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Automatic gain control (AGC) is a closed-loop
feedback
Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...
regulating circuit in an
amplifier
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost the v ...
or chain of amplifiers, the purpose of which is to maintain a suitable signal amplitude at its output, despite variation of the signal amplitude at the input. The average or peak output signal level is used to dynamically adjust the
gain
Gain or GAIN may refer to:
Science and technology
* Gain (electronics), an electronics and signal processing term
* Antenna gain
* Gain (laser), the amplification involved in laser emission
* Gain (projection screens)
* Information gain in de ...
of the amplifiers, enabling the circuit to work satisfactorily with a greater range of input signal levels. It is used in most
radio receiver
In radio communications, a radio receiver, also known as a receiver, a wireless, or simply a radio, is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. Th ...
s to equalize the average volume (
loudness
In acoustics, loudness is the subjectivity, subjective perception of sound pressure. More formally, it is defined as, "That attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud". The rel ...
) of different radio stations due to differences in received
signal strength, as well as variations in a single station's radio signal due to
fading. Without AGC the sound emitted from an
AM radio
Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...
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. In a typical receiver the AGC feedback control signal is usually taken from the
detector
A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon.
In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
stage and applied to control the gain of the IF or RF amplifier stages.
How it works
The signal to be gain controlled (the detector output in a radio) goes to a
diode
A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other.
A diode ...
&
capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of ...
, which produce a peak-following DC voltage. This is fed to the RF gain blocks to alter their bias, thus altering their gain. Traditionally all the gain-controlled stages came before the signal detection, but it is also possible to improve gain control by adding a gain-controlled stage after signal detection.
Example use cases
AM radio receivers
In 1925,
Harold Alden Wheeler invented automatic volume control (AVC) and obtained a patent.
Karl Küpfmüller
Karl Küpfmüller (6 October 1897 – 26 December 1977) was a German electrical engineer, who was prolific in the areas of communications technology, measurement and control engineering, acoustics, communication theory, and theoretical electro-te ...
published an analysis of AGC systems in 1928. By the early 1930s most new commercial broadcast receivers included automatic volume control.
AGC is a departure from linearity in AM radio
receivers. Without AGC, an AM radio would have a linear relationship between the signal amplitude and the sound waveform – the sound
amplitude
The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of amplit ...
, 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
In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has a ...
. If the circuit were not fairly linear, the modulated signal could not be recovered with reasonable
fidelity
Fidelity is the quality of faithfulness or loyalty. Its original meaning regarded duty in a broader sense than the related concept of ''fealty''. Both derive from the Latin word ''fidēlis'', meaning "faithful or loyal". In the City of London f ...
. However, the strength of the signal received will vary widely, depending on the power and distance of the
transmitter
In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna (radio), antenna. The transmitter itself generates a radio frequency alternating current, which i ...
, and signal path
attenuation
In physics, attenuation (in some contexts, extinction) is the gradual loss of flux intensity through a medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and sound at variable att ...
. 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 RF is an abbreviation for radio frequency.
Rf or RF may also mean:
Arts and entertainment
* ''Red Faction (series)'', a series of revolution video games
* Rinforzando, , in music notation
* ''RF Online'', an online RPG made by CCR
Businesses
* A ...
of the receiver on weaker signals as low gain can worsen
signal-to-noise ratio
Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in deci ...
and
blocking; 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
A communications receiver is a type of radio receiver used as a component of a radio communication link. This is in contrast to a ''broadcast receiver'' which is used to receive radio broadcasts. A communication receiver receives parts of the r ...
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. 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.
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
Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
systems, as a method of overcoming unwanted
clutter
Clutter and its derivations may refer to any of the following:
Excessive physical disorder
* Clutter, a confusing, or disorderly, state or collection, and possible symptom of compulsive hoarding
* Clutter (marketing), numerous advertisements, a ...
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
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
An audio tape recorder, also known as a tape deck, tape player or tape machine or simply a tape recorder, is a sound recording and reproduction device that records and plays back sounds usually using magnetic tape for storage. In its present ...
generates a certain amount of
noise
Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference arise ...
. If the level of the
signal
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...
on the tape is low, the noise is more prominent, i.e., the
signal-to-noise ratio
Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in deci ...
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
clip or
distort
In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal ...
the signal. In professional
high-fidelity recording the level is set manually using a
peak-reading
A peak meter is a type of measuring instrument that indicates visually the instantaneous level of an audio signal that is passing through it (a sound level meter). In sound reproduction, the meter, whether peak or not, is usually meant to corres ...
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
A microphone, colloquially called a mic or mike (), is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and public ...
of an audio recorder. Similar considerations apply with
VCRs
A videocassette recorder (VCR) or video recorder is an electromechanical device that records analog audio and analog video from broadcast television or other source on a removable, magnetic tape videocassette, and can play back the recordin ...
.
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
Dynamic range (abbreviated DR, DNR, or DYR) is the ratio between the largest and smallest values that a certain quantity can assume. It is often used in the context of signals, like sound and light. It is measured either as a ratio or as a base-1 ...
; the result can be a reduced musical quality if the signal is not re-expanded when playing, as in a
companding
In telecommunication and signal processing, companding (occasionally called compansion) is a method of mitigating the detrimental effects of a channel with limited dynamic range. The name is a portmanteau of the words compressing and expanding, ...
system.
