DC Transfer function problem...

[Tim Cockram]

No your not being stupid, good compressor designs have been some thing that many pro manufacturers have a problem with ( no I won't say who but some 
"pro" compressor/limiters are not fit to grace a $25 cassette machine).

The control side-chain can either be feed forward or feed back (dbx tend to use feed forward most others use feed back).  The feed forward loop has some 
advantages but requires more thought to get it working properly (it does have lots of scope for wave distortion though !).

What you basically need is a "modulus" device i.e. a full wave or half wave rectifier (some companies just use diodes)/comparator/rms detector as a front 
end.  The choice depends on what you want to do eg peak/average/rms control.  This is followed by a dead zone circuit (rectifier with offset) to set the 
threshold (thecomparator frontend includes this function). Add an AD time constant and finally a variable gain stage to set the ratio.

You can get loads of information from the Analog Devices texts:- Non linear circuits and the Audio Video reference manual (application notes section).
There are also some interesting patents out there, try searches for Aphex, dbx and Orban.

Perhaps the trickiest bit is the time constant section as a simple AD network can have a number of operational problems.  The worst or best? is that fast
time constants will cause distortion of the waveforms  (if the time constants are say 1mS and the input signal is 100 Hz the wave will be distorted).  There are 
many methods of over coming this From multi-time constants to multiband systems.

If you need more info I can send you a few circuits.

Tim