Gain compression is a reduction in ''differential'' or ''slope'' gain caused by

nonlinearity In mathematics and science, a nonlinear system is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathematicians, and many other ...

of the

transfer function In engineering, a transfer function (also known as system function or network function) of a system, sub-system, or component is a mathematical function that theoretically models the system's output for each possible input. They are widely used ...

of the amplifying device. This nonlinearity may be caused by heat due to power dissipation or by overdriving the active device beyond its

linear Linearity is the property of a mathematical relationship ('' function'') that can be graphically represented as a straight line. Linearity is closely related to '' proportionality''. Examples in physics include rectilinear motion, the linear ...

region. It is a '' large-signal'' phenomenon of circuits.

Relevance

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 ...

compression is relevant in any system with a wide

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 ...

, such as audio or RF. It is more common in tube circuits than

transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...

circuits, due to topology differences, possibly causing the differences in audio performance called "

valve sound Tube sound (or valve sound) is the characteristic sound associated with a vacuum tube amplifier (valve amplifier in British English), a vacuum tube-based audio amplifier. At first, the concept of ''tube sound'' did not exist, because practically ...

". The front-end RF amps of

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 are particularly susceptible to this phenomenon when overloaded by a strong unwanted signal.

Audio effects

A tube radio or tube amplifier will increase in volume to a point, and then as the input signal extends beyond the

range of the device, the effective gain is reduced, altering the shape of the waveform. The effect is also present in transistor circuits. The extent of the effect depends on the topology of the amplifier.

Differences between clipping and compression

Clipping, as a form of signal compression, differs from the operation of the typical studio audio level compressor, in which gain compression is not instantaneous (delayed in time via attack and release settings). Clipping destroys any audio information which is over a certain threshold. Compression and limiting, change the shape of the entire waveform, not just the shape of the waveform above the threshold. This is why it is possible to limit and compress with very high ratios without causing distortion.

Limiting or clipping

Gain is a linear operation. Gain compression is not linear and, as such, its effect is one of distortion, due to the

of the transfer characteristic which also causes a loss of 'slope' or 'differential' gain. So the output is less than expected using the '' small signal'' gain of the amplifier. In

clipping Clipping may refer to: Words * Clipping (morphology), the formation of a new word by shortening it, e.g. "ad" from "advertisement" * Clipping (phonetics), shortening the articulation of a speech sound, usually a vowel * Clipping (publications) ...

, the signal is abruptly limited to a certain amplitude and is thereby distorted in keeping under that level. This creates extra harmonics that are not present in the original signal. ''Soft'' clipping or limiting means there isn't a sharp ''knee point'' in the transfer characteristic. A

sine wave A sine wave, sinusoidal wave, or just sinusoid is a mathematical curve defined in terms of the '' sine'' trigonometric function, of which it is the graph. It is a type of continuous wave and also a smooth periodic function. It occurs often in ...

that has been softly clipped will become more like a

square wave A square wave is a non-sinusoidal periodic waveform in which the amplitude alternates at a steady frequency between fixed minimum and maximum values, with the same duration at minimum and maximum. In an ideal square wave, the transitions b ...

, with more rounded edges, but will still have many extra harmonics.

Compression

Compression of gain is caused by non-linear characteristics of the device when run at large amplitudes. With any signal, as the input level is increased beyond the linear range of the amplifier, gain compression will occur. A transistor's operating point may move with temperature, so higher power output may lead to compression due to collector dissipation. But it's not a change in gain; it's non-linear distortion. The output level stays relatively the same as the input level goes higher. Once the

non-linear In mathematics and science, a nonlinear system is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathematicians, and many other ...

portion of the transfer characteristic of any amplifier is reached, any increase in input will not be matched by a proportional increase in output. Thus there is compression of gain. Also, at this time because the transfer function is no longer linear,

harmonic distortion 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 ...

will result.

