A customer contacted me inquiring how to reproduce a multi-band compression preset using MultiDynamics 6. The customer was using Steinberg MultiCompressor and wasn't sure how to map the parameters from MultiCompressor to MultiDynamics. He provided this image:
Setting up four corresponding bands with the proper edge frequencies in MD6 is trivially done, but the problem is how to map the compression characteristics in each band, and also what to do with the band gains. Admittedly, MultiDynamics is a bit confusing as it takes a slightly non-standard approach to setting up compression (or expansion) characteristics. But it's not that difficult to map from traditional compressor parameters.
Let's quickly review the standard compressor characteristic curve:
Compression starts when the input signal exceeds the threshold level. As the signal level increases above threshold the compressor reduces the gain. The ratio defines the slope of the characteristic curve, but it's actually the inverse of the line slope we were taught in high school algebra. A ratio of infinity means the compression line is horizontal (brickwall limiter), and a ratio of 1 means no compression. The ratio from the above diagram is dX / dY.
Without any makeup gain, as we increase the ratio there is more compression, but the output signal level decreases. Many compressors automatically add a compensating makeup gain so that a 0 dB input will give a 0 dB output. The makeup gain we need is diagrammed above.
Note also that as signal levels continue to increase beyond 0 dB, the gain reduction continues to increase, essentially the characteristic line extends to the right along a straight line.
Now lets look at MultiDynamics' characteristic. It's actually very similar, but MultiDynamics limits the total compression (or expansion) gain between a "Lo gain" (the gain below threshold) and a "Hi gain" (the gain above threshold). The ratio and threshold are the same, but the gain characteristic doesn't continue on a straight line as with a traditional compressor.
Here we've added the makeup gain so that fullscale inputs yield fullscale outputs. Hence the Lo gain corresponds to the makeup gain in the first diagram and the Hi gain is 0 dB. Note also that the MultiDynamics user interface plots the above characteristic rotated by 45 degrees so that the y=x line is horizontal.
OK, now let's get back to the MultiCompressor preset and focus on the first low frequency band. From the characteristic plot we see the threshold is -16.6 dB and the ratio is 16.6 / 1.2 = 13.8. The makeup gain is 15.4 dB. So we would map this to MultiDynamics parameters of Thresh = -16.6, Lo gain = 15.4, Hi gain = 0, Ratio = 13.8. But there is also the band gain of -5.3 dB and the output gain of -6 dB. We add these gains to each band's lo and hi gains. In this case we add -11.3 dB giving lo gain = 4.1 dB and hi gain = -11.3 dB. It would be simpler to add only the band gains, but for this particular preset there is a ton of band gain in the midrange band and so I needed to subtract the 6 dB from each band to get the parameters to work with MultiDynamics' limit of +/-18 dB gains.
The final MultiDynamics preset is shown below (note the band lo gain is 5.1 but I screwed up and it should be 4.1):
So how does it sound? Well it had a problem that it was clipping. For some reason this preset yields more gain than the original MultiCompressor preset, about 4 dB too much, not sure why. And the MultiCompressor includes a peak limiter which is not present in MD6. So I tweaked the above output gain by -4 dB, and also added FinalPlug 6 as a peak limiter with the default preset. This combination sounded very similar to the original MultiCompressor preset and the customer was happy.
Hopefully that helps clarify how one can port traditional compressor settings to MultiDynamics.