Threshold sets the level above which compression takes place. Signals below the threshold will remain unaltered.
Ratio is the 'strength' of compression above the threshold level. The higher the ratio, the greater the effect. If the ratio is set at 5:1, it means that when the signal is above the threshold level, when the input signal rises by 5 dB, the output signal rises by 1 dB.
At a compression ratio of 2:1, the effect is mild and suitable for the subtle compression of vocals or for a complete mix. At 10:1, compression is much stronger and more noticeable. Ratios between 5:1 and 15:1 are suitable for the 'compressed' sound, used as an effect in its own right. Higher ratios are used for the control of extremely peaky signals. Above 20:1, the compression effect is so pronounced that it is known as 'limiting'. It is possible to buy a dedicated limiter.
The point where the slope of the compressor curve changes is known as the Knee. Some compressors have an adjustable knee, variable between hard (which is normal) and soft:
Attack sets the time the compressor takes to respond once the threshold has been exceeded. Attack may be set so that the initial transient of the instrument passes through unaltered, or set to a faster value so that the very start of the sound is compressed. Particularly with drum sounds, careful adjustment of attack time can make the sound more 'punchy' and 'driving'.
Release time plays a very important role in compression. During periods of high signal level, gain is reduced. When the signal level falls below the threshold, the gain will increase at a rate determined by the Release control. If the release time is short, the gain will rise quickly. A long release time will mean that the gain will stay at its reduced level, only recovering gradually:
The setting of the correct release time is a compromise. If the release time is too short, background noise can cause effects often known as 'breathing' and 'pumping'. If the release time is too long, the signal will not be compressed, but simply reduced in level. For effective compression, the release time must be set to as short a value as possible before modulation of the background noise becomes too noticeable. The gain reduction bargraph meter will show how much actual compression is going on. If it stays steady, there is little active compression, just a steady-state reduction in level. The faster the bargraph moves up and down, the harder the compressor is working.Gain Make-Up restores the level lost in the compression process. Since the compressor works by bringing down peak levels, the level of the output signal would be lower than the input if nothing were done. Sufficient gain make up should be applied so that the peaks of the compressed signal are the same level as the peaks of the inputs signal. The sections of the input signal that were quiet will now be louder.
Stereo Link: When a stereo signal is compressed, the stereo link has to be activated so that both channels provide the same amount of gain reduction. If this is not done, a loud signal in one channel will cause that channel to be lowered in level while the other stays the same. Any signal that is panned center in the mix wiill swing in the stereo image towards the unaltered channel. With stereo link selected, the stereo image is maintained.