Polycom SoundStructure C16 Design Manual page 121

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.
For example, if the compression ratio is set to 4:1, the threshold is set to -10
1
dBFS , and the input signal level is -2 dBFS (8 dB above the threshold) the com-
pressor 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.
The "attack" portion of the compressor is when attenuation is increased as the
signal level crosses the threshold, and the "decay" portion is when the attenu-
ation is reduced toward 0 dB as the signal level falls below the threshold.
Decreasing the attack time will allow the compressor/limiter to work more
aggressively but may also introduce audio artifacts.
Limiters perform just like compressors, but are typically set with higher com-
pression ratios (10:1 or more) to further limit the dynamic range of signals
levels above the threshold.
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.
Threshold
Input Level (dB)
Customizing SoundStructure Designs
No Compression
2:1 Compression
4:1 Compression
10:1 Compression
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