A compressor is an amplifier whose output decreases as its input level increases, and a limiter is a compressor which puts an absolute ceiling on the output level in spite of ever-increasing input levels. The action of compression or limiting is set to begin when a certain input level is reached. This level is called the threshold and is considered to be 0dB.
Signal peaks that rise above the threshold level are either attenuated or chopped off, depending upon the setting of the compression ratio control (which we shall discuss later).
Some musical instruments are louder in their upper registers than in their lower ones. Compressors can flatten out their signal to obtain a more uniform level. For example, the bass guitar is usually a good candidate for compression since it is renowned for often having one string louder than the others (usually the low E string). Vocalists who wander away from the microphone, are also signals that would benefit from compression. Guitarists kicking in a stomp box/foot pedal or a drummer’s over-enthusiastic snare beat are easily corrected if the compressor is set correctly.
Generally speaking, compression should be used to make a visible difference on the compressor’s meter, but not so much that the sound is audibly colored. Excessive compression can result in a squeezed or unpleasantly thin sound, and can also increase any undesirable noise elements which are present in a signal.
As a general rule, each dB of compression brings about an equal amount of deterioration in the signal-to-noise ratio of the input signal.
Some music continually varies in amplitude – one moment it is very quiet, and then it becomes very loud. Sometimes the signal will be above the threshold level, and sometimes it will fall below. The speed with which a compressor reacts is an important factor to consider.
The ratio control governs the degree of compression that is applied to the input signal. Compression ratio is defined as the amount of dBs attenuated per dB of input signal. In other words, When I’m talking about a ratio of 3:1, for example, you might wonder what that figure actually means. Put simply, for every 3dB by which the input signal exceeds the threshold, only 1dB will be allowed to pass by the compressor. Most compressors offer a choice of compression ratios typically ranging from a slight 2:1 to a more severe 20:1. When these higher ratios are reached the compressor is then considered to be a limiter although strictly speaking, the ratio for limiting is regarded to be infinity: one. A limiter acts in a similar fashion to a compressor; however, it affects only the peaks of the signal, leaving the lower levels unchanged.
Limiting is normally employed to remove occasional short term peaks that would otherwise saturate the tape or clip (distort) the digital recording. Compressors tend to work a little more naturally than limiters in terms of retaining signal integrity. The main reason for using limiting rather than compression is this: When a signal must not be allowed to exceed a certain level under any circumstances. If the compression ratio control is altered after the other controls on the unit have been set, the location of the threshold point will be affected and the input level will need re-adjusting to accommodate the new ratio setting.
The Attack is the period when the compressor is decreasing gain to reach the level that is determined by the ratio. This is the time it takes for the compressor to react to a signal which is registering above the threshold. If the attack time is set very fast, the signal will be attenuated immediately. If a medium- fast attack time is set, the initial peak will not be caught. This type of attack setting can be used creatively to add a percussive quality to a signal and make it sound punchier. If a slower attack time is set, a long sustained note will not be attenuated immediately. Initially, the note will pass through unchecked, and then the compressor will react and attenuate the remainder of the signal. Typical attack times vary from 20 to 800 milliseconds.
When the signal is above the threshold, the compressor doesn’t instantly apply the full compression to the signal but the compression ratio increases gradually from 1:1 to the maximum within a certain amount of time. This time is called the attack time.
This is the time it takes for a compressed signal to be restored to its normal uncompressed level after the level of the input signal has fallen back beneath the threshold. With a very fast release time, full gain will be restored as soon as the signal reaches a point lower than the threshold level. If the release time is set too slow, the unit may unnecessarily attenuate a soft passage that is closely following a loud, compressed passage because the gain reduction circuit has not had time to return to normal. Release times vary typically from 50 ms. to one whole second or even longer.
One significant disadvantage with some compressors is that they can also grab low level signals and raise them to the threshold level along with any unwanted background noise. This rise in background noise often sounds like a pumping or breathing effect when the signal level is continually rising above and falling below the threshold. If the release control is set correctly, this problem can be avoided. Faster release brings the signal to the front. Slow release pushes the signal in the mix.
Because the compressor is reducing the level of any signal that exceeds the threshold, the final output level from the unit will naturally be lower than the original input level. A pre-amplifier within the compressor applies an extra stage of boost to the treated signal to restore it to a usable level for re-entry into the console. The output knob (or gain control) controls this level.