The Multiband Compressor module compresses volumes that exceed the threshold in up to four frequency bands. The frequencies that separate the frequency bands can be arbitrarily set.
The parameters that can be set for each frequency band are:
- Crossover Frequency: Frequency cut-off between bands (vertical line in the graph).
- Threshold: Level at which compression starts being applied.
- Ratio: Amount of compression applied, which corresponds to the level surpassing the threshold per dB of actual gain increase.
- Attack: Time for the compression to start being applied after the level has gone over the threshold.
- Release: Time for the compression to stop being applied after level returned below the threshold.
- Knee: Determines how gradually compression is applied as the threshold is approached and passed, where a higher value means a more gradual compression.
- Gain: Make-up gain applied after compression.
Compression can be linked across channels using three different modes: No Link, All Channels and Partial Link. In those two last modes, the compression applied is based on the maximum value of compression that was computed for each channel.
- No link: Compression is applied to each channel individually.
- All channels: The maximum compression is applied equally to all channels.
- Partial link: Uses the value of Link Strength to control compression of channels, applying the maximum compression to the corresponding channel and applying the Link Strength ratio to that maximum for other channels.
The Partial link mode offers two parameters to control the compression to the channels that are linked together (those with less than the maximum compression):
- Link Strength: Ratio applied on the maximum compression value when compressing other channels than the one that had maximum compression. This allows to control how much of the maximum compression other channels receive, between 0% and 100%.
- Stereo Link: Enables the linking of pairs of channels that make up a Left-Right pair. This forces the compression applied to match between any Left-Right pair.
The following schema shows the processing applied by the compressor; the input signal is split, each band is compressed, and then joined again.
Different compression can be applied to each frequency band; therefore more detailed configurations are possible, and it is possible to remove undesired effects (such as "pumping") that can be noticeable when using a single compressor.
The compression algorithm in the Multiband Compressor is designed to smoothly change signal energy so that human ears can naturally hear output. In other words, the compression ratio is controlled based on the root mean square (RMS) value of the input signal calculated for each band. Therefore, sudden peaks are not controlled. (To control sudden peaks, use the Limiter module.)
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