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What Is Parallel Compression And How To Use It

A few weeks on back on the blog, I wrote about compressors and the basics of using them in the studio. We’re taking it a step further today by talking about a more advanced function of the compressor called parallel compression and some scenarios where the technique is most commonly used.

Parallel compression uses a send and return setup similar to how you would send signal to an effects processor. It is the combination of the dry signal mixed with a compressed version. In a parallel setup, you can use heavier compression to pull more depth out of the source signal. Since you still have the dry signal in the mix, you don’t hear as much direct compression.

The art of parallel compression plays a large part in how modern mixes sound so full and loud. While some argue that it isn't necessarily a good thing, it’s not necessarily meant to be used for all genres of music. Compression crushes a lot of dynamics when overdone, but to make a song the loudest it can be without blowing up speakers, it needs to be heavily compressed.

If you’re an engineer that works on multiple genres of music and want to compete in the current market, standard and parallel compression is something you must have a good grasp on or you will be left in the dust. If you use parallel processing in the right way, you can still achieve a nice dynamic mix that is really loud, as well as reduce the amount of overall compression on your master mix bus.

Watch our new video for a brief overview on parallel compression and continue reading to learn more about different techniques for using it in the studio. 

In a parallel setup, the signal is often compressed much heavier than you normally would as an insert on the channel. As an insert, average compression would range from -3dB to around -10dB before smashing all dynamics. In a parallel setup, compression can range from -3dB all the way to -20dB and above.

Let’s go over how to set up Parallel Compression in an analog and digital environment.

Hardware Setup
If you are using a large format console that utilizes auxiliary sends or bus outputs, setup is a little lengthier, but pretty simple if you understand basic functions of a patch bay. For a more in-depth explanation of how to use a patch bay, check out our blog “What is a Patch Bay?”

You start out with the dry signal channel. For this example, let’s say we are sending a dry vocal from channel one on a console and setting up parallel compression on channel two. You’ll want to think of parallel compression in a send and return setup, which means you are “sending” a copy of the signal from the dry channel, and “returning” that copy to another channel on the desk.

If we are using a bus send, you choose the destination from the routing matrix on your desk, let’s say we are using bus one on the console. A bus send will send a full volume copy of signal from the desk, compared to an aux that allows you to dial in the right amount of send. You would then patch out of the bus one patch point on the patch bay to the input of the compressor you are looking to use. Then patch from the output of the compressor back into the line input of channel two on the desk.

If using an auxiliary send, the setup is similar. For this example, let’s say we are using aux one on the dry channel. You first have to set up the patch for the send and return configuration, which would be the output of aux one going into the input of your compressor. You would then patch out of the compressor back into the line input of channel two on your console. You now have control of how much signal is being sent to the compressor through the auxiliary one knob on the desk. For this to work similar to a bus send, set the aux send to unity gain. Take note: Every console is slightly different in how the bus outputs and auxiliary sends function, this is the most common setup on large format consoles.

Now you have the uncompressed dry signal on channel one and a compressed version of the signal on channel two. This allows the dry signal to be unaffected by any compression, which will help stay true to the original recording and stand out in a mix. You can now blend in a compressed version of the signal, which will add overall volume to the source, as well as depth in the low end and midrange. If you choose, you can still use a small amount of compression on the dry channel to tame any harsh transients that may still be coming through from the tracking process.

DAW Setup
Just like the hardware setup, we start with the dry channel. Assign an internal bus output from the channel sends, for this example we’ll use bus one. Set the sends output to unity gain in pre-fader mode for it to work like a console bus send, or without pre fader engaged, you can dial in any desired amount for it to work like the auxiliary send setup. Note: Not every console works in that exact setup.

We then need to create an aux channel within the DAW, to mimic the hardware setup mentioned earlier, you’ll want to create a mono aux. On the insert of the auxiliary channel, assign the compressor you wish to use, then set the input of the auxiliary channel to bus one.

Now you have the uncompressed signal on channel one, and the compressed version on channel two. Blend them together to achieve the amount of parallel compression needed to make that source stand out in the mix.

With the advancement of plug-ins over the last couple years and more people using parallel compression than ever before, software companies are starting to integrate a “Mix” knob within the plug-in. The mix knob allows you to have a mix of dry to wet signal without any external busses being used. Although it’s an easier setup and can still give you great parallel compression results, a lot of us old school engineers still prefer the classic send and return system. I think overall you have more flexibility when using the send and return setup compared to the mix parameter inside of a plug-in.

