Polycom SoundStructure C16 Design Manual page 118

On the dynamics processing page there is also a fader control - the same fader control found on the
channels and matrix page- that can be used to add or remove gain from the underlying virtual channel.
The Reset button may be used to return the Dynamics processing to its default settings which leaves the
signal unprocessed.
Using Compressors And Limiters
The peak limiter monitors the peak signal magnitude and compares it to a threshold. If the peak surpasses
the threshold, the peak limiter immediately attenuates the signal with a very fast attack to bring the peak
level below the threshold.
Limiters and compressors attenuate high-level signals without changing low level signals and are typically
used to prevent loud signals from clipping, or to reduce the dynamic range of a signal to make the output
level more consistent even if the input level is not consistent. When the input signal level rises above the
compressor's threshold, the compressor applies attenuation so that the output signal increases at a rate of
one over the compression ratio past the threshold. Signals below the threshold are not modified, signals
above the threshold are "compressed" or scaled by the compression ratio.
1
For example, if the compression ratio is set to 4:1, the threshold is set to -10 dBFS , and the input signal
level is -2 dBFS (8 dB above the threshold) the compressor applies the compression ratio (in this case 4:1)
and divides the 8 dB by 4 to arrive at 2 dB. The output signal is then -8 dBFS (2 dB above the threshold)
even though the input signal was 8 dB above the threshold.
From this example, it is clear that the threshold is not a hard limit, but rather the onset of when the
"compression" or division by the compression ratio is engaged. This is shown in the following figure.
1. dBFS means dB full scale where 0 dBFS is the maximum input signal allowed. Due to the SoundStructure design of
a nominal signal level of 0 dBu with 20 dB headroom, -20 dBFS equals 0 dBu.
Polycom, Inc. 118