The compressor can make a tremendous difference to the quality of finished recordings, and its principles and operation are not as complicated as you might expect. Paul White helps you get to grips with this useful studio device.

Compressors are amongst the most useful pieces of sound processing equipment available for the sound recording studio, but judging by your letters, it's not always obvious what they do or how best to use them. Hopefully, this article will put that to rights, but before looking at compressors, it is more logical to explore the type of problem they are designed to solve. As Arthur Dent found out, you can't appreciate what the answer means without first defining the question!

In an ideal world, sound balance would simply be a matter of setting the faders for the different instruments and voices, and then leaving everything to look after itself. But there are several reasons why this is often less than satisfactory, vocals being the prime example. Most pop singers are untrained and many have limited control over the level at which they sing. They feel more comfortable with some parts of their vocal range than others, while some phrases and words are easier to sing than others. The outcome is usually a performance which fluctuates in level by a considerable margin, from phrase to phrase and even from word to word.

The simplest cure for this is to mix the tape with one hand on the singer's level fader and pull it down a touch when the level gets too high or push it up a little when the level gets too low, but this is far from satisfactory. Unless you've played the track through and memorised exactly where the loud and quiet spots are, you'll always respond too late because you can't start to move the fader until you hear the start of the offending loud or quiet sound. Add to that the reaction time of a typical human being and you can see why you'll always be chasing the problem rather than curing it. Nevertheless, this approach is far better than having no control at all, and before compressors were in common usage, manual 'gain riding' was a valuable engineering skill. Figure 1 shows a schematic diagram of what is happening when an engineer attempts to control levels manually by gain riding.

Basic Compressors

Essentially, the compressor does exactly the same job as the gain-riding engineer, but as it has much faster reaction times, and it can bring excessive level deviations under control almost instantaneously. At the heart of the compressor is an electronic gain control which replaces the fader in Figure 1. Electronic gain controls often take the form of Voltage Controlled Amplifiers or VCAs, the advantage being that they allow the signal level to be controlled electronically rather than by the physical movement of a fader. Exactly how they work or how they are controlled is of academic interest only — it no more affects how you use compressors than knowing how a TV works affects your ability to tune into Neighbours every lunchtime.

In order to react quickly enough, the compressor dispenses with the human ear and instead monitors its own output level by electronic means. A part of the circuit known as the 'Side Chain' follows the envelope of the output signal and uses this to generate a control signal which is fed into the VCA to control the gain. It works exactly like the human version, only much faster and far more precisely; when the output signal rises past an acceptable level, a control signal is fed to the VCA, which turns it down. Figure 2 shows a simplified block diagram of a typical compressor, and a little careful examination reveals that the process is almost exactly the same as manual gain riding, except that the electronic circuitry does all the work.

The Terminology

With manual gain riding, the level above which the signal becomes unacceptably loud is determined by the engineer's discretion — if it sounds too loud to him, he turns it down. In the case of a compressor, we have to 'tell' it at what level to step in and take control, and this level is known as the Threshold. If the output signal is below the Threshold, then it is passed through with no change, but if it exceeds the Threshold, then the compressor swings into action and turns it down. In physical terms, the Threshold is set by means of a knob on the front panel; how this is set will be explained later.

The amount by which the compressor turns the level down depends on the setting of another front panel control — the Ratio control. You will see the ratio expressed in the format 4:1 or similar, the range of a typical Ratio control being variable from 1:1 (no effect at all) to infinity:1, which means that the output level is never allowed to rise above the Threshold, no matter how high in level the input signal is. This latter condition is known as limiting, because the Threshold is, in effect, a limit which the signal is not allowed to exceed. The figures around the Ratio control are based on dBs, so a 4:1 compression setting means that for a 4dB increase in the input signal, the output is only allowed to increase by 1dB once the input level exceeds the Threshold we have set. If you're not entirely comfortable with dBs, don't worry — all you need to remember is that the larger the Ratio, the more gain reduction will be applied to signals exceeding the Threshold. For most applications, Ratios of between 4:1 and 8:1 are typical, and once you get past 20:1 or so, the end result is so close to absolute limiting as to make no difference. In other words, the more control you feel you need, the higher you set the Ratio control.

