An introduction to compression, taken from the ThingsYou'reMissing archives. Ironically, this is one of the longer articles here.

From the ThingsYou'reMissing archives.

Compression FAQ

Disclaimer: this is what I know about compression. I've done a lot of home recording, but I'm not a sound (or electronic) engineer so there may be inaccuracies. If you find something you disagree with, mail me at the address above.

What is compression?
Compression is a process by which the difference in loudness between the loudest and quietest parts of an audio track (live or recorded) is made smaller.

The basic concept of a compressor is as follows: up as far as a certain level (the threshold), changes in the volume level of the input produce correponding changes in the volume of the output. Above the threshold, a changes in the volume level of the input will produce smaller changes in the level of the output.
With the make-up gain (more about that later) set to zero, once the input signal is lower than the threshold the output will be the same as the input. If the input goes, say, 8dB over the threshold and the ratio is set at 4:1, the output will only go 2dB over the threshold (the ratio is the ratio between changes in input level and output level once the threshold is exceeded).

So, essentially, what a compressor is does is make the louder bits quieter. Obviously this means that the output from the compressor will be quieter than the input. You can counteract this using the make-up gain, which is just an amplifier, making the output louder. So, as well as making the loud bits quieter, you can make the quiet bits louder (by making the loud bits quieter then turning up the whole lot so the loud bits are at the same level they were before … hey presto, the quiet bits are louder).

What is gain reduction?
Gain reduction is just the difference in level between the output and the input. In the above example (input 8dB over the threshold, ratio of 4:1, therefore output 2dB over threshold), gain reduction is 6dB.

What are attack and release?
Attack is how quickly the compressor begins to act after the threshold has been exceeded – the length of time it takes the electronics in your compressor to realise that the input has exceeded the threshold and start reducing the level of the signal it's sending to the output.
Release is the reverse – it's the length of time between the input signal falling below the threshold and the compressor stopping its attenuation of the signal it is sending to the output.

How do the controls on my compressor correspond to the above?
Threshold, ratio, make-up gain, attack and release are the basic compression parameters, but they might have different names on your particular machine or be marked differently, or they might even be missing. Some basic compressors that you might find say on a bass amp just have one knob that says 'Compression' – this probably controls threshold (with ratio, make-up gain, attack and release all fixed). In this situation your best bet probably just to twiddle the knob until it sounds good.
Threshold units are usually dB, and will run from minus some number to zero. Basically a lower number (ie bigger minus number) means the threshold is lower – -24dB means the compressor will kick in sooner than -6dB. It's possible that threshold could be marked 0-10 – in that case I'd guess that lower numbers correspond to a lower threshold, but it's conceivable that it'd be the other way around, depending on the machine.

Make-up gain can be marked in dB (from zero up) or 0-10, bigger numbers means more gain.

Ratio is easy to spot, it's a ratio (2:1, 8:1, 100:1 or whatever). I have also seen it called 'slope' – in this case a steeper slope would presumably mean a lower ratio.

Attack and release can be difficult to figure out – sometimes they are marked in time units (smaller number=faster attack), sometimes just 0-10 (higher number=faster attack? … only way to tell is to play with it), sometimes just 'fast' and 'slow'.

What would I want to use a compressor for?

  1. Compressing!
    The most simple use of a compressor is where there are big variations in volume in something you're recording (or mixing, or putting through a PA) – on the loudest bits you're getting distortion, but if you turn it down the quiet bits are too quiet. You need to make the loud bits quieter relative to the soft bits, so you set your threshold so that the loud bits trigger compression, then set your ratio so that you're no longer getting distortion. If the quiet bits are still too quiet, turn up the gain a bit, then maybe turn up the ratio to take a bit more off the loud bits. You'll want to have your attack set quite fast, cos you want the compressor to kick in before the signal gets loud enough to cause distortion.
  2. Adding punch
    You can also use a compressor as a kind of an effect. Set a low threshold with a fairly high ratio so that more or less any input signal will be squashed, then set a not-so-fast attack. Because the attack is a little bit slow, the start of a input sound gets past the compressor, then the compressor kicks in and reduces the gain. This emphasises the start of the sound, and can add 'punch' to percussive sounds like drums or ska guitar or picked bass. You might want to use a second compressor with a higher threshold and fast attack to keep all the 'punches' around the same level.
  3. Making things louder.
    There's a limit to how loud a sound on a tape or CD or piece of vinyl can be. For your recording to be as loud as everyone else's, you'll want to try and make sure that the loudest sound on your recording is getting close to that limit. However, if the loudest sound on your recording is a lot louder than everything else, you might find that everything else seems too quiet. You can use a compressor to make the loudest bits a bit quieter, then crank up the make-up gain to make the loudest bits approach the limit of your tape/CD/vinyl/harddisk again and hey presto your recording is LOUDER. Don't go too overboard on this though, it's fairly easy to compress something so much that is no longer sounds musical – your ears and brain expect variations in loudness, so if you've compressed something so much that there's none, it's going to sound odd.

