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All about Compressors

What they are, and when, where and why we use them

By Tweak


Close up of the DBX 1066, a full featured dual compressor/limiter/gate.  Note the sections "expander/gate", "compressor" "output gain" and "limiter" for each channel. Not all compressors have gates or limiters

What is a Compressor?
OK, Imagine you had a vocalist who would whisper words quietly and then belt out some loud screams.  You,  the engineer, has to record such a take. You find the whispers are too quiet unless you boost the fader and the screams require you to quickly lower the fader so you don't overload the recorder. That is the time-honored technique called "riding the gain" (gain is another word for volume).  A compressor is an audio circuit that automatically rides the gain. It pumps up the volume when things get quiet and slams down the peaks when they get loud. The goal of the compressor is to achieve a more uniform, more consistent audio signal that is optimum for recording and listening. 
Q: I noticed some compressors have gates and others do not.  What is a gate?
A gate is a switch that turns off the audio when it goes below a defined threshold. It's great for removing room noise, and makes the recording dead silent when it kicks in.  It's a valuable feature and if you want it, double check the specs before you buy.  
What is a sidechain?
A sidechain is an output and input back into a compressor.  You can connect other signal processors to it.  The connected processor only gets the signal when the compressor is working.  Sidechains are often used with eqs to make a de-esser, which will soften the loud "SSS" and "PP" sounds from a vocalist when they exceed the compressors threshold.  But you can connect delays, reverb anything you want to a sidechain for unusual, program  loudness dependent effects. It is not needed for typical compressor operations, so many manufacturers leave these off.


Universal Audio UAD2 Solo Flexi DSP Card (Macintosh and Windows)
The UAD-2 SOLO Flexi DSP Accelerator Card gives producers and engineers a simple, powerful entry point into the award-winning UAD Powered Plug-Ins platform on Mac or PC -- plus a $500 plug-in voucher to build your own personalized bundle. This single-processor PCIe card provides access to the world's most authentic analog hardware emulations and audio processing plug-ins, including licensed emulations from Neve, Roland, BOSS, Empirical Labs, EMT, Fairchild, Harrison, Helios, Little Labs, Pultec, SPL, Teletronix and more.



Compressors at zZounds

See the Compressor Price List

  ART Pro VLAII 2-Channel Compressor  The ART Pro-VLA II is a tube driven Vactrol-based Compressor and Leveling amplifier designed to excel in any professional audio application, including tracking, mixing, mastering, live sound or broadcast situations.

Tweak:  This is a tube compressor.  It uses tubes to warm up the sound.  Nice meters.

PreSonus ACP88 Eight-Channel Compressor/Gate  Stemming from their popular ACP-8 technology, the ACP88 comprises 8 channels of compression, limiting, and noise gating for a variety of applications. The ACP88 is the highly anticipated replacement for the ACP-8. 

Tweak: Who needs 8 compressors? Those who want to mix off of an analog console, without going through a computer.  Or those who want a compressor on each of their 8 busses that will catch the signal before it gets to the computer. 

Empirical Labs EL8X Distressor Compressor

Empirical Labs EL8 Distressor Compressor
The Distressor electronically controls the volume of just about any source in a very pleasing, and "musical" manner - adding fullness, intelligibility, and especially in the Distressor's case - excitement

Tweak:  A guitar pedal compressor is in a different league than a  studio compressor.  The impedance is at instrument, rather than line level, the controls are simpler, noise is more apparent and it will usually survive a good kick.

Avalon VT747SP Class A Tube Stereo Compressor/Equalizer
The Avalon VT-747SP combines a creative STEREO tube-discrete Class A spectral-opto-compressor with a musical six band program equalizer, L-R output level and gain reduction metering and internal regulated power supplies in a 2U space. Ideal for high performance DAW input signal conditioning, stereo buss compression-EQ, stereo keyboards and analog mastering applications.

