The auto-pan effect performs an automatic, repeating pattern with a virtual panning potentiometer. This creates the perception of a sound moving from side to side across the stereo field.
Typically, a low-frequency oscillator (LFO) signal is used to change the pan value. Specifically the amplitude of the LFO at a given sample sets the pan value. Different types of LFOs will create different repeating patterns. Common examples include a sine wave, triangle wave, and square wave.
Because the panning potentiometer is fundamentally an amplitude control, the auto-pan effect processes a signal by increasing the amplitude in one stereo channel and decreasing the amplitude in the other channel. As the effect continues, the LFO will continue to change the amplitude in each channel. Therefore, the auto-pan effect is the stereo version of the tremolo effect.
Besides stereo effects using the panning potentiometer, another strategy for stereo effects is based on Mid-Side Processing.