There is another aspect to dynamic range, apart from the fact that even modern equipment is not as 'hi-fi' as the ear. Do we actually need such a wide dynamic range when we play back a recording at home? If the loud parts of the music are too loud, then might not the neighbours become just a little fractious (and possibly litigious)? And if the quiet parts are too quiet, will you still be able to hear them above the background noise inevitable in almost any domestic setting? The only answer is to control the dynamic range to make sure that it isn't too much, either for the equipment to handle or for the listening environment. This doesn't always mean using a compressor; sensible mixing and use of the faders is one of the best ways of making sure that the dynamic range of a recording is as you want it.

If one function of the compressor is to control dynamic range, then a completely separate function is its use as an effect. Take 'effect' to mean changing a sound from its natural state into something intentionally different and artificial, either to improve it or simply make it more interesting. As will be seen later, the noise gate has 'corrective' and 'creative' uses too. And when we put the two together then we will really find out the power of dynamic control.

COMPRESSING VOCALS

One of the main uses of compression as an effect is recording vocals. Trained singers can sing at a consistent level and keep the difference between loud and soft passages within acceptable limits. Untrained singers usually don't have the same degree of breath control. Indeed, singing according to the rule book involves breathing from the diaphragm rather than the chest, and this does not form part of the vocal style of many of the most popular vocalists, so we need to compress. Compression is used to bring down the highest peaks, above the threshold level, leaving the lower levels just as they were. After that the level is restored so that the peaks are the same level as they were to start with, but the overall dynamic range is reduced. The result is a much more controlled sound.

Most compressors have pretty much the same controls:

1. Threshold sets the level where compression starts to take effect. Sounds below the threshold pass through unaltered and only sounds above the threshold are compressed.

2. Ratio sets the degree of compression above the threshold level. A ratio of 2:1 represents mild compression and means that when the incoming level (above the threshold) rises by 10dB, the outgoing level will only rise by 5dB. Ratios of up to 5:1 are regularly used for vocals and other instruments, and can pass by unnoticed by the listener if the other controls are set properly. Higher ratios are used for more serious limiting, where the level needs more severe control. Ratios of 10:1 and higher are nearly always noticeable to the listener.

3. Attack is measured in milliseconds and determines the time taken for the compressor to start working once the signal has passed the threshold level.

4. Release sets the length of time it takes for the compressor to return to its normal state once the signal has gone back below the threshold.

5. Gain is provided because compression always reduces the peak level: the more compression, the lower the level of the outgoing signal. This control is sometimes referred to as 'make-up' gain because it makes up the level that is inevitably lost during the compression process.

Of these five controls (there may be more on a more sophisticated device), Threshold, Ratio, Attack and Release are the most important. With these you can either improve a sound beyond all recognition or you can ruin it beyond salvation. The compressor was a powerful device. Thinking about vocals and single instruments for the moment, it is useful decide what ratio to use just by listening, then start on the other controls. It's not the kind of procedure where you can set one control, fine tune it and then go on to the next. All the controls of the compressor seem to interact with each other and need careful balancing. Having fairly arbitrarily set the ratio, next up is the threshold level. This is where the important gain reduction meter on the compressor comes into play. The gain reduction meter tells you what the compressor is doing to the signal from moment to moment and gives you a visual check on what you are hearing. A gain reduction of from 6 to 12dB is usually OK. Above this much compression you might hear that the sound is being squeezed too much (although it can be useful for an effect, as opposed to simply controlling the sound). Anything much below 6dB compression and you'll have a job hearing the difference. An important point which often isn't mentioned is that at some occasions during the track, there should be no gain reduction (as indicated by the meter) at all. If the only time the meter reads zero dB gain reduction is when the instrument isn't playing, then when it does start you'll get a 'crack' as the compressor takes effect. Setting the threshold to a lower value normally gives more compression, but setting it to a lower value than necessary just gives you the undesirable side effects.

There is a balancing act between the Ratio and Threshold controls to give the right amount of compression, according to what your ears perceive, and there is also a balancing act between the Attack and Release controls and the signal, to reduce the potential obtrusiveness of the compressor. The principal reason why compressors have Attack and Release controls is so that you can make the compression come and go unnoticed. The settings have to be judged against the attack and decay envelope of the signal. Drums for example naturally have a fast attack and decay, so if the attack and release settings are too slow, the compression will not start quickly enough and the initial transient will get through uncompressed, and after the drum sound has gone you will hear the compressor gradually reducing the amount of gain reduction as it returns to normal. This last effect is known as 'breathing' or 'pumping', and occurs when you can hear background noise changing in level unmasked by the signal. If the Attack and Release controls are set properly, the signal should hide any level changes in the background. The best way to set attack and release is to match the attack and decay characteristics of the signal itself. That way you won't get more pumping than necessary.

Having said how to set the attack and release times correctly, let's explore how they can be set incorrectly, but to good effect. One use of the compressor is to make drum sounds more punchy. This is done, just as mentioned above, by setting a slow attack. The initial transient of the drum then gets through before the compressor clamps down on the remainder of the waveform. This is a very useful technique for guitars as well. Curiously enough, the compressor is actually working to expand the dynamic range around the time of the initial transient. What a strange world! The Release control, as well as having an effect on how much pumping you will hear, can also change the amount of compression going on. In fact the Release control has as much effect on that as have the Ratio and Threshold controls. Let me give an example: at an audio exhibition several years ago where a salesman was demonstrating a compressor - the best compressor in the world, needless to say. "That's 30dB compression", he was saying, which is the same as saying 30dB gain reduction, which is a lot, "Can you hear it working?". Listeners couldn't hear any pumping at all, nor could they hear the characteristic 'compressor' sound which you would certainly expect from a unit working that hard. Looking a little closer at the gain reduction meter, it was right at the end of the scale as expected, but more importantly it wasn't moving. This is a crucial point about compression. Unless the gain reduction meter is moving then no compression is taking place. The demonstrator had craftily set a very long release time and virtually all that the compressor was doing was lowering the gain on a steady state basis. To get any use out of the compressor, not only do you have to see the LEDs of the gain reduction meter light up, you also have to see them move. The faster they move up and down, the more effective compression you are getting. (And the more you are likely to hear pumping).

The other important control you will always find on a compressor is the Stereo Link switch. This is quickly explained: If you are compressing a stereo signal, the levels in the two channels will be unequal and will change from moment to moment. Unless something is done the amount of compression will be different in the two channels, making the stereo image shift from one speaker to the other according to which channel is being compressed the most. To avoid this, the Stereo Link switch mixes the control signals for the gain elements together so that each channel is compressed to the same extent.