You have no doubt heard people talking about compressors and recording.  Perhaps you heard of albums or tracks being "compressed" to make the sound better.  You may have also heard about audiophile albums boasting that "no compression" was used as a positive thing.  Huh? What gives here? We'll get to that and many other issues in this article, designed to make you fully conversant about the compression process and where to use and how much to use and when not to use.  We'll end up with a discussion of software vs. hardware compressors and when software is appropriate.

Compression, ideally, is an "invisible" sort of effect that can bring your audio material up to spec with professional recordings.  Most audio professionals do use compressors in every piece and sometimes on nearly every track in every piece. 

And sometimes compressors are overused.  Ever listen to a radio broadcast talk show and notice that when no one is talking you hear noise and hiss coming through until someone talks?  That's a compressor doing that.  Radio stations, especially those with weak transmitters, pump the gain so they can get every ounce of volume out of FM radio's limited bandwidth. They know that the loudest channel will attract and keep more listeners than the ones at lower levels.

And the same is somewhat true of the music we buy and listen to.  Top 40 music is always compressed, polished and buffed so when it comes across the radio or TV, even on tiny speakers, it's fully listenable and accessible.


 3 Ways to Use a Compressor in your Studio

There are 3 places in the audio chain where compression can be used to enhance your work of art.  They are the recording chain, the tracking chain and finally the mixdown chain. We'll spend a little time on each one.

The Recording Chain

Here the compressor is put on a direct out or insert of the mixer which takes the microphone signal after it is boosted by the preamp. Other methods are to place the compressor "in between" a mic preamp and an audio interface, or on the inserts of an audio interface or preamp.


An important tip for those going mixerless: Before you buy an audio interface with built-in preamps consider if you will ever want to add a compressor. If you think you will, make sure the audio interface has inserts or sends. If it does not, there will be no way to add a compressor to the rig without buying yet another preamp. Those buying a mixer should take care that the mixer has direct outs or inserts, though you can get by using the alt 3-4 bus. To understand inserts, direct outs and alt 3-4 busses see the mixer class.


The purpose here is to optimize the material for the recorder.  You want to make sure all low volume passages actually do have a strong enough level where they won't bring in noise later, and you also want to stop and loud "peaks" from overloading the recorder's input, which will ruin the track.  That is compressor theory 101.

However, there is a strong bias among those recording to computer sequencers not to record with with compression, but to record at 24 bits.  The idea is that 24 bit audio offers such a significantly lower noise floor it is best to simply record at at full dynamics (louds and softs) at a level so low that the highest peak will never approach 0db fs.   When you have the audio recorded as pristinely as possible, then you apply compression in the digital domain, usually, with a plugin. 

Even recording to analog tape, or 16 bit files, you can decide to avoid compression while recording, if you are good at riding the gain or you have performers that understand how to position themselves with the mic. (That is, they back off a few feet before letting out the loud, and eat the mic when they whisper).  However, the more out-of-control your performers are, the more likely you will need compression as you record.  Its also true that some people like to record through compressors because they want to work that way.  Finally, if you are recording live audio direct to a 2 track stereo feed, say, for live TV, you may simply have to have a whole lot of compressors working for you, particularly on the vocal channels. 

There are many products specifically designed for the task of compression.  If you see a mic preamp on a single channel compressor, these are designed for this part of the chain. Sometimes these are called vocal compressors.  But like any other gear, you can use it for other uses too, such as guitars, acoustic instruments, etc.

 ART Pro VLAII 2-Channel Compressor


The Tracking Chain

Once you have your audio tracks recorded on your computer or multi-track, you will be in the process of tweaking each track to make it sound the best it can, in reference to all the other tracks.  Here the compressor is added as an insert on a mixer.  That is, the signal goes out of the fader, goes through the compressor, then goes back to the fader's channel.  If you recorded your vocals and acoustic instruments without compression, and you are mixing on an analog board, you almost certainly have to use one here to get the track up to spec.  This can be done in the computer sequencer's mixer with a plugin, in the multi track if it has onboard compressors, or you do at at an analog board on inserts or busses.  No matter how you mix, the idea is to get the tracks uniform, so you don't have instruments or vocals suddenly dropping out because they went soft on you.  

