A customer writes wondering how to measure a compressor transfer response for compressor output versus input response. That measurement can be made with the AMP Gain and Distortion plug-in, shown in the plug-in menu here:
The settings on that plug-in look as follows:
Note that the “Plot as Gain” is disabled. That will let you see output versus input. The resulting plot appears as follows (this measurement was made on a guitar pedal compressor):
We can adjust the knob on the compressor to “max sustain” (it was at 12 o’clock position) and add that curve to the graph too by re-running the plug-in an opting to add it to the existing graph. With a bit more clean-up on trace names and colors, we get the following graph created a minute or two of measurements and tweaking colors/axis/titles/etc:
From the plot above, we can precisely see what the compressor is doing as the sustain knob is swept. At max sustain, the compressor is acting linearly up until about -50 dBV input, and which point it begins to clamp down rather hard on the output level. Similar plots could be generated for the other knob settings.
You can look in the time domain at the attack characteristics. For the plot below, the output of the QA401 was split: One went into the guitar pedal, the other went into the right channel. In the plot below, you can see the red (input to compressor) and the yellow (output of compressors) in the time domain. As the QA401 output ramps, the compressor allows it to grow for about 6-8 mS, and then the compressor clamps down hard on the output and holds it there for the duration.
