All about Studio Compressors

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.

Compressors at zZounds

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ART Pro VLA Compressor (Model 212) The PRO VLA (Vactrol-based Leveling Amplifier) is a multi-purpose tool for audio engineering and recording. Enclosed in a 2U (3.5" high) rack-mountable chassis are 2 independent channels of analog leveling/compression designed to work seamlessly with any recording, sound-reinforcement, or electronic instrument setup.

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 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

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 his 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. 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.

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. You can decide to avoid compression here, 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). There are many products specifically designed for this task, and some may add tubes to impart a sense of warmth. 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.

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, you almost certainly have to use one here to get the track up to spec. This can be done in the computer with a plugin, in the multi track if it has onboard compressors, or you do at at the board. 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 also need to clamp down on those pesky peaks. 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 guitar track to be boosted to near 0db, where it can, at least, be heard.

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.

The Mixdown Chain

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.

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. 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 the radio.

Hardware vs. Software Compressors

Just a new 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.

There are some great software compressors available today. As usual, the better ones will cost you. Take a look at the Universal Audio UAD-1 Studio Pak which features 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"

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

By Tweak

3 Ways to Use a Compressor in your Studio

The Recording Chain

The Tracking Chain

The Mixdown Chain

Hardware vs. Software Compressors

Questions and Answers

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