Now that we’ve gone over the setup process, let’s dive into some ways to apply this to your mixes. I’ll go over some examples on drums, vocals and sending the entire mix to “Rear Bus” Compression.

Drums
A very common use for parallel compression is on drums. 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.

Other times after you apply the compression, you turn up the make-up gain to offset the gain reduction, which will end up blowing out your mix bus in the beginning of the mix process and mess up gain staging for the rest of the song.

Using parallel compression on drums can help keep all the dynamics of the original source material but add volume, depth and power to them without crushing dynamics or adding too much make up gain that will overdrive the mix bus. Since the volume added by the parallel compression is coming from a heavily compressed signal and blended to taste rather than always at full volume, it’s not adding as much peak volume to the mix bus as make-up gain from the output of a compressor.

In the hardware world, some of the top parallel drum compressors are the Empirical Labs Distressor, Empirical Labs Fatso, Shadow Hills Mastering Compressor, Undertone Audio UnFairchild 670m, AMS Neve 33609, Universal Audio 1176, SSL Bus compressor and API 2500.

Most engineers will use multiple parallel compressors to complement different aspects or frequencies of the drums. When mixing in the box, I actually use five compressors in parallel, with each of them hitting between -3dB and -6dB. I use sends at unity gain from my summed drum bus to feed each of the parallel compressors, the input of each bus is controlled by a master fader to control the input to the compressor without having to mess with the level of the send or adjust the balance of the initial mix. One of the compressors is for attitude, one is for warmth, one for punch of the kick, snare and toms, one brings out more the room tones, and the last one adds excitement for the sizzle and sustain of the cymbals.

Having this much parallel compression tends to let the natural sound of the recording breath a little bit more and I never need to surgically EQ any of the close microphones or room sources. It always helps to have a good source recording when using this method, if the source material doesn’t sound good to begin with, this method could amplify all of the poor qualities of the recording. You may want to look into high end samples for the kick, snare and toms if that is the case, then use a blend of the live drums and samples to achieve the tone you are looking for.

Another common use for drum parallel compression is the “Kick/Snare Crush” setup. This is a compressor setup that is specifically focused on the kick and snare drum which is used to add depth, length and attack. This can be used in conjunction with the other full kit parallel processors, but most of the time you won't send this K/S crush to the group of parallel busses, you would send them straight to the mix bus. The send and return setup works the same as explained before, but this time you are sending both the dry kick and snare to a single compression channel. Very common compressors for this setup are the Empirical Labs Distressor, Universal Audio 1176, DBX 160 and Teletronix LA-2A.

The kick and snare typically happen on different beats within a bar, kick usually on one and three and the snares on two and four. Sending both of them to the same parallel processor will make the compressor respond to the rhythm of the song. The kick drum will typically be the first to trigger the compressor, once the kick is fully compressed and heading back to zero gain reduction, the snare drum will hit and re-trigger the compressor. This will make the gain reduction meter dance rhythmically to the groove of the song, which will result in a more musical and transparent form of compression rather than them working independently.

Once the desired amount of compression is set, use the channel fader to blend in the right amount of parallel processing to enhance the sound of the kick and snare. Sometimes the volume will be at unity gain if you are looking to have the kick and snare drive the mix, sometimes it will be set to -20dB or below and just used to add a little more depth and presence then there was before.

Some engineers will then EQ the parallel channel to add specific frequencies to make them cut through even more, such as 220Hz to add more depth to the snare and thud to the kick drum, or frequencies like 2kHz or 7kHz to add crack and presence to the attack of the kick and snare.

Vocals
Vocals can be one of the hardest things to make sit on top of a loud mix. Most mix engineers focus on making the band as big and loud as possible, then the vocals are the last thing to be added on top of everything. Most every other instrument in a recording has multiple microphones. Drums usually have 10 plus as well as any electronic drum layers, guitars will have two or more mics per track then layered multiple times, two channels or more of bass, multiple layers of synth, percussion, and so on. It’s not uncommon for a pop or rock track these days to have well over a hundred instrumental tracks once all the layers are in place.

Then you have the lead vocal which is one track, maybe two or three if you have some doubles or triples in certain sections of the song. Not to mention all the layers of harmonies the lead has to sit on top of. In a modern production, engineers will usually triple every layer of harmony, so if you have two part harmony happening on top of the lead vocal, that’s an additional six tracks the lead has to compete with.

