Dynamic range compression, also called DRC simply compression reduces the volume of loud parts or amplifies quiet parts by narrowing or "compressing" an audio stream’s dynamic range.
A compressor reduces the level of an audio signal if its amplitude exceeds a certain threshold. It is commonly set in dB, where a lower threshold (e.g. -60 dB) means a larger portion of the signal will be treated (compared to a higher threshold of −5 dB).
The amount of gain reduction is determined by ratio: a ratio of 4:1 means that if input level is 4 dB over the threshold, the output signal level will be 1 dB over the threshold. The gain (level) has been reduced by 3 dB:
Threshold = −10 dB
Input = −6 dB (4 dB above the threshold)
Output = −9 dB (1 dB above the threshold)
The highest ratio of ∞:1 is often known as 'limiting'. It is commonly achieved using a ratio of 60:1, and effectively denotes that any signal above the threshold will be brought down to the threshold level (except briefly after a sudden increase in input loudness, known as an "attack").
Attack and release
A compressor might provide a degree of control over how quickly it acts. The 'attack phase' is the period when the compressor is decreasing gain to reach the level that is determined by the ratio. The 'release phase' is the period when the compressor is increasing gain to the level determined by the ratio, or, to zero dB, once the level has fallen below the threshold. The length of each period is determined by the rate of change and the required change in gain. For more intuitive operation, a compressor's attack and release controls are labeled as a unit of time (often milliseconds). This is the amount of time it will take for the gain to change a set amount of dB, decided by the manufacturer, very often 10 dB. For example, if the compressor's time constants are referenced to 10 dB, and the attack time is set to 1 ms, it will take 1 ms for the gain to decrease by 10 dB, and 2 ms to decrease by 20 dB.
In many compressors the attack and release times are adjustable by the user. Some compressors, however, have the attack and release times determined by the circuit design and the user cannot adjust these. Sometimes the attack and release times are 'automatic' or 'program dependent', meaning that the times change depending on the input signal. Because the loudness pattern of the source material is modified by the compressor it may change the character of the signal in subtle to quite noticeable ways depending on the settings used.
Because the compressor is reducing the gain (or level) of the signal, the ability to add a fixed amount of make-up gain at the output is usually provided so that an optimum level can be used.