The compressor is an indispensable processor in sound engineering and is used in many different ways. In this section we will describe its most common and codified uses, bearing in mind the possibility of a more "personal" approach and experimentation, so long as this takes place with method and criteria , and not on the basis or hope that "anyhow something cool will come out of it"...!
Compression of the whole mix: By applying a stereo compression to an entire mix we can obtain a more homogeneous sound, in that its sudden volume variations become leveled and each sound becomes part of a single body of sound. The compression of an entire mix also allows us to limit its dynamic range, and this will be a desired effect either because the musical genre requests it (for example dance music requires no more than 30 dB, therefore if we were to record a real drum for a dance music track, we'd have to compress it heavily), both for technical reasons (for example, radio-transmission of the music consents a maximum dynamic range-level of 15 dB, therefore a heavy compression is applied to the tracks before they are sent on-air). Sometimes a light compression is already applied during recording so as to avoid saturation problems. Obviously what has been said with regards to a mix, applies to sub-mixes too. For example if we have a wind-section with 8 units playing, we can choose to sub-mix and compress them separately to obtain a homogeneous wind-section before they get mixed with the rest of the instruments. The same is true with a drum-kit recorded with many microphones. The values used in this context are: light compression ratio (2:1) (to not add manipulations that are too loud), low threshold (in that we want to bring about an overall reduction of the dynamic range and not only the upper part), medium attack (to react to more or less all the sounds in the mix) long release (up to 2 seconds, the compressor remains practically always in action and this avoids ugly volume changes). If a compressor is used in stereo modality, the two sidechaininputs are linked by an apposite switch in order to have a synchronized function on two channels. In this case the sidechain input controls are unified on the left compressor whilst the right ones get deactivated.
Modification of the signal envelope: In this case we want to modify the sound of a single instrument. The choice of parameters depends entirely on the kind of input signal and the result we wish to obtain. Let's take a look at two opposite examples of this operative modality. As our first example, let's imagine that we want to increase the sound in question's attack. We will therefore use a slow attack time to allow the first sound transient to pass through the compressor unaltered. The release time will have to be longer than the duration of the produced sound, in order to allow the compressor to remain activated until the sound has died out. The threshold and the compression-ratio depend on the input signal's amplitude and on how heavily we wish to manipulate the sound. The opposite example is a sound we wish to elongate in time, such as when we want to have a very long sustain on an electric guitar note. In this case the attack time shall be set at its lowest point, to not modify the initial part of the sound envelope. The peak shall be put quite high, the release time shall be long and the compression-ratio low. This combination of technical choices assures us that when the sound reaches its threshold, it goes beyond it and thus the compressor is activated. However, as a result of its low compression-ratio, the sound's amplitude shall remain almost completely unchanged. After this phase the original sound decays but the release time being long makes sure that the compressor maintains the sound's amplitude at its initial value for all its duration.
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