically adjusting the gain of the receiver to maintain the output level within an acceptable range. For a very weak signal, the AGC operates the receiver at maximum gain; as the signal increases, the AGC reduces the gain.

It is usually disadvantageous to reduce the gain of the [[RF front end]] of the receiver on weaker signals as low gain can worsen [[signal-to-noise ratio]] and [[blocking (radio)|blocking]];<ref>[http://www.qsl.net/va3iul/Files/Automatic_Gain_Control.pdf Automatic gain control in receivers] by Iulian Rosu, VA3IUL</ref> 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 receiver]]s 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.<ref>Langford-Smith 53, page 1108</ref> 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.<ref>Langford-Smith 53, chapter 25 page 1229</ref>

=== FM radio receivers ===
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 (radar)|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.
Many [[Electronic warfare#Electronic attack .28EA.29|radar countermeasures]] use a radar's AGC to fool it, by effectively "drowning out" the real signal with the spoof, as the AGC will regard the weaker, true signal as clutter relative to the strong spoof.

=== Audio/video ===

An [[audio tape]] generates a certain amount of [[noise]]. If the level of the [[Signal (electronics)|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 level should be set as high as possible without being so high as to [[clipping (audio)|clip]] or [[distortion|distort]] the signal. In professional [[high-fidelity]] recording the level is set manually using a [[peak-reading]] meter.  When 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 such as classical music, 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 when playing, as in a [[companding]] system.

Some [[reel-to-reel]] [[tape recorders]] and [[cassette deck]]s have AGC circuits. Those used for high-fidelity generally don't.

Most VCR circuits use the amplitude of the [[vertical blanking interval|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.

===Vogad===

A voice-operated gain-adjusting device<ref>[http://www.its.bldrdoc.gov/fs-1037/dir-039/_5820.htm Vogad at Federal Standard 1037C]</ref> or volume-operated gain-adjusting device<ref>
{{cite journal
 | journal = Popular Mechanics
 | title = Roar and Whisper Equalled by Radio Voice Leveler
 | page = 236
 | date = Feb 1939
 | url = https://books.google.com/books?id=f9sDAAAAMBAJ&pg=PA236
 }}</ref> 
(vogad) is a type of AGC or [[Audio level compression|compressor]] for [[microphone]] amplification. 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. In [[telephony]], this device takes a wide variety of input amplitudes and produces a generally consistent output amplitude.

In its simplest form, a limiter can consist of a pair of back-to-back [[Clamper (electronics)|clamp diode]]s, which simply shunt excess signal amplitude to ground when the diode conduction threshold is exceeded. This approach will simply clip off the top of large signals, leading to high levels of distortion.

While [[Clipper (electronics)|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. As well as preventing overmodulation, it boosts the level of quiet signals so that undermodulation is also avoided. Undermodulation can lead to poor signal penetration in noisy conditions, consequently vogad is particularly important for voice applications such as [[radiotelephone]]s.

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. In practice the attack time will be a few milliseconds, so a clipping limiter is still sometimes needed to catch the signal on these short peaks. A much longer decay time is usually employed, so that the gain does not get boosted too quickly during the normal pauses in natural speech. Too short a decay time leads to the phenomenon of "[[breathing (noise reduction)|breathing]]" where the background noise level gets boosted at each gap in the speech. Vogad circuits are normally adjusted so that at low levels of input the signal is not fully boosted, but instead follow a linear boost curve. This works well with [[noise cancellation|noise cancelling]] microphones.

=== 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 is also found in biological systems, especially sensory systems.  For example, in the [[vertebrate]] [[visual system]], calcium dynamics in the [[retina]]l [[photoreceptor cell|photoreceptor]]s adjust gain to suit light levels.  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.  Similarly, in the [[auditory system]], the [[Olivocochlear system#Effects of electrical stimulation|olivocochlear efferent]] neurons are part of a biomechanical gain control loop.<ref>{{cite book
 | chapter = Functional roles of the inner-and outer-hair-cell subsystems in the cochlea and brainstem
 | title = Hearing science: Recent advances
 | editor = C. I. Berlin
 | author = D. O. Kim
 | publisher = College Hill Press
 | year = 1984
 | pages = 241–262
 | url = http://kimdolab.uchc.edu/Kim-84.PDF
 | access-date = 2010-10-13
 | archive-url = https://web.archive.org/web/20100701024647/http://kimdolab.uchc.edu/Kim-84.PDF
 | archive-date = 2010-07-01
 | url-status = dead
 }}</ref><ref>{{cite book
 | chapter = Automatic Gain Control in Cochlear Mechanics
 | title = The Mechanics and Biophysics of Hearing
 | editor = P. Dallos
 | author = R. F. Lyon
 | publisher = Springer-Verlag
 | year = 1990
 | pages = 395–402
 | url = http://www.dicklyon.com/tech/AGC_MOH1990.pdf
 | display-editors = etal
 }}{{Dead link|date=October 2019 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>