A Beginner's Guide To Compression - Focusrite Trakmaster Bedienungsanleitung

A BEGINNER'S GUIDE TO COMPRESSION

Compressors are probably the most widely used signal processors in the audio industry. A
compressor can be thought of as an automatic volume control. Once the volume of the
input signal exceeds a certain level (called the threshold), the compressor reduces the gain
(or, in other words, 'turns the volume down'), causing the signal to be less loud than it
would otherwise have been.
The amount by which the compressor reduces the gain is determined by the ratio. The
ratio is conventionally expressed as a numerical value, e.g. '4:1', which represents the
amount by which the gain is reduced when the volume of the signal rises above the
threshold. This is based on dB - if a ratio of 4:1 is set, an input signal exceeding the
threshold by 4 dB will cause only a 1 dB increase in level at the output.
Compression has the result that any variations in the volume of the signal (in other words,
the signal's dynamic range – the difference between the minimum and maximum levels
of a signal) are reduced. The amount of this reduction is determined by the threshold (the
level above which the gain is reduced) and the ratio (the amount by which the gain is
reduced.) Higher ratios are referred to as hard ratios; lower ratios are called soft ratios.
Because compression causes a reduction in volume level of loud signals, gain must be
applied after the compressor to bring the overall volume level back up, so that the
maximum volume before the compressor is the same as that after the compressor. This is
called makeup gain, and is necessary so that the maximum level of the signal is always the
same, for correct level matching with any further processing or other equipment.
Once makeup gain has been applied, the quieter parts of the signal that were lower than the
threshold volume (and hence not compressed) will now be louder than they were before
entering the compressor. This will cause any compressed instrument to sound louder. One
use for this phenomenon is to give guitars more sustain.
In most pop music, the backing instruments (such as drums, bass guitars, rhythm guitars etc)
tend to be compressed heavily (using a fairly hard ratio and low threshold), so that they
remain at a consistent volume level throughout the track. This will provide a solid backing,
without occasional drum hits or bass notes poking through (or disappearing from) the mix
untidily.
A soft ratio tends to be used on instruments such as lead guitars or vocals that 'sit' on top of
the mix. In this situation it is desirable to preserve more of the dynamics of the original
performance, to retain more expression. A reduction in variation of volume level is still
required (for the reasons mentioned above), but not to the same extent.
Here's an example of the effects of threshold, ratio and makeup gain using some real
numbers. Suppose the threshold of the compressor is set to –10 dB and the ratio is set to
4:1. An input signal with a peak at –6 dB (4 dB above the threshold) would be output from
the compressor with a peak at –9 dB (1 dB above the threshold).
Signal levels below the threshold are unaffected, so if the level of the signal in the above
example varied between –20 dB and –6 dB before entering the compressor, it will vary
between –20 and –9 dB after being compressed. In other words its dynamic range (the
difference between the quietest and loudest parts of the signal in dB) is reduced from 14 dB
to 11 dB. Following this compression, 3 dB of makeup gain would be applied to bring
the peak back up to –6 dB. Note that this will bring the minimum signal level up from –20
dB to –17 dB, and so the perceived loudness of the signal will thus be increased, although
the peak will still be at –6 dB.
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