A Comprehensive Guide to Parallel Compression (…and Other Forms of Parallel Processing)

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What is Parallel Compression?

“Parallel Processing” is a term that can describe a number of techniques engineers use in pursuit of punchy, relentless backbeats, rock-solid bass-lines, and vocals that speak with power and nuance.

At its most basic level, this family of techniques is at play whenever we duplicate an audio feed, and treat each copy of that feed differently.

The parallel processing approach: another path to the sounds we hear in our minds’ ear.

Say the word “parallel” to an audio engineer or music producer, and the first effect that comes to mind will likely be parallel compression.

When applied to a drum bus (often with a healthy dose of EQ), this technique carries the nickname “New York-style Compression.”

Properly executed, parallel compression on a drum bus can help us achieve a consistent minimum audio level while retaining some organic dynamic range, allowing us to obtain powerful, up-front sounds that still maintain clear and natural transients.

But the game doesn’t end there: For decades, bold engineers have turned to an endless variety of parallel effects wherever they need to sculpt new tones that merge the best features of two or more sounds. Compression, limiting, distortion, even parallel EQ can be used on a wide array of instruments to great effect.

There are applications here for not just drums, but vocals, bass, keyboards and more.

HOW DO I DO IT?

In a broad sense, the way mix engineers traditionally route signal to a reverb is a form of parallel processing: The audio signal effectively gets split in two, with some of the sound running through the sonic spin cycle before it’s blended back into the mix underneath our unadulterated “dry” sound.

In essence, we apply dramatic effects to a duplicate of a sound, and leave the original relatively untouched.

Unlike reverbs or delays (which are traditionally fed through an aux send) instruments earmarked for other types of parallel processing are most commonly routed via buses, patchbay “mults”, or by simply duplicating a track within a DAW.

However you chose to route and duplicate your sounds, the concept behind all parallel effects is basically the same: fearlessly crush, smash, carve, distort, or sonically sculpt your duplicate. Then, fold this hot twisted nugget of audio back into the mix alongside your original sound.

PARALLEL COMPRESSED DRUMS

For the iconic sound of “New York-style” compression on drums, get a good blend of drums together for your main drum bus. Then, simply route all your drums through a second bus, squeeze them to taste, and fold them back into the main mix alongside your clean tracks.

The goal here is to make sure your parallel drum mix is compressed to the point where it has very little dynamic range left… Don’t be afraid to make it sound over-the-top by itself!

Many mix engineers who use this technique will add big boosts in the lows and highs along with these hefty gobs of compression. Shelving boosts as extreme as 6-10dB as low as 100Hz and as high as 10kHz are not unheard of.

For this “New York” compression technique—and many of our parallel compression techniques—a very fast attack time often works best. Don’t be afraid to chop off some of the transients and create a big, consistently thumping backbeat. You’ll get some of your transients back when you blend this parallel compressed track with the original sound.

Slow attack times are much less common with this any many other parallel compression techniques. If you use slow attack times with heavy parallel compression, you’ll actually end up exaggerating your transients, and even exaggerating dynamic inconstancies in the performance. This is usually not what we are looking for with parallel compression.

Faster release times that pump and breathe with the rhythm of the music are fair game. Though longer release times that create a smooth and consistent decay aren’t unheard of either.

Depending on your tastes, the parallel compressed drums may sound a bit dramatic by themselves—but that’s okay. When added to your original mix, this parallel bus acts as a supporting element, allowing you to bring up the beef and average level of your drums while preserving the original transients and much of the larger dynamic changes.

A similar approach can be applied to individual drums as well. Snare tracks are an especially common recipient of the parallel treatment. To bring more attack and consistency out of a snare drum or its corresponding reverb send, it can be fun to crush, EQ, and perhaps gate the original sound in parallel. Feel free to go for an unusual, even unnatural drum-machine-like sound to add impact and excitement to your clean snare.

But be careful, and make sure to maintain perspective! These techniques can be a quick path to big, punchy and relentless drums. It’s easy to get drawn into the addictive process of creating ever more exciting sounds. But sometimes a little subtlety is key.

PARALLEL COMPRESSED VOCALS

As with drums, many of today’s producers and engineers feel compelled to create distinctive and ever-more-present vocal sounds.

But this isn’t a contemporary phenomenon. Back in the 1960s, Motown engineers Mike McClean and Lawrence Horn were using a technique on vocals that they called “Exciting Compression.” This was effectively a parallel compression technique, similar to what we have seen with drums.

Simply put, a vocal track would be duplicated in the console. One instance of that vocal would be treated with typical EQ, compression and reverb, and the other would be squashed heavily with compression, and then brightened considerably with heavy-handed hi-shelf EQ.

