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The process of taking an input signal that is louder than a given threshold, and making it somewhat quieter on the way out.
The "somewhat" is known as the ratio of compression.
- A ratio of 4:1 means that for every 4dB the input signal rises above the threshold, the compressor will allow the output signal to rise by 1dB.
- A ratio of 1:1 means the signal isn't compressed at all; each 1dB increase above the threshold results in a 1dB increase in the output level. Although it doesn't seem useful, it's sometimes available on a compressor for one of two reasons:
- If the ratio can be varied continuously (instead of just being able to select from a few presets) it lets you make the compression as light and subtle as you want.
- Most compressors, especially classic older models from the 1960s-1980s, are not electrically neutral - they add their own flavour to the signal passing through them. In some cases, such as the UREI 1176, that flavour is often desirable by itself. A ratio of 1:1 enables you to apply this flavour without applying compression.
- Ratios between about 20:1 and infinity:1 effectively stop the signal from rising above the threshold at all. In these cases, it's usually referred to as
limiting instead of compression because it "limits" the signal to the threshold level.
If the signal is below the threshold, the compressor lets it pass without changing its level.
Note It does not make quiet sounds louder. That's why you never see an explanation of how that part works; see this commentary for more depth.
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