Automatic gain control
Encyclopedia
Automatic gain control is an adaptive system
System
System is a set of interacting or interdependent components forming an integrated whole....

 found in many electronic devices. The average output signal level is fed back
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...

 to adjust the gain
Gain
In electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...

 to an appropriate level for a range of input signal levels. For example, without AGC the sound emitted from an AM
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...

 radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...

 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.

AM radio

In 1925, Harold Alden Wheeler
Harold Alden Wheeler
Harold Alden Wheeler was a noted American electrical engineer.-Biography:Wheeler was born in Saint Paul, Minnesota, to William Archibald Wheeler and Harriet Marie Alden Wheeler , graduated in 1925 from George Washington University with a Bachelor of Science degree in physics and was awarded the...

 invented automatic volume control (AVC) and obtained a patent. Karl Küpfmüller
Karl Küpfmüller
Karl Küpfmüller was a German electrical engineer, who was prolific in the areas of communications technology, measurement and control engineering, acoustics, communication theory and theoretical electro-technology....

 published an analysis of AGC systems in 1928. By the early 1930s essentially all broadcast receivers included automatic volume control.

AGC is a departure from linearity in AM radio receiver
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...

s. Without AGC, an AM radio would have a linear relationship between the signal amplitude and the sound waveform — the sound amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...

, 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
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...

. If the circuit were not fairly linear, the modulation
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...

 could not be recovered with reasonable fidelity
Fidelity
"Fidelity" is the quality of being faithful or loyal. Its original meaning regarded duty to a lord or a king, in a broader sense than the related concept of fealty. Both derive from the Latin word fidēlis, meaning "faithful or loyal"....

. However, the strength of the signal received will vary widely, depending on the power and distance of the transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...

, and signal path attenuation
Attenuation
In physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, X-rays are attenuated by lead, and light and sound are attenuated by water.In electrical engineering and telecommunications, attenuation affects the...

. 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 an approximately constant average output level. For a very weak signal, the AGC has no effect, allowing the receiver to operate at its maximum gain; 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
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...

 and blocking
Blocking (radio)
In radio, and wireless communications in general, blocking is a condition in a receiver in which an off-frequency signal causes the signal of interest to be suppressed....

; 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
Communications receiver
A communications receiver is a type of radio receiver used as a component of a radio communication link.-Features:Commercial communications receivers are characterised by high stability and reliability of performance, and are generally adapted for remote control and monitoring...

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
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

 systems, as a method of overcoming unwanted clutter
Clutter
Clutter may refer to any of the following:*Excessive physical disorder** Clutter , a confusing or disorderly state or collection, and possible symptom of compulsive hoarding** A type of light pollution...

 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
Magnetic tape sound recording
The use of magnetic tape for sound recording originated around 1930. Magnetizable tape revolutionized both the radio broadcast and music recording industries. It did this by giving artists and producers the power to record and re-record audio with minimal loss in quality as well as edit and...

 generates a certain amount of noise
Noise
In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...

. If the level of the signal on the tape is low, the noise is more prominent, i.e., the signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...

 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
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...

 or seriously distort
Distortion
A distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice...

 the signal. In professional high-fidelity recording the level is set manually using a peak-reading
Peak-reading
A peak-reading electrical instrument or meter is one which measures the peak value of a waveform, rather than its mean value or RMS value.As an example, when making audio recordings it is desirable to use a recording level that is just sufficient to reach the maximum capability of the recorder at...

 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
Microphone
A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...

 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
Dynamic range
Dynamic range, abbreviated DR or DNR, is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 or base-2 logarithmic value.-Dynamic range and human perception:The human senses of sight and...

; the result can be a reduced musical quality if the signal is not re-expanded on playback, as in a companding
Companding
In telecommunication, signal processing, and thermodynamics, companding is a method of mitigating the detrimental effects of a channel with limited dynamic range...

 system.

Most reel-to-reel tape recorders and cassette deck
Cassette deck
A cassette deck is a type of tape recorder for playing or recording audio compact cassettes. A deck was formerly distinguished from a recorder as being part of a stereo component system, while a recorder had a self-contained power amplifier...

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
Vertical blanking interval
The vertical blanking interval , also known as the vertical interval or VBLANK, is the time difference between the last line of one frame or field of a raster display, and the beginning of the first line of the next frame. It is present in analog television, VGA, DVI and other signals. During the...

 to operate the AGC. Video copy control schemes such as Macrovision
Macrovision
Rovi Corporation is a globally operating, US-based company that provides guidance technology, entertainment data, copy protection, industry standard networking and media management technology for digital entertainment devices and services...

 exploit this, inserting spikes in the pulse which will be ignored by most television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...

 sets, but cause a VCR's AGC to overcorrect and corrupt the recording.

Telephone recording

Devices to record both sides of a telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

 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
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

 visual system
Visual system
The visual system is the part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light to build a representation of the surrounding world...

, calcium dynamics in the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

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
Auditory system
The auditory system is the sensory system for the sense of hearing.- Outer ear :The folds of cartilage surrounding the ear canal are called the pinna...

, the olivocochlear efferent neurons are part of a bio-mechanical gain control loop.

See also

  • Squelch
    Squelch
    In telecommunications, squelch is a circuit function that acts to suppress the audio output of a receiver in the absence of a sufficiently strong desired input signal.-Carrier squelch:...

  • Companding
    Companding
    In telecommunication, signal processing, and thermodynamics, companding is a method of mitigating the detrimental effects of a channel with limited dynamic range...

  • Dynamic range compression
  • Gain compression
    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 device beyond its linear region...

  • VOGAD
    VOGAD
    In electronics systems, a VOGAD or voice-operated gain-adjusting device is a type of automatic gain control or 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...

    (Voice-operated gain-adjusting device)
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