Parallel Compression – what’s really going on?

By Giles Reaves

There is a lot of talk about parallel compression these days, and what it does that cannot be achieved any other way. Lets look at the basics of this effect to see what’s actually going on under the hood. Check the quiz below before watching the video!

What is parallel compression?

Parallel compression, aka Detroit/Motown Compression, aka New York Compression, is simply splitting the audio into two channels and compressing one of them and not the other. Folks give many reasons for using this technique over putting the compressor in an insert; from the idea that it’s a form of upwards compression, to the concept that it somehow preserves the transients, etc. But before we continue, a quick quiz based on information and claims I’ve heard made about parallel compression over the years.

There is a simple way to test all of these theories, that if true will reveal the fundamental nature of parallel compression in its simplest form. I’ve heard all of these claims at one time or another, but the one that seemed the most difficult to believe was that parallel compression was simply a different way to affect the compression ratio. Turns out there’s an easy way to test this.

The setup

Here is the setup I created to test the “ratio” theory. It involves comparing a simple insert compression setup with a parallel compression setup. Here’s what it looks like in Reason:

We start with a simple audio source, one that everyone has access to: the old “College Drums” REX loops in the Reason Factory Sound Bank. We split the output of the Dr. Octo Rex into two Mix Channels. One of those mix channels has an MClass Compressor in its insert. This is the traditional “insert” compression setup (on the left in the image above). For the second Mix Channel we create a Parallel Channel and put the MClass compressor in its insert (on the right in the image above). Then we lower the fader of the parallel channels by 6 dB to account for the fact that a parallel channel adds roughly 6 dB to the overall signal (6.0206 to be exact). This will allow us to compare the two paths at equal levels. Finally, we bus the two parallel channels to a Mix Bus Channel so we can mute/solo them “as one.” So far so good. Now for the fun part. If we bypass both compression inserts we can then invert the polarity of one of these two paths and we should see an almost perfect null (cancellation). Why is it not a totally perfect null? Because we don’t have the fader resolution to achieve this – it requires a very precise 6.0206 dB setting to perfectly account for the doubling of the number of audio channels, and Reason doesn’t allow this much resolution. But for the purposes of our test, we only need to compare the “before” and “after” to see if there’s a difference. Read on to see how. When I compare the null with compression bypassed, I see peaks around – 66 dBFS on the Big Meter. This is our base line for further comparisons. If the “ratio theory” is correct, I should be able to find a setting on both compressor’s Ratio knobs that roughly equals this bypassed null – with me so far?

The result

So I’ll cut to the chase here – when I did this test initially I found that if the parallel channel’s compression ratio was Infinity to one (∞:1) and the insert channel’s compression ratio was 2:1 (or as close as the MClass would allow, 1.99:1), I would see the same null as with both compressors bypassed. Wow, mind blown. Was it really that simple? Was parallel compression really the same thing as “ratio”? Turns out, yes. Are there any exceptions? YES! In practice, most folks add EQ or saturation or similar when using parallel compression, and in that case there really is a difference and an advantage to using these effects in parallel. But for simple cases such as above, when using parallel compression, all you are affecting is the compression ratio. Does this null test work with all compressors? No, because some compressors affect the sound differently. The Softube FET compressor is one example. In the same setup as above, it will null when bypassed. But even with ratio at 1:1 on BOTH compressors, it will not null due to subtle saturation and possibly other behind the scene’s ‘magic’ going on. In contrast, the MClass will null in the above setup if you set both ratios to 1:1, the same as it does in bypass or with the settings I outlined above. There are also compressors that use auto-makeup gain which is often tied to multiple controls, including ratio. This makes it impossible to create a null test with those devices, but does not negate the earlier conclusions that at the heart of parallel compression, it’s the ratio that’s being affected.

 

So why ever use parallel compression on it’s own when adjusting the ratio achieves the same thing? One obvious reason would be when using a compressor that has no ratio control, such as an LA-2a or clone thereof. Another may be in cases where your compressor only gives you a few preset ratios such as a hardware 1176.

The answer

So my conclusions are that at a basic level, parallel compression is not another form of upwards compression. Nor is there any evidence parallel compression preserves the transients of your audio signal any more than using an insert compressor. If either of these were happening, there could be no null in the above tests. So what does that mean for our quiz?

Answer: for the sake of this discussion, the correct answer is “C”. If any of the other choices were true, we would not be able to null in the above tests.

As always, I’m open to suggestions as to how to improve this test or reasons why my test is not valid – let the conversation begin! Did you answer the quiz correctly?

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