Compressors are processors that allow you to control the dynamics of a signal — and boy, are they ever versatile. Compressors have a series of dials that allow you to adjust several parameters. They are as follows:
Threshold: The threshold setting dictates the level that the compressor starts to act on the signal. This is listed in decibels (dB). For the most part, you set the threshold level so that the compressor acts only on the highest peaks of the signal.
Ratio: The ratio is the amount that the compressor affects the signal. The ratio — such as 2:1, for instance — means that for every decibel that the signal goes over the threshold setting, it is reduced by two decibels.
In other words, if a signal goes 1dB over the threshold setting, its output from the compressor will only be 1/2dB louder. The ratio is the one parameter that varies considerably from instrument to instrument because the level of the transient varies.
Attack: The attack knob controls how soon the compressor starts, well, compressing. The attack is defined in milliseconds (ms), and the lower the number, the faster the attack. For the most part, you’re trying to control transients, and they happen at the beginning of a note. Therefore, you set the attack to act quickly.
Release: The release parameter controls how long the compressor continues affecting the note after the note starts. Like the attack, the release is defined in milliseconds. Because transients don’t last for very long, you usually use a short release time when using compression on the front end.
Gain: The gain knob allows you to adjust the level of the signal coming out of the compressor. This is listed in decibels. Because adding compression generally reduces the overall level of the sound, you use this control to raise the level back to where it was going in.
Hard knee or soft knee: Most compressors give you the option of choosing between a hard knee and a soft knee (or they do it for you based on the setting that you’ve chosen). Hard knee and soft knee each refer to how the compressor behaves as the input signal passes the threshold. More detailed descriptions are as follows:
Hard knee applies the compression at an even rate, regardless of the level present over the threshold. So if you choose a compression setting of 4:1, the compressor applies this ratio for any signal over the threshold limit. Hard-knee compression is used for instruments like drums, where you need to quickly clamp down on transients.
Soft knee applies the compression at a varying rate depending on the amount the signal is over the threshold setting. The compressor gradually increases the ratio of the compression as the signal crosses the threshold, until it hits the level that you set. Soft-knee compression is used on vocals and other instruments where the signal doesn’t have fast peaks.
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Source:http://www.dummies.com/how-to/content/sound-compressor-parameters-in-home-recording.html
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