Expression Pedals are used to control effect parameters. For example, the pedal might control the level of distortion, delay time or phaser rate. One of the best known effects controlled by an expression pedal is an adjustable bandwidth filter, known as a Wah-Wah. Other controllable effects include Pitch, Reverb, Ring modulator and various rates and depths such as those related to Vibrato, Tremolo, Flanger and Chorus.
Typical Expression pedal
Exactly what can be controlled depends on the effect unit in question. Some effect units, particularly single effects (stomp boxes) often have no controllable effect parameters. Whilst most multi-effects units do provide control by an expression pedal.
Often the effect parameter being controlled is also available as a knob on the effect unit, particularly so for single effect units. And so an expression pedal can be described as a type of remote control. Where a foot can be used to control an effect parameter, rather than a knob on the effect unit.
For example, a Tremolo unit might have a RATE knob where the rate of trembling can be increased and decreased. This is set to one position and stays there through out a performance. It would be inconvenient to reach down to the knob and move it back and forth as a musician was playing their instrument, however doing so can create some interesting sounds. So having a remote control function via a foot controllable unit known as an expression pedal enables a musician to easily modify the parameter as they continue to play. Ultimately, this adds to a musicians ‘EXPRESSION’, hence the name ‘Expression Pedal’.
Oz Inventions Xpression fX controller is an Expression box, and works in the same way as an expression pedal. But rather than a pedal, there is a sensor that detects motion. The sensor can be placed on an instrument, arm, leg, foot or anything else that moves and provides remote control over an effect parameter. The Expression Box contains many features not available with an expression pedal.
Expression Box & Sensor