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Old 10-20-2015, 10:35 PM   #3
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Join Date: Dec 2010
Location: Sydney
Posts: 137

Originally Posted by kitkonis View Post
knee size
high pass
Seeing as no one else has answered directly, I'll tackle this...
Pre-comp... this is a temporal look ahead function. If you set it for say 5ms, the compression circuit will always be looking 5ms ahead for any rapid transients which would otherwise sneak through without getting caught. Longer settings might cause latency issues. Suck it and see.

Release... like any compressor, this is the time it takes for the compression circuit to STOP controlling the dynamics, AFTER the signal level has fallen back below the threshold..

Attack... again, same as any other compressor. The time it takes for the compression circuit to react to an input signal ONCE that signal has exceeded the threshold.

Ratio... like it says on the box, this controls the ratio of input level to output level, after compression has been applied. Maths lesson coming at ya shortly....

Knee size... usually expressed in dB and referrs to a softening between signal NOT being compressed and signal which IS being compressed. Think of it as a smudging of the THRESHOLD value.

Lowpass... wherever you set this value, frequencies ABOVE wil not have any influence on the compression circuit.

High pass... just the opposite. Frequencies below whatever value you enter here will not have any influence on the compression circuit. This is far more critical than the lowpass value, as high amplitude low frequency content CAN seriously mess with your compression if you don't really understand how compression works (not saying you don't... just sayin').

OK, some maths....

Let's say you have a signal which peaks at exactly 0dBFS.
You set a ratio for say 5:1.
This means that for every 5dB of dynamic range you had in your signal BEFORE comrepssion was applied, you're going to have 1dB of dynamic range AFTER compression is applied.
So if you set your THRESHOLD for -5dBFS, in theory, your highest output value after compresison will be -4dBFS.
If you set your THRESHOLD at -10dBFS with a 5:1 compression ratio, then your highest level after compression should be -8dBFS.
Think it through. You'll get there.
Now, I say "SHOULD" because it all comes back to attack time. If your attack time is 10ms, but a drum transient reached 0dBFS in 8ms, guess what? You're shutting the gate after the horse has bolted. So, does that mean we should just default to the shortest attack time. No, it doesn't. But the answer to why is for another day/thread.
Have fun!
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