How do we inspect hardware det filters?

[robrokken]

I had this idea of how to find out what the filter was like on the detector circuits of hardware or even other plugin compressors. The thought was to run a tone through it and watch for changes in gain reduction (if there was a hp or adip in the filter(eq) you would see less GR in the region. But the test has failed.

Any ideas on how maybe we can find out what the "classics" are using for detector filters? I have to test this more, maybe I'm using the wrong tone..

~Rob.

[asomers]

Most good compressors have a key input (aka sidechain). Put the compressor on a track with a constant sine tone on it. Then run a constant amplitude sine sweep from 0Hz to (SampleRate / 2) Hz through the key input. The changes, if any, in the frequency response of the input to the detector circuit will be visible as amplitude changes in in the resulting waveform (the processed constant frequency sine tone). You may also want to watch the GR level on the graphical display as you are doing, however, since other parts of the compressor may have frequency dependent characteristics. BTW the only real way to do this is to open one up and have a look inside.

[transphere]

Doing a sine sweep won't be very accurate due to the time constants of the compressor unless the sweep is sufficiently slow. If you see a change in GR you'll know the threshold was exceeded somem moments before and thus a lower frequency, but it is quite hard to determine that specific frequency.

IMHO it is more usefull to have tone bursts (low amplitude below threshold followed by high amplitude above threshold followed by low amplitude again) at discrete frequencies. In this case you can even determine frequency dependent attack/release patterns.