A harmonizer shifts pitch in real time by varying playback rate with spliced grain boundaries

A harmonizer is a real-time device that shifts the pitch of an input signal without altering its duration. The technique reads input samples at rate SR_in and outputs at rate SR_out; the ratio SR_in/SR_out determines the pitch change. To maintain continuous output, the device must occasionally skip samples (for downward shifts) or repeat samples (for upward shifts). When the output address pointer overtakes or is overtaken by the input pointer, the device splices to a new memory location. To make this splice inaudible, the jump size is estimated from the pitch period of the input signal, and smoothing fade-in/fade-out envelopes are applied across the splice point. The Eventide H910 (mid-1970s) was the first commercial device of this type.

Examples

Pitch shift up one octave: SR_out = 2 times SR_in. The output pointer runs twice as fast as input, creating frequent splice points every half-period of the input fundamental.

Assessment

How does a harmonizer maintain continuous audio output while reading samples at a different rate than they arrive? What artifact can occur if the splice timing is not pitch-synchronized?