Some
reel-to-reel
Reel-to-reel audio tape recording, also called open-reel recording, is magnetic tape audio recording in which the recording tape is spooled between reels. To prepare for use, the ''supply reel'' (or ''feed reel'') containing the tape is plac ...
tape recorders
An audio tape recorder, also known as a tape deck, tape player or tape machine or simply a tape recorder, is a sound recording and reproduction device that records and plays back sounds usually using magnetic tape for storage. In its present- ...
and
cassette deck
A cassette deck is a type of tape machine for playing and recording audio cassettes that does not have a built-in power amplifier or speakers, and serves primarily as a transport. It can be a part of an automotive entertainment system, a part of a ...
s have AGC circuits. Those used for high-fidelity generally don't.
Most VCR circuits use the amplitude of the
vertical blanking pulse to operate the AGC. Video copy control schemes such as
Macrovision
TiVo Corporation, formerly known as the Rovi Corporation and Macrovision Solutions Corporation, was an American technology company. Headquartered in San Jose, California, the company is primarily involved in licensing its intellectual property ...
exploit this, inserting spikes in the pulse which will be ignored by most
television
Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of television transmission. Television is a mass medium for advertisin ...
sets, but cause a VCR's AGC to overcorrect and corrupt the recording.
Vogad
A voice-operated gain-adjusting device or volume-operated gain-adjusting device
(vogad) is a type of AGC or
compressor for
microphone
A microphone, colloquially called a mic or mike (), is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and public ...
amplification. It is usually used in radio transmitters to prevent
overmodulation
Overmodulation is the condition that prevails in telecommunication when the instantaneous level of the modulating signal exceeds the value necessary to produce 100% modulation of the carrier. In the sense of this definition, it is almost always con ...
and to reduce the
dynamic range
Dynamic range (abbreviated DR, DNR, or DYR) is the ratio between the largest and smallest values that a certain quantity can assume. It is often used in the context of signals, like sound and light. It is measured either as a ratio or as a base-1 ...
of the signal which allows increasing average transmitted power. In
telephony
Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is i ...
, 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
clamp diodes, 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
clipping limiters are often used as a form of last-ditch protection against
overmodulation
Overmodulation is the condition that prevails in telecommunication when the instantaneous level of the modulating signal exceeds the value necessary to produce 100% modulation of the carrier. In the sense of this definition, it is almost always con ...
, 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
A radiotelephone (or radiophone), abbreviated RT, is a radio communication system for conducting a conversation; radiotelephony means telephony by radio. It is in contrast to '' radiotelegraphy'', which is radio transmission of telegrams (mes ...
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
Breathing (or ventilation) is the process of moving air into and from the lungs to facilitate gas exchange with the internal environment, mostly to flush out carbon dioxide and bring in oxygen.
All aerobic creatures need oxygen for cellular ...
" 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 cancelling
Active noise control (ANC), also known as noise cancellation (NC), or active noise reduction (ANR), is a method for reducing unwanted sound by the addition of a second sound specifically designed to cancel the first. The concept was first develop ...
microphones.
Telephone recording
Devices to record both sides of a
telephone
A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into e ...
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
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
visual system
The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight (the a ...
, calcium dynamics in the
retina
The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...
l
photoreceptors 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 auditory system is the sensory system for the sense of hearing. It includes both the sensory organs (the ears) and the auditory parts of the sensory system.
System overview
The outer ear funnels sound vibrations to the eardrum, increasin ...
, the
olivocochlear efferent neurons are part of a biomechanical gain control loop.
Recovery times
As in all automatic control systems, the temporal dynamics of AGC operation may be important in many applications. Some AGC systems are slow to react to the need for gain changes, while others may react very rapidly. An example of an application in which fast AGC recovery time is required is in receivers used in Morse code communications where so-called
full break-in or QSK operation is necessary to enable receiving stations to interrupt sending stations mid-character (e.g. between dot and dash signals).
See also
*
Companding
In telecommunication and signal processing, companding (occasionally called compansion) is a method of mitigating the detrimental effects of a channel with limited dynamic range. The name is a portmanteau of the words compressing and expanding, ...
*
Clipping (audio)
Clipping is a form of waveform distortion that occurs when an amplifier is overdriven and attempts to deliver an output voltage or current beyond its maximum capability. Driving an amplifier into clipping may cause it to output power in excess ...
*
Dynamic range compression
Dynamic range compression (DRC) or simply compression is an audio signal processing operation that reduces the volume of loud sounds or amplifies quiet sounds, thus reducing or ''compressing'' an audio signal's dynamic range. Compression is ...
*
Gain compression
Gain compression is a reduction in ''differential'' or ''slope'' gain caused by nonlinearity of the transfer function of the amplifying device. This nonlinearity may be caused by heat due to power dissipation or by overdriving the active devic ...
*
High dynamic range
High dynamic range (HDR) is a dynamic range higher than usual, synonyms are wide dynamic range, extended dynamic range, expanded dynamic range.
The term is often used in discussing the dynamic range of various signals such as images, videos, au ...
*
Squelch
In telecommunications, squelch is a circuit function that acts to suppress the audio (or video) output of a receiver in the absence of a strong input signal. Essentially, squelch is a specialized type of noise gate designed to suppress weak s ...
*
Glossary of video terms
This glossary defines terms that are used in the documen"Defining Video Quality Requirements: A Guide for Public Safety" developed by thVideo Quality in Public Safety (VQIPS) Working Group It contains terminology and explanations of concepts relev ...
References
{{reflist
Analog circuits
Biophysics
Communication circuits
Radio technology
Radar signal processing