Bend it, shape it, squeeze the treble and make it go wobbly. Many are the things you can do to recorded music. Martin Sheehan begins a series explaining the hows, whats and whys of FX treatments open to the home recordist. This month, compression.

TICK WHICHEVER of the following you feel to be true:
- All compression is theft
- Compression is a form of mental illness
- Compression is nine points of the law
- Compression is fun to do to music

Well, the first is true in terms of dynamic range. The second is correct for some sound engineers. The third is designed to help prove responsibility for the first, and the last is indeed true of all we have come to know and love in Rock and Pop.

Years ago, in the days of analogue tape and vinyl discs (or was it shellac), lots of music veered between being too quiet or too loud to be faithfully recorded. Quiet parts became drowned in hiss and rumble, and loud parts caused unpleasant distortion and bent VU needles. So one day, a record producer went up on to the roof of Abbots Way Studios and, three days later, came down with a compressor under his arm.

He used this device to restrict the dynamic range of his sounds. It imposed an electrical 'ceiling', turning down the volume whenever an incoming signal was too loud, then turning it back up to normal for the quieter signals coming along next. He could consequently record a higher average level on to tape, and thus lift the quiet parts above the hiss.

Following this, everybody else developed compressors on their rooftops. Some of the recently discovered ones are totally automatic while others have many variable parameters such as Threshold, Ratio or Slope, Attack, and Release.

The Threshold is the level above which the compressor begins to compress. The Ratio or Slope is the degree to which any increase in level above the Threshold is compressed, For example, if a ratio of 2:1 is selected then a 10dB increase in input level will result in only a 5dB increase in output level. The Attack is the time taken by the compressor to act on a signal once it rises above the threshold, and the Release is the time taken for normal gain to be resumed once the signal has dropped back below the threshold.

Very boring would be a fairly accurate description of the use of compressors just for controlling the dynamic range of sounds on recording media. Far more interesting are the effects they can have on the various instruments of Rock. A good "thocka thocka" bass sound relies heavily on the use of compression, as does the swelling "boooweem" of the fretless.

A bass string which is repeatedly heaved away from the body then released to crash back against the fretboard is somehow expected to vibrate gently and produce a musical note. A compressor will curb the otherwise ear shattering noise of string against body and then restore gain to allow the comparatively feeble vibration of the string to achieve a respectable volume. A fairly fast attack time is necessary to react to the rapid transient of the slap, but beware excessively fast attack and release times on basses as this can cause distortion at low frequencies.

For the swelling sound of the fretless bass, a fast release (in terms of bass sounds) of around 100mS allows the gain to return quickly to emphasise the sustain of the notes. Compression is also useful on bass guitars generally as they can vary so much in volume over their different registers. It also follows that the worse the guitar and the lousier the player, the more beneficial the compression will be.

What of other instruments? The super tight "wacka-wacka-wack" of the rhythm guitar is aided by a high ratio, long release compression setting (10:1, two seconds) enabling a steady and well controlled sound to be produced. The lead guitar is also encouraged to be thrusting and prominent by using a fast attack and a quick release (aren't we all).

Manifold are the uses of compression for percussion, although unless specific effects are required, an initially good drum sound will often translate to tape better untreated. However, we like a nice tight bass drum with a good slap to it, don't we? A fast attack in the order of 1mS can help achieve this, and a slow release of a second or more will keep boominess at bay. That which gives us a slap on the bass drum, however, may take the edge off our crispy snare, so lengthening the attack here can prove useful. Shorter releases on the toms can help to swell them in the same way as occurs with the sustained notes of the fretless.

And so we move on to vocals. The duffer the singer, the more urgent the compression requirements. Without excellent microphone technique even the most well trained of voices can bob up and down in volume. The use of a gentle compression ratio of around 3 or 4 to 1 with the threshold set low enough to cover most of the voice's dynamic range, is a good starting point. Some consonants require a fast attack to catch their rapid transients, but beware of using a rapid release in conjunction. A low threshold combined with fast attack and release can cause 'pumping' — the sound of any background noise surging up and down as the compressor alters the gain. The setting of the release time on a voice is often a compromise between avoiding this and still enabling the gain to recover quickly enough so as not to lose any of the quieter parts of the vocal.

If compression, to you, seems like too much knob twiddling then there are some fixed and some fully automatic units on the market. The compromise here is that the fixed models are less versatile, and the ones with all the brain, which are continuously altering their parameters in order to give unintrusive results, are obviously more expensive. It does mean, however, that there is a compressor to suit all pockets and purposes. Knowing that they are operating in some fashion on the vast majority of the music we hear, can you afford to ignore them? Indeed, Soft-knees, Side-chains and Ducking just peep into the bottom of this piece, and are just some of the myriad facets of this effect we call Compression. An effect which has become omni-present as a consequence of the sojourn on the roof.

Compressing matters

Soft knee: a useful characteristic of some compressors whereby full compression is not imposed as soon as the threshold is crossed, making the onset of compression less obvious.

Side chain: access to a compressor's level detection circuitry. This allows certain tricks to be performed such as... Ducking: an effect often used by DJs. A feed taken from the voice signal is patched into the side chain causing the music input to automatically drop in level whenever the DJ talks into the mike.