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Compression has to be one of the most confusing and elusive effects out there. It's easy to know you need it just by watching your meters, but what does each knob and button really do and how does it all work? This article should answer those questions, and will touch on the "whens" and "whys" of compression.

Let me first start by explaining the basics of dynamic ranges in recording. First, we have the noise floor. This is the lowest level, where tape hiss and electrical hum reside at. Next we have the nominal level, which is the level that is best for recording your incoming signal in order to minimize distortion and overcome the noise floor. The distance between the noise floor and the nominal level is called the signal-to-noise ratio. Next is the maximum level, which is where distortion occurs at when your incoming level reaches it. This is the highest level in the total dynamic range. Distortion is something that you definitely want to avoid unless you are versed in the skills of good tape saturation (sometimes engineers will try to slightly distort the signal by pushing it over the maximum level because this will give a stronger sound to an originally weak one. However, in digital recording, any distortion due to overpeaking is distasteful.). Now the difference between the nominal level and the maximum level is referred to as your headroom. This is your safety zone, and this is needed to account for some stray peaks here and there without hitting the maximum level. And to wrap this up...the whole thing, from noise floor to the maximum level is called the dynamic range.

Okay, lets cover how compressors work. Imagine a recording scenario where you are starting to record some tracks on your multitrack recorder. You have set a good recording level for your instrument which is at or near the nominal level, but you notice that the incoming signal occasionally jumps up into the red. That is typically going to be the nature of either the instrument, your playing, or both. So, you don't want those distortions going to tape and ruining an otherwise fine performance. This is where the compressor comes in handy.

The Alesis Company a while back issued a brochure on how compressors work, and it gives the analogy of the compressor acting like your own dedicated engineer for that one track. It will monitor all the incoming signals and then act like it is pulling down the fader the instant that high volume peak occurs. In a more technical explanation, what the compressor is actually doing is reading the incoming signals, and then according to the compression ratio that you set, it knocks the hot signal down by that ratio. This allows you to keep the level down to one that is manageable and recordable, without the wild peaks.

Compression ratio you ask? Well, let me explain the 5 main controls. First, we have the threshold. Think of this as the decibel level where the compression will start working. I visualize the threshold as a line that is lowered onto the offending noise peak, and the lower the threshold level, the more the incoming signal will be compressed. This is because more of the noise peak is now ABOVE the threshold level so there is more to squash.

Next we have the ratio settings. This knob has different ratios on it like 2:1, 3:1, 4:1, and usually any combination in between. Okay, assume you set your ratio to 3:1. What this does is that for every 3dB your incoming signal goes over your threshold line, the compressor will allow only 1dB to pass. The level still goes over the threshold, but assuming that you set the threshold low enough and used an appropriate ratio, the peak will never reach the maximum level and distort. This is also due to the amount of headroom you have. Typically, I set my ratio first, and then use the threshold knob to find the point that the incoming levels are being compressed. This is done while watching the meters on the mixer, and you will see the offending peaks all falling within the same lower range which is nearer to the nominal level. Keep in mind that if your incoming signal is lower than the threshold level, (or the threshold is set too high), then none of the signal will be affected.

Next we have the attack parameter. Think of this as how fast the compressor acts on the peaks once they pass the threshold. Some instruments have a really quick attack sound as soon as they are played, and most peaks arise from this attack. Therefore, on instruments like bass and kick drums, you would want to set a quick attack.

The release parameter works by setting how fast the compressor lets go of the incoming signal once it has gone below the threshold level (where the signal doesn't need to be compressed anymore.) You could set the release to fast and cut off a signal quickly, or set it to slow which results in a longer sustain. Many guitar players use this to sustain their notes.

The last main function is the output setting. Typically, when you lower the threshold and the compressor kicks in to squash the signal, your nominal level will be lowered slightly depending on the amount of compression being used. You can then use the output knob to bring the input level back up to nominal. Be careful though, because by raising your signal back to the nominal level, you are also increasing the noise floor due to added noise from within the compressor itself. You may want to increase the trim on your channel or master fader so more pure signal is getting to the compressor instead. Everytime you patch your signal through another pathway (such as another processor), you are also adding the inherent noise of that pathway.

