Compressors and compression (updated 8-10)

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 so 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.

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" and they might keep it turned down for a while - that's the release time.

Lets 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" is tricky. Ears are non linear. They compress too. If the lowest level of a sound is above the threshold (causing the compressor to compress), then all levels of sound will be reduced the same amount with a normal compression curve. The "curve" in a ratio is often called the "knee" meaning the ratio will change at different levels according to the amount the signal goes above the threshold. Simply, the more you exceed the threshold, the more compression will occur relatively. Soft knee compressors are considered desirable because they sound more natural - more like your ear handles volume. Some compressors have a built in knee - and some let you select one. Opto type compressors have a built in knee due to the response of the optical element controlling the volume. Opto compressors are particularly nice if an important part (vocal?) is all over the map dynamic range-wise. They will smooth it out nicely - but the same effect can be had by controlling the curve which can be done with most PC based compressors.

"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 its not a note, and its 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 problem 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.

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. If you like how the low end sounds without compression but hate the midrange - you can just split a signal, run one half unaltered through the main signal path, and take the other half and use it for the side chain via an EQ. Just filter out the bass, and the bass will not trip the threshold but other frequencies will. The possibilities are endless.

A limiter is a type of compressor. Basically, limiting is high ratio, low threshold compression. It flattens the peaks only. Limiting is nice because it is assumed to usually not be impacting the signal at all - but coming in to the picture only when some thing goes way out of control. All big live rigs have limiters all over the place. Any compressor can be a limiter depending on how you set it, but some boxes are preset to limit. Some of these sound great. In the digital world the Waves L1 limiter is wonderful. They even make a rack mount box that runs their PC algorithm. There's a switch! 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.

“Common” compressors: The Universal Audio (UREI) (http://en.wikipedia.org/wiki/Universal_Audio) 1176, now produced in every conceivable form factor … reissue hardware, software, etc. It produces a gritty sound that perks up a vocal track and makes it jump out of a mix. It’s a virtual standard on voice over. Its behavior is hard to control in the software versions (imo), and it was built as a limiter. So, basically, while it is a standard, it’s not the right tool for a lot of things.

Universal Audio LA2a. This is often used as a leveler. It has an intelligent release that tracks vocals well, and a lot less grit than the 1176. I find them dull, but a lot of very respected people love them, so, use your own ears.

Manly varimu. This is the modern example of a varimu compressor. The famous and pricey retro comressors (Fairchild), and very expensive modern clones of it use varimu type compression. Basically, these devices use a tube, directly as a volume control (tubes are valves!). They deliver a remarkable lush tone, and they cost a lot. The Shadow Hills compressor is the supreme example of this technology produced today, imo. Pendulum makes a vari mu that is reasonable priced, and solid.

Just because a compressor has tubes does not mean its vari mu. Tubes are also used for makeup gain in opto and fet compressors.

Tube Tech makes several types of tube compressors that are common in the best studios. The CL1B is the standout, a single channel vocal compressor that is simply beautiful. Its an opto compressor. The LCA2b is their Fairchild clone. I’ve used it for mastering and it’s almost like cheating. The only ding on tube tech gear is that it requires maintenance to be at its best, and that’s a pain.

Anyway, those are some of the studio standards. There are dozens more, and a million clones. There are also a bunch of compressors designed to sit on a master buss, or drum buss. They often carry the names of famous consoles. Neve 33609 was a great buss comp. SSL made a great buss comp. These are oriented to unobtrusive low release low ratio compression, generally. Many of them have ways to keep bass out of the side chain so that low end does not trigger the compression. Buss compressors are an odd thing, but they turn out to be practical for several reasons.. but they are also dangerous because they mask what you might hear otherwise. I only use them in the final stages of mixing.