2 Signal processing

2.7.1 Dynamics

These are signal processors that, in one way or another, react to and/or alter the playing volume. The basis of all dynamics controllers is the envelope follower. What it does is take a look at the input signal, and then output a signal which perfectly tracks the volume of the input. With that signal the actual effect is controlled, as if you were turning an effect knob while you're playing. In advanced multi-effects processors it can be found as a separate module with its own controls, and can be linked with different effects, like a wah-filter or a phaser.

Noise gate

As the name implies, its main function is to prevent noise from being heard. This is accomplished by muting the sound when the volume is below a certain level (called "threshold"). The idea is: if you're not playing, then why listen to the internal processes of your equipment? More sophisticated noise gates may have several controls:

Threshold level: the volume level at which the gate opens
Attenuation: how much the volume is being cut when below the threshold level
Attack: the time it takes for the gate to go from the attenuation value to fully "on"; prevents popping
Hold time or sustain: the minimal "on" time; prevents constant opening and closing of the gate, for instance when playing short, separate notes.
Decay or release: the time it takes for the gate to go from "on" to the attenuation value; prevents popping

Please note: a noise gate can't block all noise. If the power amplifier section puts noise on the speakers when no signal is present, a noise gate can't suppress that noise, because a power amplifier can't be muted. Also, when there's noise while you play, obviously the noise gate won't have any effect on that either.

Compressor

This device is used to make volume changes less dramatic. It gradually turns down the volume when playing louder and resets it when playing with less intensity. Controls are:

Theshold: the volume level at which the compressor will engage.
Compression ratio: how much the volume is cut. Say you have a given rise of input volume. A 2:1 ratio will increase the ouput volume only half of that, as soon as the volume level exceeds the threshold level. A 4:1 ratio will only increase the output for a quarter of that.
Attack: the time it takes for the compressor to adjust the volume. With a larger attack time, you can keep sudden peaks in the signal. For slap playing this may be desirable.
Release: the time it takes to return to the normal volume.
Gain: an extra gain stage is used to bring back the compressed signal to counteract the cut volume.

Limiter

A variation of the compressor. A limiter usually has a fixed high compression ratio, a high threshold, and a very fast attack and release. Its purpose is to avoid very high peaks that would, for instance, overload an amplifier or recorder. Professional amplifiers have this feature built-in. An advanced compressor can be used as a limiter with the proper control settings.

Envelope filter

A filter changes its frequency at command of an envelope follower (which follows your playing volume). The most common type is the touch-wah, which is an envelope-controlled low-pass filter with a peak just before cut-off. When playing softly, the wah is closed, giving a very muffled sound, and playing louder gradually sweeps the wah to a higher cut-off frequency, giving a more "ah"-type sound. The touch-wah is best used with very dynamic playing, causing the device to continuously shift from low to high frequency cut-off and back.