You may also need to clamp down on those pesky peaks. Compression helps.  If you have a single guitar note that peaks 15 db higher than the rest of the material, for example, your whole track will have to be mixed 15db down which will definitely put it in the background. The compressor, by clamping down on that peak, allows the whole guitar track to be boosted higher in the mix, where it can, at least, be heard.

A classic compressor such as the UA LA2A is a nice choice for a vocal track.  it helps keep the vocal above the band in a very pleasing way.  But let me tell you you won't be finding too many of these at your local pawn shop. That's to software modeling you can have an authentic replica of the LA2A on your sequencer track. Or you can get it in hardware

Universal Audio LA2A Classic Leveling Amplifier
Universal Audio now announces the rebirth of the Teletronix LA-2A, a Universal Audio Classics product. Painstaking care has been taken to ensure that every new LA-2A provides the performance and characteristics of the original. Each unit is hand built, each component carefully evaluated for authenticity. No expense has been spared to guarantee that this LA-2A will bring that classic sound to your recording. Demand the original. Accept no copy.


Compressors can also be used as effects in their own right on drum tracks.  Drums are "peaky" by nature and by clamping down on the peaks, you can make the drums louder and fuller sounding.  If you have ever heard any strong rock drums on the radio, you are hearing drums squashed down with compression and then boosted with volume.  Drums without compression cannot hold up next to screaming vocals and distorted guitars.  The same is true even for light jazz, where the engineer might only compress enough to tame the peaks, but not affect the transparency of the audio. 

SM Pro Audio TB202 2-Channel Tube Microphone Preamp/Compressor


The Mixdown Chain

In the mix, a variety of compression techniques may be used.  Compressors can be put on busses or even on sends and returns to affect (and effect!) certain parts of the mix. 

An advanced mix technique is often called Parallel Compression, where the uncompressed source tracks are mixed in with the compressed signal coming back on a return or on a bus.  The advantage here is that the compressor fattens the overall sound yet the peaks (which come from the source signal) remain clear and "on top" of the compressed signal.  Parallel compression can work for drums and vocals, or anything really.  It can also be done with groups of tracks.

But sometimes there is a temptation to put the compressor on the master bus (the main outs) particularly among newbies.

OK, a compressor may be added here too, and can have a dramatic affect, for better or worse.  Some professionals advise against using compression here.  Particularly if you are sending the mix to a mastering house for cd replication, let them use their gear.  However, if this is a home cd production, you will have to master it yourself.  But again, more cautions.  See if your mastering software has any software tools for the finalizing task.  Mix to wave without compression and use a mastering processor there.  But if you are mixing down direct to a cd recorder or DAT and this is the last stop, then go ahead, compress the mix. If done properly, the whole thing will come out louder and stronger.

There are some exotic compressors like the Fairchild, which has been modeled by UAD and Waves, that is designed to be strapped on a 2 channel mix.  These impart a character on the whole mix in a pleasing way.  Universal Audio has released a hardware replica of their famous 1176 Limiting amplifier.   You can also get an 1176 in software in the UAD2 system.


Universal Audio 2-1176 Twin Vintage Limiting Amplifier
Finally, a true stereo 1176! With over 2000 units already sold to date since the year 2000 re-birth of Universal Audio's 1176LN, it is clear the public's love affair with the legendary sound of the 1176 is still as passionate as ever.


Dynamic Range at Mixdown

This is a good thing, right?  Not if you want to be loud.  Dynamic range is the difference between the softest passage and the loudest passage in a song.  Compression shrinks dynamic range.  it makes the soft part louder and the loud part the loudest it can be.  So.   Got to be Loud?  It's at this point where you would consider multi-band compressors, like the TC electronics Finalizer and brick wall limiters, like the Waves L2, or the UAD Precision limiter. These will let you use every bit of space in the audio bandwidth and you will be able to maintain consistent loudness.  Because those writing top 40 hits all seem to do this, you may need to go this route if that's your bag.  Want to be soft and loud? You might consider not using compression or just extremely light limiting at all at this stage and preserve the dynamics of the material.  Orchestral and ambient works benefit from this approach as it makes for great dramatic passages when the orchestra does get loud. This is where some producers boast, "no compression was used".  Of course, they are not aiming to get played on car radios around the globe.  

