Measuring Compressor Response

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.

Hi Matt,

thanks for the quick reply!

Is it also possible to edit the vertical and horizontal scales to other units such as Vpp, Vrms, dBu? Or any custom dBreference?

Does the generator also provides those units?

Hi @AudioGarden, the scales are almost always fixed in dB. The plots you see above with white background are automated measurements and if you are measuring THD(N) then the left axis will shown dB and the right axis will show %.

In the analyzer (dark background) you can switch between dBFS, dBV, dBU, and dBR. The dBFS is useful if you want to understand your measurements relative to full scale on the converter. The dBR is, of course, relative measurements. You can also switch between freq domain and time domain. But the time domain shows things relative to dBFS. In other words, the time domain is primarily for sanity checking what you are seeing in the frequency domain.

In the measurements shown on the top of the display, those are most all in dB (as specified by user). The one exception is the RMS measurement. You can opt to display that as a linear measurement (eg 0 dBV displayed normally, but 1 Vrms displayed if you select linear). And the peaks can be displayed in watts if you specify a load impedance. THD(N) are always shown as dB and % together (no way to turn one off).

The generator voltage level is specified in dB always.Let me know if you have other questions or would like to see other measurements.