Fig. 3 The compressor in Ableton Live 6 has a threshold slider (left). Lowering the threshold will cause the signal to be compressed more. Ditto for the Ratio control, which tells the compressor how much you want to squash the signal when it exceeds the threshold. The attack and release knobs control how quickly the compressor cuts in when the signal pops up above the threshold, and how quickly it lets go when the signal drops back. Make-up gain is controlled by the Out slider (right). The G.R. indicator is strictly a meter, not an adjustable parameter. It shows how much gain reduction is being applied from moment to moment.
Compressors and limiters control the dynamic level of a track. They do this in an interactive way by lowering the level of the loudest peaks without touching anything else. When the loudest peaks are tamed, the level of the whole track can be raised (using the comp/limiter’s “make-up gain” control). The track then sounds louder than it did before, even though its loudest peaks are the same.
A limiter has a hard “ceiling.” The output signal is never allowed to get any higher than the ceiling, no matter how loud the input is. A compressor operates in a more gentle way: As the input gets louder, the output also gets louder, but the increase of the output isn’t as steep as the increase of the input.
Limiters are useful with digital audio because it’s important to keep the overall level of a recording from clipping. Clipping distortion, which is nasty, happens when the signal is too loud for the digital system to handle.
Compressors and limiters have a “threshold” parameter (Fig. 3). This is the point in the loudness curve at which the comp/limiter starts to do its job. Signals that are quieter than the threshold level pass through the effect without being altered in any way. But when a signal rises above the threshold, the comp/limiter starts to work. When the threshold is set fairly high, most of the track will pass through, with only the loudest peaks being affected. When the threshold is low, the comp/limiter will be “riding” the output most of the time. With a low threshold, the make-up gain control will bring up the noise floor of the recording. This can be useful with more in-your-face types of music, as it can make a sterile drum booth sound a bit trashy.
Compressors typically have three more controls: ratio, attack, and release. The ratio knob sets the amount of compression. When it’s set to a ratio of 2:1, for instance, as the input signal rises 2dB above the threshold, the output signal will rise only 1dB above. Gentle compression might use a ratio of only 1.4:1, while a ratio of 10:1 or more will squash the peaks ruthlessly.
The attack knob determines how fast the compressor cuts in when the signal rises past the threshold. Attack is usually measured in milliseconds (ms). When the attack is less than 1ms, the signal will be compressed very quickly. A longer attack will allow part of the attack transient of a sound to pass through the compressor before the compression kicks in. This can be useful with drum sounds, because it allows the snap of the stick to be heard more clearly.
The release knob controls how fast the compressor “lets go” when the signal falls back below the threshold. Typical release settings are in the 10—100ms range. A release that’s too quick can cause the compressor’s output to pump or flutter in an unnatural way, while a release that’s too slow can squash a note that follows a peak. For best results, use your ears.
Some engineers put compression or limiting on the output of the DAW’s mixer. This allows them to mix the whole song louder. If you have time, you may find that you get more musical results by compressing and/or adjusting the levels of certain tracks manually.
A multiband compressor splits the incoming signal into a number of separate frequency bands. Each band is compressed separately. Multiband compressors are often used to balance the frequency range of an entire mix.
Fig. 4 Wizooverb 2 (available from M-Audio) is a convolution reverb. The miniature browser on the right side of the central display area is used for loading impulse files. An impulse is a recording made in an actual acoustic space (typically by firing a starter pistol and recording the echoes). To create reverberation, the impulse file is “convolved” with the input signal from your track using a mathematical process. The other parameters of Wizooverb 2 (Wet/Dry mix, Predelay, and so on) are fairly standard.