Compressors and compression

Warning: Compressors can ruin your recording. They kill the thing most people strive for: realism. They are seductive because they smooth things out. They will suck the soul out of a recording if you use them improperly and if you compress to tape it may be too late. There’s no realistic “uncompressor” – although expanders do the opposite of compressors they can’t fix over-compression.

Compressors control dynamic range. Dynamic range is the one thing reproduction systems including recording can't faithfully reproduce. There are a lot of reasons for this. Dynamic range is the main difference between a recorded sound and a real sound. Generally compressors are used to take out dynamic range. Why would you want to take out something desirable? There are several reasons.

The most important has to do with limitations in the recording process. Since most gear cannot accommodate the dynamic range of real sounds, we need to protect it from those sounds to it won't distort. More to the point however, is not the dynamic range of the actual sound being reproduced but the imperfection of the humans making those sounds for a microphone. They move, they get emotional etc. A singer can open up her pipes and blow away carefully set levels. In the sterile environment of a studio it's easy to say that this drum hit or that guitar chord was too soft or too loud - so we just fix it with a compressor. The best performers are very aware of this and don't need as much compression as inexperience performers. Another reason compressors were important was that they were useful in getting a better signal to noise ratio when tape noise was a problem. A little compression could allow the engineer to push the levels hotter without as much risk of overloading the tape.

Every compressor is just a volume control that responds to some electronic signal. The way it responds gives the characteristic of the compressor. The character of different compressors is a subject of lore and debate among engineers. All compressors start to turn down the volume when the input hits a threshold set by you the user. They turn it down by an amount determined by the "ratio". They might wait for a while before turning it down – that’s "attack time" and they might keep it turned down for a while - that's the release time.

Let’s talk about these knobs. The threshold is important knob since it determines if the compressor is doing anything. It’s also the simplest. If you lower the threshold to some point the compressor will always be compressing. That's not generally good since when the compressor is engaged, the signal is impaired by the gain reduction. You will lose some transient accuracy, dynamic range (obviously) and usually high end. Good engineers are gentle with compressors unless they are being used as an effect.

Ratio. This knob determines how much reduction is applied when the signal goes over the threshold. Threshold and ratio work together to determine when and how much gain is reduced. Sometimes the relationship is a curve and sometimes there is a “knee” in the curve. These shapes change the ration depending on how much a signal goes over the threshold. Very handy for a lot of reasons.

"Attack" is tricky. Many engineers will use a slow attack (>20ms) because it sounds more natural. Some compressors give you no choice. (Especially vintage compressors). For some things, especially drum hits, you have to use a fast attack because the problem you are solving requires it. The most interesting thing about attack is that for low frequency sounds a fast attack happens before the wave even develops. A problem with fast attack is that the perception of high end in stringed instruments is often contained entirely in the attack of the string. Like a pick attack. This is "noise" in the sense that it’s not a note, and it’s all over the spectrum. The attack of a classical guitar string has plenty of stuff at 30 hz - and 10k. So if the attack of the compressor is fast, it will compress out the string attack making the sound very dull, lifeless, and often severely damaging all the wonderful sensitivity the musician brings to the instrument with their hands.

"Release" time is a good creative tool. Nominally the release time should mimic the decay time of the program material being compressed. Fast sound (drum hit) fast release. However, the release time can be manipulated to fix a lot of problems or create effects. For example, long release times create sustain on a plucked instrument. Steven Stills cranked it in "Black Queen" to create a cool effect. On drums, you can control how much "ring" is in the mix with release times. Many vintage compressors automatically set the release varying according to the attack in the program material. This produces a natural sound that accounts for some of what people like about vintage compressors - but the effect can be easily emulated. This is what you get when you press "auto" on a Behringer composer.

The other thing that people like about vintage compressors is the coloration of the signal produced by the compression process. Tubes, and transformers produce coloration that can sound great - and many old compressors (or clones thereof) have both. Basically, there are a million possibilities for distortion in a circuit and some of them sound good.

Most compressors have a "side chain". This means simply that a signal other than the input sound will be used to control the behavior of the compressor. This is useful for all sorts of tasks. You can split the original sound, filter it, and use only the filtered sound in the side chain with the compressed changing the original sound according to a filtered version of itself. This is basically how de-essers work.

A limiter is a type of compressor. Basically, limiting is high ratio compression that only happens when a sound is really loud. It flattens the peaks only. Limiting is nice because it is assumed to usually not be impacting the signal as much - but coming in to the picture only when some thing goes way out of control. Most big live rigs have limiters in several spots. Any compressor can be a limiter if you can set the ratio and threshold and some boxes are preset to limit. Some of these sound great. In the digital world the Waves L1 limiter and its cousins are wonderful – and there’s always someone trying to improve on those. Limiters are used in situations where maximum output is essential but going "over" is bad - like mastering a CD or the main output of a radio station. Because limiters were used in critical situations many of them especially vintage have a great sound and high-grade components. Fairchild and Telefunken compressors and limiters sound great and you can easily pay 25 grand for one.

Noise performance. Anyone who is experienced with compressors will tell you that if you have a noisy track/source, compression will make it worse. This is because the net effect of a compressor is to make loud sounds quieter, ergo soft sounds relatively louder. Its easy to hear the noise floor come up as a compressor begins to release. Careful use of gates can help in some circumstances, but its best to keep noise to a minimum if you think you will need compression.

I have used a lot of compressors and here’s a one liner on some of them in no particular order: Like all generalizations there are lots of exceptions.

Pendulum : Amazing, transparent, fast, lovely.

Millenium: High class pro gear, smooth.

Focusrite: Usually amazing but I don’t like the silver series.

Meek: Funky and cost effective buy not pristine

LA2A: Everything vintage, cool tone, not flexible

Behringer: Workmanlike and cost effective. Not “cheap”.

DBX: Expensive ones are good, cheap ones… look elsewhere.

RNC compressor: Cheap, and acts like it but serviceable for live work.

Tubetech: Gorgeous sounding, all pro, but a tad pillowy when you start taking out more than a few db.

SSL: Transparent but dangerously so.

Aphex: Pretty good for live work, some “character” in their tube-essence (or whatever they call it) circuit.

And some compressors I have not worked with but are noteworthy:

Manly: Great but many people like the Pendulum better.

Cranesong: I hear nothing but good – and the best studios have them.

Drawmer: Cool funky clean pro. I’ve always wanted a 1961.

Imperative distressor: Appearing in a lot of Nashville producer racks.

.. And compressor plug-ins?

The LA2A has the same vibe as an LA2a, for example, and they get the tone about right – but I’m sorry they just don’t respond like the real thing. This could be because of the dynamic interactions between the components within an analog box. This should not be a problem, however for even the best recordings since, after all, we are looking at the quality of something that’s basically an impairment in the first place. J