Intentional compression

In intentional compression (sometimes called

automatic gain control Automatic gain control (AGC) is a closed-loop feedback regulating circuit in an amplifier 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 inpu ...

audio level 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 ...

) as used in devices called 'dynamic range compressors', the overall gain of the circuit is actively changed in response to the level of the input over time, so the transfer function remains linear over a short period of time. A sine wave into such a system will still look like a sine wave at the output, but the overall gain is varied, depending on the level of that sine wave. Above a certain input level, the output sine wave will always be the same amplitude. The output level of Intentional compression varies over time, in order to minimize non linear behavior. With gain compression, the opposite is true, its output is constant. In this respect intentional compression serves less of an artistic purpose.

Radio-frequency compression

Gain compression in RF amplifiers is similar to soft clipping. However, in

narrowband Narrowband signals are signals that occupy a narrow range of frequencies or that have a small fractional bandwidth. In the audio spectrum, narrowband sounds are sounds that occupy a narrow range of frequencies. In telephony, narrowband is usua ...

systems, the effect ''looks'' more like gain compression simply because the harmonics are filtered out after amplification. Many data sheets for RF amplifiers list gain compression rather than distortion figures because it's easier to measure and is more important than distortion figures in nonlinear RF amplifiers. In

wideband In communications, a system is wideband when the message bandwidth significantly exceeds the coherence bandwidth of the channel. Some communication links have such a high data rate that they are forced to use a wide bandwidth; other links ma ...

and low-

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

systems, the nonlinear effects are readily visible, e.g. the output is clipped. To see the same thing at 1

GHz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one he ...

, an

oscilloscope An oscilloscope (informally a scope) is a type of electronic test instrument that graphically displays varying electrical voltages as a two-dimensional plot of one or more signals as a function of time. The main purposes are to display repetiti ...

with a

bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...

of at least 10 GHz is needed. Observing with a

spectrum analyzer A spectrum analyzer measures the magnitude of an input signal versus frequency within the full frequency range of the instrument. The primary use is to measure the power of the spectrum of known and unknown signals. The input signal that most co ...

, the fundamental compressed and the

harmonic A harmonic is a wave with a frequency that is a positive integer multiple of the ''fundamental frequency'', the frequency of the original periodic signal, such as a sinusoidal wave. The original signal is also called the ''1st harmonic'', t ...

s picking up.

Examples of RF compression

A low-noise

RF amplifier A radio-frequency power amplifier (RF power amplifier) is a type of electronic amplifier that converts a low-power radio-frequency signal into a higher-power signal. Typically, RF power amplifiers drive the antenna of a transmitter. Design goa ...

, if fed by a

directional antenna A directional antenna or beam antenna is an antenna which radiates or receives greater power in specific directions allowing increased performance and reduced interference from unwanted sources. Directional antennas provide increased performanc ...

to a consumer 900 MHz receiver, should improve the transmission range. It works, but the receiver may also pick up a couple of

UHF Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter (on ...

stations around 700 MHz. For example, if channel 54 is transmitting 6 MW of AM, FM, and PM, the RF front end, expecting −80 dBm, would be grossly overloaded and generate mixing products. This is a typical effect of gain compression.

High-power loudspeakers

Power compression is a form of gain compression that takes place in

loudspeaker A loudspeaker (commonly referred to as a speaker or speaker driver) is an electroacoustic transducer that converts an electrical audio signal into a corresponding sound. A ''speaker system'', also often simply referred to as a "speaker" or ...

voice coil A voice coil (consisting of a former, collar, and winding) is the coil of wire attached to the apex of a loudspeaker cone. It provides the motive force to the cone by the reaction of a magnetic field to the current passing through it. The te ...

s when they heat up and increase their resistance. This causes less power to be drawn from the amplifier and a reduction in

sound pressure level Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average or equilibrium) atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in water with a hydropho ...

References

{{reflist Audio engineering Signal processing nl:Compressie (elektrische gitaar)