A stereo compressor without a sidechain can be used as a mono compressor with a sidechain. The key or sidechain signal is sent to the first (main) input of the stereo compressor while the signal that is to be compressed is routed into and out of the second channel of the compressor.
Threshold:
Threshold is the level above which the signal is reduced. 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).
Ratio:The ratio determines the input/output ratio for signals above the threshold। For example, a 4:1 ratio means that a signal overshooting the threshold by 4 dB will leave the compressor 1 dB above the threshold। The highest ratio of 8:1 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 (unless some attack is in force).
A compressor might provide a degree of control over how quickly it acts। The 'attack phase' is the period when the compressor is increasing gain reduction to reach the level that is determined by the ratio. The 'release phase' is the period when the compressor is decreasing gain reduction to the level determined by the ratio, or, to zero, once the level has fallen below the threshold. The length of each period is determined by the rate of change and the required change gain reduction. For more intuitive operation, a compressor's attack and release controls are labelled 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 reduction to rise from 0 dB to 10 dB, and 2 ms to rise from 0 dB to 20 dB[4].
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 these cannot be adjusted by the user. 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.
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