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Parallel Compression

When talking with sound-engineering students of all ages and experience, I often find that one area where most struggle is compression - Hugh Robjohns.

I certainly resonate with this statement, compression is something that I'm familiar enough at least to be able to effectively use it, but my understanding of how it actually works is still limited to fundamentals. A compressors basic purpose is to "squash" an audio signal and reduce it's dynamic range, meaning that there is less difference between the loudest and quietest sounds in a signal, giving the engineer more control over the output level. This is achieved by attenuating the level of a sound by a particular db range over a set threshold, so only signal that passes the threshold set by the user with be brought down in level, bringing it "closer" to the quieter sounds.

This kind of compression is referred to as downward compression because, although compression is generally perceived to make a signal louder, in all actuality the compression-induced attenuation is lowering the output. This is where “output gain”, or “make-up gain”, comes into play. You can use the output gain to “make-up” for the attenuation done by the compressor - M, Hicks.

To summarise, Downward Compression is a process used to attenuate any part of the signal that passes a set threshold and effectively squashes the sound, this of course brings down the level of all transients in a mix, which is what gives instruments like drums their bite. "If you are trying to tame drums in a mix and insert compression on every channel, you’ll likely destroy all the dynamics and end up making them sound very tiny once all the other instruments are played over top of them" - Reilly, B. This is where upwards compression comes in handy.

I like to use an upward compressor on any room-based track that isn’t quite roomy enough. Let’s take the room mics of a drum set: in the studio, perhaps the drum booth was rather small, resulting in a closed-in sound. Using an upward compressor in this scenario can allow you to bring out the ambience—the space between the transients—and get a roomier feel in your track - Nick Messitte.

Upward Compression is similar to downward compression but instead of attenuating and squashing sound, it amplifies quieter signals without affecting the louder sound, effectively leaving your transients untouched. This is perfect if you want to decrease your dynamic range without completely washing out your sound. From what I have been able to find there are a few different virtual plugins that can achieve upward compression, such as iZotope's Neutron 2, or The Curve for Reason, but so far it seems like this isn't really something that has been built into hardware processors. Now if you don't want to go out and buy yourself Neutron 2 (which is about $249AUD) or any other processor that uses upward compression, you can in fact emulate this affect with any compressor you want!

Parallel compression refers to the technique of duplicating a signal, compressing the copied signal, and then blending it back in with the uncompressed signal. Some compressors also have a ‘mix’ knob where you can blend the compressed and uncompressed signals - Matthew Weiss.

Parallel Compression is a way of emulating the effect of upward compression by splitting or duplicating a signal and only compressing one instance, leaving an untouched version that you can then mix the compressed signal into.

To demonstrate this, I will apply general compression to a drum tracking to show what ordinary downward compression does to a signal and then I will take the same tracking and apply parallel compression. For the audio examples I will isolate the snare track only, just so it's easier to hear the effect the compressor has.

Here is a snippet of the snare without any processing;

The snare transients are quite loud and prominent, it's punch and snappy. But say we added some compression to the snare on it's own. I've used quite an extreme example of compression for this but only to highlight exactly what it does to the transients.

Here is a snippet with compression on only the snare;

And here's what it looks like in Pro Tools. I have simply put a compressor on both the tracks that make up my snare. (I have already set up my bus for parallel compression for later but in this example it is bypassed and muted);

You can hear how squashed those transients are starting to get, even if we are getting a decrease in dynamic range. This might be the sound you're looking for, but let's say I wanted to bring up that room sound without messing with transients, and did so by applying parallel compression, here's what it would sound like;

You can hear an increase in the volume of all background noise and thus a reduction in dynamic range, while preserving those all important transients.

Now here's how it now looks in Pro Tools;

In this example I've split the drum tracks into a separate channel where I can apply compression independently from the individual tracks. This means I can blend in the compressed signal to my taste.

While doing this I noticed that it's hard to get an idea of what the compressed signal sounds like on it's own because you can't solo an auxiliary or a send without hearing the original tracks as well. This is due to the fact that soloing an auxiliary on it's own will essentially mute all other tracks meaning there is no signal entering the auxiliary. However this is because the default function for soloing in Pro Tools is SIP (Solo In Place), which simply mutes all other tracks than the one soloed. You can however change the Solo Mode to AFL (After Fader Level) and PFL (Pre Fader Level) as well. This Sound on Sound article explains these functions best in context of a mixing desk;

"PFL stands for Pre-Fade Listen. It allows you to monitor the channel in question's signal level at a point immediately prior to the channel fader, and will therefore include any EQ or dynamics that might have been applied on that channel. Thus when setting up a channel's input gain using PFL, it's important to bypass any EQ and dynamics processing, otherwise you won't know what the actual headroom is at the front end. On mono channels, PFL is mono. On Stereo channels PFL should be stereo, but some cheap desks derive a mono PFL signal for both mono and stereo channels.

AFL, which stands for After-Fade Listen, is similar to PFL in function, but takes its signal from a point immediately after the channel fader, showing the level of the channel's contribution to the mix. AFL is also mono on mono channels" - Hugh Robjohns.

While I'm still trying to wrap my head around exactly how these functions work in Pro Tools, it seems as though AFL solo mode allows me to listen to just the signal going in to the auxiliary without having to also solo the original signals. This makes it a lot easier to hear the compression before I start mixing it in.


Parallel Compression is a fairly easy and useful way of applying compression, however it can be implemented in a number of different ways. To bypass the soloing issue I mentioned earlier you could just simply duplicate the track you wish to apply it to in Pro Tools and then use that as your split signal for compressing, I suppose you could also use to completely separate recordings of the same guitar track for example and use one of them as your parallel compression bus, mixing it in solo to suit the main guitar track. It's definitely something that I will now be messing around with a lot more in my mixes.


Reference List;

Hugh Robjohns (2013). Parallel Compression - Sound on Sound

Mason Hicks. Audio Compression Basics - Universal Audio

Hugh Robjohns (2003). Q. What do Solo, PFL and AFL do? - Sound on Sound

Enrique Ferrer (2018). How to Use Solo Modes in Pro Tools - Obedia

Matthew Weis (2011). 2 Effective Ways to Use Parallel Compression - Pro Audio Files

Nick Messitte (2018). Expanding on Compression: 3 Overlooked Techniques for Improving Dynamic Range - iZotope


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