I recently mixed a song that had 150+ tracks, which consisted of over 100 tracks of instruments, about 45+ tracks of vocal harmonies and a single lead vocal track. How was I going to have that single vocal track compete with 150 other tracks you say? You guessed it, parallel compression.

I added a little bit of compression on the “dry” lead vocal track. Lately, I've been loving the way the Universal Audio Rev A blue stripe compressor sounds. It can be hit really hard to control all the dynamics and never seems to have an over compressed pumping sound. It also adds a nice presence in the upper mid range that isn't harsh but can make a vocal cut through the track.

I then sent that signal through a send and return system to a couple different parallel compressors. The first one it sends to is a Waves CLA-76 on the black setting with the ‘ALL” setting engaged. This setting is commonly used to add attitude to the vocal, but also fills in a lot of depth to the low end. I was hitting the compressor with about -20dB or so of gain reduction, if you listened to it by itself, it would sound absolutely horrible, but since I was combining that with the “dry” signal, you don’t hear the actual compression happening, just the depth and vibe it’s adding to the track.

The next compressor I went to was a Universal Audio UAD-2 Fairchild 660 plug-in. This plug-in sounds great even if no compression is happening, as they did an excellent job of modeling the warmth and glow the hardware model provides from all the tubes and transformers. Since the CLA-76 was adding all the aggression and quite a bit of volume, I was using the Fairchild plug-in to add some additional warmth and the final bit of level I needed to have the lead compete with the rest of the band. The compression was hitting around -6dB and the time constant was set to three. I boosted the output level a bit more than I normally would if used as an insert, driving the output of the Fairchild plug-in adds some nice harmonic distortion that works well when blended into the dry signal.

Once I finished dialing in all the parallel settings, I added my reverb, delay and chorus effects. I used the sends from the dry track rather than the parallel tracks, and once they were all in place I had plenty of room to make the vocal sit into the track just right.

Rear Bus Compression
Rear bus compression is a technique used by a lot of today’s top mix engineers. There are a lot of different names for this technique, but this one is the most common. Some time in the 1970s or 80s, companies like Neve, API and SSL began building recording consoles to handle quadraphonic sound, which would allow you to expand the mix from two stereo speakers to four speakers, two in the front and two in the back.

The process never really caught on for the mass production of records. This was mainly due to engineers not wanting to adopt the new style, pressing plants not upgrading their equipment and average music listeners not having a quadrophonic system at their disposal. Once the idea was dead in the water, engineers had amazing consoles with all these extra features they weren't using.

A quadrophonic desk allowed you to assign a track to the main left and right speakers, which were controlled by their own designated master fader, which had patch points for insert send and return. The channel strip would also allow you to send an exact copy of the signal to the “Rear Bus” which would have been the rear left and right channels in a quadrophonic mix, which also had their own patch points for insert send and return. Most consoles allow you to assign the rear outputs to the main mix bus, some other consoles had to be modified to have all channels send to the main stereo output.

So what engineers began doing was using the main stereo bus as they always had, which was the combination of the final mixed source material. They would then send a copy of all the tracks (most of the time everything except the drums and bass) to the rear bus outputs, which was then patched into a stereo compressor or two mono compressors. It is best to use this setup in multiple mono rather than having a linked compressor.

Now you have a “dry” version of the mix as well as a “wet” version that can be compressed to taste. Typically on a rear bus compressor, you don’t want to smash dynamics too much. -3dB to -5dB tends to be the sweet spot. Turn up the rear bus compressor until it begins to fill out the mix. This method will bring all the instruments forward and much closer to the front of the mix, as well as bring out harmonic content that was unheard before. It works as a glue that holds all of the instruments together, since everything in the mix is working to trigger the compressor. If one source gets too loud, that source will be compressed by engaging the compressor, which will then bring up the softer elements.

Some great rear bus compressors are the Universal Audio 1176, Manley Vari-Mu, Fairchild 670, Spectra Sonics 610, API 2500 and Neve 2254. Rear bus compression can really damage a mix if it is overused, so make sure you practice on some demo sessions before bringing it into your everyday workflow for professional clients.

I hope this shed some light on how to use and set up a parallel compressor in your workflow. It is something that takes a bit of getting used to, often times it won’t sound magical the first couple times you implement it into your way of mixing. But just like everything else, practice makes perfect, and it is a technique I highly recommend getting very familiar with.

If you have any questions regarding the right hardware or software parallel compressors that would be the perfect fit for your workflow, be sure to contact your Audio Consultant at Vintage King by email or phone at 888.653.1184.

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