TC Electronic C400XL Dual Gate Compressor

Tweak:  A compressor with presets?  You bet.  For those that don't want to learn how to tweak a compressor, the TC C400XL offers presets to get you in the right ballpark. 


Hardware vs. Software Compressors

Just a few notes here. Most modern sequencers have software compressors these days. These come in many different styles and types and many of them sound quite good. However, these are mainly for post-recording. You apply them to an audio track or soft synth as a plugin, after the recording has been made. Software compressors do not help as you record, so they cannot limit the peaks coming off the microphone through the preamp and into the converter. Hardware compressors, on the other hand, when setup correctly, modify the signal before it is recorded, thus preventing the overloads that can ruin a take. If you don't want to use a hardware compressor here you simply have to be careful about overloads. With 24 bit recording you can record at a lower level to avoid overloads, however, it is a great idea to have the protection of a hardware compressor all the same.


The UAD-1 Fairchild 670 Software Compressor is modeled after the famous Fairchild compressor designed for compressing the signal for vinyl LP.  You can use it in your software sequencer and it can impart an interesting sheen over the mix.   It is one of my favorites.


There are some great software compressors available today. As usual, the better ones will cost you. Take a look at the Universal Audio card which allows you to run software models of the classic 1176LN, LA-2A, Pultec EQP-1A, and Fairchild 670 hardware devices. Also check out the Waves Platinum compressors. But as always, use what you have first, get as much as you can out of them, and then consider upgrading.  One of the more unique compressor bundles you can get today is the Focusrite Liquid Mix which is a combination of software and hardware DSP in a unified control surface.  The package includes 20 different EQs and 40 classic compressors you can use right inside your sequencer, yet it uses a firewire connected DSP for the horsepower. 

OK, you should be up to speed on what a compressor is and why, how and where they are used. I suggest continuing your reading with the article on "The Perfect Mix", and "The Art of Recording Vocals"


Focusrite Liquid Mix FireWire Mix Processor

large product image

Each of Liquid Mix’s 32 channels provides EQ and Compressor emulations selected from a huge pool of high-quality vintage and modern day classics. 20 EQs and 40 compressors

Tweak:  Check out the 60 machines they modeled


Questions and Answers

Q) Tweak!  "Everyone" says hardware compressors are no longer needed!  What's your take on this?

A) These are guys who record in the box to their computers and stay in the box or in the digital realm.  However, "everyone" does not mix in the box.  Ok here we go, here's the...

Top Ten Reasons to Use a Compressor

1. You are recording on reel to reel
2. You are recording on cassette
3. You are using bare-boned 16 bit multi track recorders
4. You work on live shows
5. You are  a Hardware tweak who chains up devices in the hardware realm to make effects
6. Guitarists rigs that need a gate and a control for uncontrollable peaks
7. You use large analog boards to mixdown from Daw or Multi-track and want hardware compression on groups.
8. You like to do analog bounces from mixer to DAW who want to shade the bounce with compression.
9. You record wild sources with unpredictable levels.
10. You want to fatten, flatten and gate a signal in the analog domain--i.e., a snare or kick in an analog mix. Vocals too.

If you are recording (and staying) in the box with Cubase, Sonar, Logic, DP, PTLE you really don't need a hardware compressor (but hey, you still might want one).  That is what these guys are talking about.  People who mix on an analog board need multiple compressors just as people who mix in the box need multiple software compressors.



Cool links

Understanding Compressors and Compression  by Barry Rudolph

How does Parallel Compression work?  SOS March 2009

Great Threads


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