A modulation wheel — often referred to as a “mod wheel” is a controller found on keyboards such as MIDI controllers and synthesizers, which takes its shape in the form of a wheel mounted perpendicular to the surface of the keyboard. The wheel itself is imbedded in the surface such that only the top half protrudes. The mod wheel is used to add expression or to modulate (change) various elements of a synthesized sound or sample. One typical use is to modulate an LFO in order to produce vibrato. Another would be to control the speed of rotary speaker emulation. There are many other applications as well depending upon the architecture of the instrument being controlled.
In order to create such effects, mod wheels send continuous controller messages (CC), which send the movements of the wheel as well as knobs, sliders, pedals etc. (See MIDI Control Change) For example, your synth’s modulation wheel or lever will almost always send CC1 messages. Each CC has a possible range of 0-127, so when you move the mod wheel down to its rest position, it should send a CC1 with a value of 0, and when you push it up to its highest point it should send a CC1 with a value of 127. CC values are not smooth, they’re stepped, that is, a standard mod wheel can send a value of 56 or a value of 57, but it can’t send 56.329 or 57.1. Depending on what sound parameter CC1 is controlling, you may hear a slightly grainy, stair-stepped effect (zipper noise) when you move the mod wheel while holding a note.