Compressors have anywhere between 2-6 controls on them, depending on the model. However, more controls does not necessarily mean that it is a better compressor. Each compressor has its own distinct tone, and strengths and weaknesses. The only way to really figure out the differences is by trial and error, listening to each until you find the sound you're looking for.
The 6 parameters that can be found on a compressor are:
The threshold of a compressor determines the point at which it starts to compress. Once a signal (in dB) goes over the threshold, the compressor kicks into action. There are typically two different ways that the compressor determines whether the threshold has been crossed. There is Peak mode, in which the compressor compresses when the signal's peak goes over the threshold. There is also RMS mode, RMS stands for Root Mean Square, which essentially means that it takes the average signal level, and when that average level goes over the threshold, then the compressor starts working. Some compressors have a set threshold, and an input gain option, essentially pushing the signal closer and closer to the threshold the more you gain it up.
Once a signal goes over the compressor's threshold, the compressor has to know how much to compress it. This is where the ratio comes in. Ratio is a very simple term, a 4:1 ratio means that for every 4 dB that the signal goes over the threshold, 1 dB comes out.
It can be noted that the ratio is where we find the difference between a compressor and a limiter. A limiter generally is a compressor with a ratio of greater than 10:1. If a limiter's threshold is set low enough, then it will squash the waveform giving it a "brickwall" look.
The attack time is the time that it takes for the compressor to reach its full ratio. Attack time settings can typically be set to very quick, down to microseconds.
If a compressor's ratio were set to 4:1, with a 2ms attack time, then once the signal goes over the threshold it will take 2ms for the compressor to fully reach the 4:1 ratio.
The release time of a compressor is the opposite of the attack time. Once the incoming signal drops below the threshold, the release time determines how long it'll take for the compressor to stop compressing. Release times generally are longer than attack times, going so far as lasting for full seconds.
If a compressor's release time was set to 200ms, then once the signal drops below the threshold, it will take the compressor 200ms to return to its normal, non-compressed state.
Obviously, when a compressor compresses a signal, there is a loss in volume on that track. To accommodate for this, compressors typically have a makeup gain feature. This is a simple gain knob that boosts the outgoing signal back up to the original incoming signal (or higher if you want).
Some compressors have a "knee" setting. This setting is tied in with the ratio. When you have a hard knee on a compressor, then right as the signal hits the threshold, the compressor immediately jumps to the full ratio in the time that the attack time is set to. With a soft knee, the compressor actually starts compressing before the threshold is reached, but at a ratio less than the full amount. As the signal approaches and crosses the threshold, the ratio is gradually increased until it is at its full value.
Types of CompressionEdit
Hard Knee Compression is a type of compression that acts only when the threshold is reached and has a sharp output slope on the input vs output graph.
Soft Knee Compression is a type of compression in which the compressor starts compressing before the threshold is reached, giving a much smoother output slope on the input/output graph.
Sidechaining is a popular form of compression in EDM. A sidechain is simply a compressor that acts on one track, but monitors another for when to compress. For example, if you have a synth that you want to have sidechained to get out of the way of the kick (or just want to have that 'pulsing' feel that comes with a sidechain), then you set the sidechain track to listen to the kick for when it should compress. Ducking is simply a less extreme form of sidechaining, in which the compression is just enough to make room for the trigger track, but not enough to have that pulsing effect on the compressed track.