The Logic Pro (not Logic Express) Plugin Reference has all the details you want starting on page 52...
Multipressor
The Multipressor (an abbreviation for multiband compressor) is the epitome of an audio mastering tool. It’s a pretty complex tool; good sounding settings require quite a lot of listening experience.
Functional Principle of Multi-band Compressors
The multi-band compressor splits the incoming signal into two to four different
frequency bands before applying compression. These frequency bands are then
compressed independently. After compression, the frequency bands are mixed back
together.
The aim of independent compression of different frequency bands is to reach high
compression levels on the bands that need it, without the pumping effect that is
normally heard at high compression levels.
Much higher Ratios, and therefore, a much higher average volume is possible before
the unwanted artefacts of compression will be heard.
Downward Expansion
Strong multi-band compression allows you to raise the overall volume level—resulting
in a dramatic increase of the existing noise floor. Downward expansion allows you to
reduce or suppress this noise. Each frequency band features a downward expander.
This works as the exact counterpart to the compressor: while the compressor
compresses the dynamic range of the higher volume levels, the downward expander
expands the dynamic range of the lower volume levels. With downward expansion, the
signal will be reduced in level when it is lower than the defined Threshold level. The
effect can be compared to a noise gate, but rather than simply cutting off the sound, it
smoothly fades the volume using an adjustable Ratio.
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Multipressor Parameters
Bands
This parameter (on the right side) determines the number of independently
compressible frequency bands, and has a crucial impact on the amount of computing
power needed for the effect. Classic multi-band compressors use three Bands.
Lookahead
Lookahead (just below the Bands parameter) determines how far the processor looks
into the future, in order to react earlier (thus better) to peak volumes.
Set Lookahead to higher values, when the Peak/RMS control (see below) is set further
towards RMS.
Peak/RMS
Adjusting the control between Peak (full left) and RMS (Root Meantime Square; full
right) is dependent on the type of signal you would like to compress. An extremely
short Peak detection setting is suitable for compression of short and high peaks of low
power, which do not typically occur in music. The RMS detection method measures the
power of the audio material over time and thus works much more musically. This is
because human hearing is more responsive to the overall power of the signal than to
single peaks. As a basic setting for most applications, the centered position is
recommended.
Attack
Allows you to define the time (in milliseconds) required before compression is faded in.
Release
Sets the time required by the compressor (after compression) to release the effect. Just
as with the other values, the best setting for this parameter depends greatly on the
material to be compressed.
Multi-band Graphic
The graphic editor to the left of the Multipressor displays several settings, both
graphically and numerically.
• Crossover Frequencies
The crossover frequencies (vertical borders) between the bands are variable. To
change a band’s crossover point, grab the borders directly within the graphic and
move them to the left or to the right. The frequency is displayed numerically at the
bottom of the graphic.
• Absolute Volume
The horizontal line in the middle of each band displays its current level
(default: 0 dB). By grabbing the area below this line and moving it up and down, you
can set the absolute volume level of the corresponding band. The level is displayed
numerically at the bottom of the graphic. This ability allows the Multipressor to serve
as a basic equalization tool, dependent on how the crossover frequencies are set.
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• Threshold Display
The horizontal lines (up to three) in the lower area of the window represent the
Threshold values for Compression (upper line), Expansion (middle line), and
Reduction (bottom line). You can set these values by using the controls of the same
name (see below).
Note: You can select the frequency band that you wish to edit by clicking in the lower
section of each band.
Comp. Ratio
This, in conjunction with Compression Threshold, is the central parameter for
compression. The Comp. Ratio determines the strength or rate of level reduction in the
range you want to compress. In most cases, the most useful combinations of these two
settings are either 1) low Compression Threshold and low Comp. Ratio or 2) high
Compression Threshold and high Comp. Ratio.
Exp. Ratio
This, in conjunction with Expansion Threshold, is the central parameter for controlling
downward expansion. It determines the strength of expansion applicable to the range
that you wish to expand.
Graphic Curve
The graphic curve in the middle of the Multipressor shows the Ratio between input
level (horizontal scale) and output level (vertical scale) of all bands. The colors
correspond to the colors of the frequency bands in the left graphic area. Adjustments
to the Ratio and Threshold controls allow you to change the curvature of the selected
frequency band.
Thresholds
• Compression
Here, you can set the minimum level at which the compressor will begin to work. If
the control is set all the way to the right (0 dB), the entire compressor section of the
Multipressor is off-line. The further you move the control to the left, the lower the
level above which the compressor will work.
• Expansion
Here, you can set the maximum level at which the expander should work. If the
control is set all the way to the left (−50 dB), expansion will only occur on signals that
fall below this level. (The Exp. Ratio can be set to a minimum of 1,2:1; below −50 dB
the expansion always takes place at this low ratio.) The further the control is moved
to the right, the higher the level below which the expander will work.
Reduction
Allows you to define the amount of noise level reduction (this is not a threshold value).
If you move the control all the way to the left, the reduction will be at its maximum
value (−50 dB). If the control is set all the way to the right, no reduction will occur.
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Level Meter
In the Level Meter to the right, you may monitor either; the change of level caused by
compression, or the output volume of each band, depending on whether you have
selected Gain change or Output (see below). You can individually switch the bands on
and off, in order to listen to single bands, by using the switches below the meters. If the
switch below a band’s meter is lit (light green), the band will be audible. If a switch is
unlit, the band is muted.
Gain Change/Output
The Gain change and Output buttons can be used to switch the operating mode of the
meters.
• If Gain change is selected, the meters indicate the strength of level reduction (to the
audio material) by the compressor.
• If Output is selected, the meters show the absolute output level of the corresponding
frequency band.
Master Gain
Allows you to reduce the overall gain increase (or increase the overall gain reduction),
resulting from the Multipressor’s settings.