A decade later, Curt Knoppel would design the first Aphex Aural Exciter, a box whose basic function would owe much of its heritage to this tactic.

The beauty of parallel compression on vocals is that it allows us to add clarity, detail and dynamic control to the vocal, but without adding sibilance.

There’s an unfortunate set of tradeoffs with traditional compression on vocals: Compress them too little and you they may never sit right in the mix. Compress them enough with fast attack compression and you can sometimes lose detail, life and intelligibility. But compress them too much with slow attack compression and the “sibilance” (your S and T sounds) will jump out of the speakers.

A similar thing happens with EQ: Don’t brighten your vocals and they may not sit right. But if you do brighten them enough, those nasty sibilants can jump out once again.

The beauty of parallel compression and EQ on vocals is that it can help overcome these tradeoffs. You can use a super fast attack, and then bring back the life, detail and clarity with brightening EQ. This is like having a separate “brightness” fader for your vocals, but you are able to add that brightness without exaggerated sibilance. If you haven’t tried it yet, you simply must!

In addition to parallel compression, parallel distortion is a common trick for vocals.

Often, applying distortion directly to a lead vocal can leave the track sounding thin, harsh, undecipherable and over-processed. Add some distortion to it in parallel however, and your have a gritty vocal that maintains much of its original body and natural growl.

BASS

A similar approach is common on bass guitar.

Sometimes, a touch of distortion is just the thing to keep the instrument articulate, defined, and audible throughout a mix. But it can also mess with your low-end definition. Adding some grit and gain in parallel however, can help ensure you don’t lose too much bottom-end power and clarity in the process.

Some mixers are known for taking a more complex approach to parallel bass, creating a blend from 3 or more parallel treatments. One channel provides solid low end (usually the most heavily compressed), one delivers a nice midrange growl, while the third channel is voiced to maximize the instrument’s upper midrange snap and articulation.

While such drastic measures may seem like overkill to some of us, bass guitar is one of those instruments that almost always benefits from at least basic parallel compression or distortion to help it gain a solid foothold in the song.

PIANO, STRINGS, ORCHESTRA, EVEN MIX BUS

The techniques discussed above work on all sorts of instruments. Almost the same exact approach mentioned for vocals works wonders on pianos for instance—often coupled with some additional low cut on the parallel track.

This allows you to blend in a bright, tightly controlled version of the piano that “cuts through” the mix in dense sections, and back off on it in sparser sections that ask for a more modest and natural piano sound.

It’s impossible to say who first made use of parallel compression in the mix process, but Motown engineer Bob Olhsson maintains that his crew stole the trick from classical engineers at DGG records who used the technique to combat the high noise floors and limited dynamic range of early recording technology.

Although parallel compression can be used to dramatic effect, it can also be a suitable approach for the audio purist. Once they’re mixed in with a full-spectrum sound, parallel tracks can have the effect of bringing up the quietest portions of the signal unobtrusively.

No discussion of parallel styles is complete without a mention of Michael Brauer, a New York-based mixer who is often associated with the term.

However, contrary to popular references to his work, the key feature of the Brauer style is in the use of multiple bus compressors for different sound sources, and not in the use of parallel treatments as we’ve defined them here. Although parallel compression may result from this style, it doesn’t paint the overall picture. But that’s enough ammunition for another post.

WHY BLEND IT?

Parallel processing isn’t just a technique that allows us to combine sounds. It can also help us find workarounds to the compromises multifaceted arrangements can impose upon us.

Alex Newport described this approach well in our recent May interview:

“The obvious example is on a vocal. Someone’s singing in the verse – you set your EQ, compression and effects, and everything sounds great. Then, the chorus hits, they start belting, and all of a sudden you’ve got a nasty 2kHz buildup. You could find an EQ setting somewhere in between as a compromise, but that’s not really acceptable.

“So, the better choice is to mult [the vocal] to two channels and develop a sound for each section. At some point I figured, there’s no reason you couldn’t do that with a kick drum or an effects return. Those moves help create these subtle dynamics that really make the song come alive.”

In the end, parallel processing is a general approach, rather than any one specific tactic. However you chose to use it, the parallel approach is just another option in the professional’s toolkit. Once practiced, it affords us yet another path we can use to chase after the sounds we hear in our minds’ ear.

Justin Colletti is a mastering engineer who writes and talks about music and how we make it. Visit him at http://www.justincolletti.com.

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This article was originally published June 23rd, 2011. We have updated and republished it to help our newer subscribers.

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