A circuit which reduces the dynamic range of an input signal. A basic compressor circuit consists of an envelope follower and a VCA. The envelope follower produces a control voltage corresponding to the volume of the input signal, which is then controls the VCA which is controlling the gain of the output signal. Processing circuitry modifies the control signal such that, as the volume of the input exceeds a certain point, the control signal is attenuated, reducing the VCA's gain and hence reducing the volume of the output signal.
Typically the modification of the control signal is expressed as a "compression ratio". For instance, if the ratio is stated as 2:1, it means that when the input volume rises past a threshold (called the "knee"), the envelope follower's output is only allowed to rise half as much as the increase in the input volume, which causes the output volume to be reduced by that ratio. When the compression ratio is "infinite", it means that past the knee, as the input volume goes up the control voltage actually goes down, which means the output gets no louder no matter how much louder the input gets. This is referred to as "limiting", and a dedicated device that does this is called a limiter.
Because compression tends to reduce the average level of the output signal, an additional feature called the "make-up gain" is provided for the output. This allows the output signal to be raised back up to the average level of the input signal. The effect is that the signal is as "loud" as it was before, but the dynamics are more under control. Extreme settings of compression and make-up gain are often used to increase the apparent sustain of instruments that are plucked or struck, notably guitars.
Compressors may have many additional features. A lag processor for the control signal is sometimes provided so that the user can control abrupt changes in the VCA gain, which may be noticeable as artifacts in the output; typically an attack and release rate control is provided. Some compressors have a mechanism known as a "side chain", which allows a signal separate from the main input to be routed to the envelope follower, which will make the output track the dynamics of the side chain input.
Compressors are often used to "fatten" bass signals, and may provide a noticeably different timbre vs. the uncompressed signal. This occurs because at low frequencies, the envelope follower has a tendency to follow the actual waveform instead of just the signal envelope, which in turn causes the VCA to distort the output signal in a characteristic manner.