Apple Using WaveBurner Logic Pro 7.1 - Manual (Page 33)

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Multipressor
The Multipressor (short for multiband compressor) is used as a mastering tool in many
different situations. The Multipressor splits the incoming signal into two to four
different frequency bands, each of which is compressed independently. This permits
higher levels of compression without producing a pumping effect.
As the name suggests, the Multipressor is like a set of compressors, each working
independently on a different part of the frequency range of an input signal. You can
independently set the Threshold, Ratio, Attack, and Release parameters for each
frequency band. Turning the Peak/RMS control clockwise changes the detection
method continuously between Peak and RMS detection.
The output Level and frequency Range for each band can be adjusted graphically. In the
display on the left hand side, click inside the colored bar that represents the frequency
band you wish to change, and drag it vertically to lower its output level. Drag the bar’s
borders horizontally to adjust the frequency range of the band.
The Multipressor allows you to raise the overall volume level drastically. This may
increase the amount of low-level noise (known as the noise floor). To counteract this
effect, each frequency band features Expansion (downward expansion). Compression
decreases the dynamic range above the Compressor threshold while Expansion
increases the dynamic range below the Expansion threshold—and in the process,
further lowering the low level signals (the noise floor). The amount of Expansion is set
with the Reduction parameter. The effect is similar to that of a noise gate, but it
smoothly decreases the signal level, rather than cutting it off abruptly.
The Multipressor has several other important parameters, including Bands and
Lookahead. The Bands parameter lets you choose whether two, three, or four
independent frequency bands are available; the higher the number, the more
processing power the Multipressor uses. Classic multi-band compressors typically use
three bands.