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Something that has taken me far too long to appreciate, is the wisdom behind the way SSC scales the output gain of their Kyma Compressors. With unity output gain, these act just like a compressor with automatic makeup gain, but that gain is such as to limit the maximum output level to around -10 dB. A maximum level signal into the Compressor would result in an output signal of -10 dBFS.

Somewhere, SSC does state that they view the -10 dB level as the nominal 0 dBVU level. I know I have seen that. But I spent far too long trying to overcome this margin provided by SSC, in an effort to get my signals back up to the input levels.


What happens during a transient peak going through a compressor? The compressor at that moment has some additional gain applied, over and above unity gain. Compressors don't generally act instantaneously, although you can set an attack time of 0 in the Kyma compressors. But with any non-zero attack time, it takes a while for the compressor to realize that it must change the gain. Meanwhile the transient makes its way through the compressor with that initial higher gain applied.

During the time that the transient sneaks through, you can view the compressor as a linear device -- nothing more than a gain block. So a 6 dB rise in the transient level will produce a 6 dB rise in the output level as well. Not 6 dB divided by the compression ratio. That ratio only tells you what the eventual steady-state gain of the compressor will be, but until it can reorient itself to a stronger steady signal, it actually applies a unit slope to that signal, on top of the extra gain already provided by the compressor in its pre-transient state.

Hence, by allowing an output margin of 10 dB, SSC has allowed your transients to be as high as 10 dB or more than the rest of the signal without going into output limiting or clipping.

So the lesson here is that one ought not to try to work the compressor post-gain too hard, lest transients actually clip. The only time it is safe to provide the remaining 10 dB of makeup gain is when you are using an attack time of 0, but that generally squashes the sound so much that it doesn't sound so good.

Headroom, headroom, headroom....

-- DavidMcClain - 16 Nov 2004

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