Figure 3 shows a graph of the input level of a typical compressor compared with the output level at different Ratio settings. Note that until the Threshold has been reached, the input and output levels are identical.

Conventional compressors as just described can sometimes be a little obtrusive in use, because as soon as the signal reaches the Threshold, gain reduction takes place straight away — one moment the signal is untreated, the next it's been electronically squashed. This can produce an exciting, punchy sound which is much favoured for certain vocal styles, drums, bass guitar and so on, but there are occasions when you want the sound to remain as natural as possible, even when compressed quite heavily. To this end, somebody dreamed up the Soft Knee compressor which, instead of having a well-defined Threshold at which everything happens, has a more 'blurred' Threshold region which allows the gain reduction to be piled on progressively over an input signal range of around 10dB. What happens is that when the signal comes within 10dB or so of the Threshold set by the user, the compressor starts to apply gain reduction but with a very low Ratio setting so there's very little effect. As the input level increases, the compression Ratio is automatically increased until at the Threshold level, the ratio has increased to the amount set by the user on the Ratio Control. Again, don't be put off by the technicalities; the term Soft Knee simply means that the compression is applied more subtly and is less likely to make individual sounds or complete mixes sound obviously processed. Figure 4 shows a graph of input against output for a Soft Knee compressor with a Ratio setting of 8:1. Note that some of the simpler Soft Knee compressors don't have a Ratio control at all — the higher the input signal level, the higher the Ratio becomes, quite automatically. An example of this approach is the highly popular Drawmer LX20, on which the amount of compression is governed by a single control.

The Other Knobs

So far we've looked at the Threshold control and the Ratio control and established the main difference between a conventional Ratio compressor and a Soft Knee model, but there are other knobs adorning the front panels of most compressors that need to be explained.

Most compressors have an Attack control, though it would be more logical to call it a reaction time control, as it sets how long the compressor takes to react to an increase in signal level. A well-designed compressor can react almost instantaneously, but that's not always what we want. By slowing down the reaction time of the compressor (by setting a longer Attack time), we can create some interesting percussive effects. This trick is often used on drums and bass guitar, though it can be applied to many more sounds than just those two. Increasing the Attack time allows the loud peak at the beginning of the sound to pass through the compressor unchecked, and then the compressor swings into action and pulls down the level. The result is that the transient attack of the sound is emphasised giving it more definition.

On the other hand, when treating sounds such as vocals, a fast attack time generally gives the best results because it brings the level under control very quickly, producing a more natural sound. Figure 5 shows a percussive sound being processed by a compressor with a fast Attack time, and then again by the same compressor with a longer Attack time set.

Teamed up with the Attack control is usually a Release control though, again, this might be better described as 'recovery time'. Once the input signal has dropped back below the threshold, the Release control sets how long the compressor takes to allow the gain to rise back to normal. If the gain is turned back up too quickly, the effect of the compression becomes rather too obvious and the result is known as 'Gain Pumping', an effect where you can actually hear the levels being turned up and down in a rather disconcerting way. On the other hand, if the Release time is set too long, any quiet sounds following the one that triggered the compressor will also be reduced in level, which is not what we want. The trick is to find the shortest possible setting that doesn't cause noticeable gain pumping; a value of around half a second is usually adequate for most purposes.

Many modern compressors have an Auto setting, which constantly adjusts the Attack and Release times to suit the material being processed. In the case of complex mixes or unpredictable sounds such as vocals or virtuoso bass playing, the Auto setting often gives the best results and shouldn't be regarded as a cop out!

Next month I'll be looking at the more advanced aspects of compressors, including stereo linking, external control and built-in gates.