CHAPTER 21. LIVE AUDIO EFFECT REFERENCE
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A compressor reduces gain for signals above a user-settable threshold. Compression re-
duces the levels of peaks, opening up more headroom and allowing the overall signal level
to be turned up. This gives the signal a higher average level, resulting in a sound that is
subjectively louder and punchier than an uncompressed signal.
The latest incarnation of the Compressor effect merges the functionality of the former
Compressor I and Compressor II devices into a single unit, and adds additional functionality
such as external sidechaining, adjustable knee, more EQ options, an improved peak mode,
and a new feedback model, which is lovingly based on some classic hardware compressors.
The new Compressor is fully backward-compatible with Compressor I and Compressor II
presets.
A compressor's two most important parameters are the Threshold and the compression
Ratio:
The Threshold slider sets where compression begins. Signals above the threshold are
attenuated by an amount speci ed by the Ratio parameter, which sets the ratio between
the input and output signal. For example, with a compression ratio of 3, if a signal above
the threshold increases by 3 dB, the compressor output will increase by only 1 dB. If a signal
above the threshold increases by 6 dB, then the output will increase by only 2 dB. A ratio of
1 means no compression, regardless of the threshold.
The Knee control adjusts how gradually or abruptly compression occurs as the threshold
is approached. With a setting of 0 dB, no compression is applied to signals below the
threshold, and full compression is applied to any signal at or above the threshold. With very
high ratios, this so-called hard knee behavior can sound harsh. With higher (or soft )
knee values, the compressor begins compressing gradually as the threshold is approached.
For example, with a 10 dB knee and a -20 dB threshold, subtle compression will begin at
-30 dB and increase so that signals at -10 dB will be fully compressed.
The easiest way to visualize Compressor's behavior is by observing how the graph changes
when you adjust the threshold, ratio, and knee values. Input level is measured on the
horizontal axis, while output level is represented vertically. Next to the graph is the Gain
Reduction meter, which shows how much the gain is being reduced at any given moment.
The more reduction, the more audible the effect; a gain reduction above 6 dB or so might
produce the desired loudness, but signi cantly alters the sound and is easily capable of
destroying its dynamic structure. This is something that cannot be undone in later production
steps. Keep this in mind especially when using a compressor, limiter or sound loudness-