There is one other feature that not all compressors have, and this is the option to compress with "hard knee" or "soft knee". Hard knee is where the signal is compressed the moment it goes above the threshold to the full extent of the ratio that is set. Soft knee is where the compression is applied more softly so as not to sound so abrupt. This is similar to using the attack knob, and I use hard knee compression on signals like bass and kickdrum.

Hooking up a compressor is a simple procedure involving an insert cable. This is a Y configuration cable with one 1/4" TRS connector that splits out to two 1/4" connectors. One of these connectors is an RS and the other the TS. (I should mention here that TRS stands for TIP -RING-SLEEVE, with the tip being the send and the ring being the return. This way, the TRS connector allows signals to go both ways, and the TS connector allows on signals to send FROM the compressor to the mixer while the RS connector returns the signal from the mixer to the compressor.) The TRS end is plugged into the insert jack on one channel of your mixer, the TS to the compressor send, and RS to the compressor return. This creates a loop where the original signal leaves the mixer, goes to the compressor, is then compressed, and finally returns to the mixer.

As for using compression, that is a matter of personal preference. I use it only when needed. Unless I am going for a certain type of effect by heavil y compressing the signal, then I use it only for stray peaks, since putting a signal that isn't peaking through a compressor will only introduce more noise. Some people think that even though the signal is peaking out during recording, they can compress the signal in the mix and it will be the same. I used to think that myself but I realize now that when you put a distorting signal to tape, the damage is already done to that signal's sound. The track is already saturated with distortion and no amount of compression during the mix will make it sound as if it were compressed during tracking. That is why you should definitely fix stray peaks with the compressor when recording. Also, final mixes may also need a little compression even if you used it on tracks during recording. This is due to the summation of all the track signals.

The following are just suggestions of where to start setting your parameters for certain instruments. As I mentioned earlier, how YOU want to use compression is your personal preference.

Bass: Try starting out with a ratio of 4:1, and a fast attack and release. I usually use the hard-knee type of compression here since bass is such an attack-oriented instrument. But if you were playing smooth jazz bass, then you may want to try soft-knee. It depends on the sound you are trying to get.

Guitar: This depends on the type of sound you are using, but a good general place to start is 2:1 on acoustic, and maybe 3:1 on distorted guitar (although you may need 4:1 here.) To get a good sustain, try a 4:1 ratio, use a fast attack and slow release. Then play the note you want to sustain, and raise the ratio until the sustain is as long as you want it.

Drums: Drum signals are often compressed due to their hard-hitting attack volumes. If nothing else, compress the snare drum, because each hit will likely peak higher than other hits. Try starting out with a ratio of 3:1, and use a fast attack and release. If the signal is still peaking, try using a ratio of 4:1. This method could also be applied to the toms. As for cymbal hits, try starting with a 2:1 ratio (moving to 3:1 if needed), using a fast attack and a slow release (to preserve the natural decay time of cymbals).

Vocals: As with drums, compression is also commonly used on vocals. The ratio to start at varies for each singer, since some may be very strong and loud singers, and others quieter, having a smaller dynamic range. Try starting out with a 2:1 ratio, with a fast attack, and medium to slow release. Keep increasing your ratio until you get your peaks under control.

Compression is not typically something that can be heard. You can hear it if you really spank all the knobs to full-on, but usually that technique is used more for an effect, rather than to control the level of the individual signal. Compression should be applied and monitored by using the peak display meters on your compressor or mixer. As I mentioned earlier, compression is something of an art, and you will have to play with it to find your personal preferences, so don't be afraid to tweak all the knobs to find out how they affect your sounds. Just remember that mastering compression techniques will help to make all of your recordings sound more professional.

(c) 2000, Ken Lanyon,
All rights reserved.


(You are allowed to copy and use this essay for your own non-professional use. You are prohibited from distributing copies to others for a fee or for no-charge. You may not publish or quote this essay without obtaining the written permission of the author.)