Apple Using WaveBurner Logic Pro 7.1 - Manual (Page 32)

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Compressor
The Compressor tightens up the dynamics of a signal by lowering the volume when it
rises above a certain level, called the Threshold. This decreases the difference between
the softest and loudest parts of the music, increasing the perceived volume. This can
give the sound more focus, by making the key parts stand out while preventing the
accompanying parts from becoming lost or inaudible.
In addition to the Threshold parameter, the most important parameter for the
Compressor is the Ratio. The Ratio parameter determines the amount that a signal
above the threshold is lowered. The Ratio is expressed as a percentage of the original
signal. As an example, if you set the Threshold to –12 dB and set the Ratio to 2:1, a
signal at –7 dB (5 dB above the Threshold) is reduced by 2.5 dB, and a signal at –2 dB
(10 dB above the Threshold) is reduced by 5 dB.
The other parameters are Attack and Release. The Attack parameter controls how
quickly the Compressor reacts when the signal passes the threshold. Longer Attack
time values ensure that the original attack transients (the sound of a pick or finger
striking a guitar string, for example) remain intact and audible.
The Release parameter determines the amount of time it takes for the Compressor to
stop dampening louder passages, once the signal falls below the threshold.
Undesirable “pumping” can be minimized by adjusting the Release parameter.
As the compressor affects signals above the threshold, the overall output signal is often
lower than the input signal. You can compensate for this decrease in output level by
raising the Gain parameter. As an alternative, you can activate Auto Gain. This facility
automatically matches the input and output levels.
The Compressor can use one of two input level detection methods: either Peak or RMS.
RMS approximates the signal’s perceived loudness. Depending on the audio material,
RMS is often the more “natural” and “musical” sounding option of the two. You should
note, however, that the Compressor does not react to every single signal peak, which
may cause distortion. In this situation, turn Auto Gain off and/or switch the Compressor
to Peak detection.