Note that the particular settings on your compressor will vary not only with what you want it to do, but with whatever it is that you're compressing. A very fast attack tends to distort bass, and compression affects the sound (no matter now 'transparent' the manufacturer says your compressor is), so you'll probably have to fiddle with the setting for, say, a vocal so it doesn't sound too unnatural.

When would I not want to use a compressor?
If you plan to get a recording mastered, it's not a good idea to compress the mix as a whole (compress away on individual tracks). It's likely that the mastering engineer has much better compressors than you, and if you've already compressed the mix you're limiting what he can do …… or at least that's what I thought. However, Mixerman (don't know his real name, but apparently he's a big-name mixer, shows up a lot on rec.audio.pro and www.prosoundweb.com) says:

"If the mix is ever going to be compressed, it should be compressed while mixing. Even if you're not very good at it right now, or even if you don't have a great compressor, I'd rather see you compress it before mastering. That's because compression will change your levels within a mix. If you are happy with your balances, then you will likely be unhappy with your new balances once compressed at the Mastering stage. Limiting does not destroy your balances as readily, That removes depth of field from your mix."

Some people prefer not to compress as they are recording, because what's on tape can't be undone, whereas if you compress while mixing you've more freedom to adjust things. Sometimes you don't have enough compressors to be able to do this though, and sometimes (say a singer that's whispering one minute and roaring the next) you just have to compress as you're recording just to get a manageable signal.

here's not much point in compressing distorted electric guitar (unless you're trying to add punch to the starts of the notes/chords) because the distortion effectively compresses the sound anyway.

What are pumping and breathing?
Pumping and breathing are weird effects that can be caused by compressors. I've never had them pointed out directly by people who KNOW what they are, but I think breathing is when, if you have the make-up gain up very high and you suddenly come into a bit of silence, as the compressor releases you'll hear the background hiss getting louder and louder. Sounds kind of like someone taking a breath. Sounds really bad say on drums where you can hear the BANG of the drum then ssssSSSSS then BANG then ssssSSSSS. In this case you can get rid of it by making the release longer (so the compressor stays on between hits on the drum) or lowering the gain.

Pumping (again I'm not sure, but this is what I think) is when you're compressing a few things at once (say a whole mix) when everything gets suddenly quieter for a while, then gradually louder, suddenly quieter, gradually louder, etc. This can happen cos something, say a bass drum, exceeds the threshold and gets compressed but you've got the release set slow so the compressor takes a while to switch off (especially if you're using a high ratio, giving you a lot of gain reduction) so after the bass drum hit everything else suddenly gets quiet too, then gradually louder as it's released, then suddenly quiet again (another kick on the bass drum), then gradually louder etc. A faster release would help in this case.

What's a limiter?
A limiter is basically a compressor with a ratio of infinity to one. It stops the level going over the threshold, end of story, no messing about.

What's hard-knee and soft-knee compression?
In hard-knee compression the compressor kicks in as soon as the threshold is exceeded (after the attack time has elapsed). Soft-knee compression kicks in gradually around the threshold.

What's the difference between different compressors?
Electronic compression can be carried out in a few completely different ways. Also, compressors can have very different electronic componenets inside (some of them are just software running on computers). Therefore they all sound different, even if they have the same knobs with the same shit written on them. If you want to know what's a good compressor for you to buy I can't help you – have a look on rec.audio.pro or talk to the guy/girl in your local music shop.

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