Apple has patented (#7,176,374) a music synthesizer and a method of generating a synthesizer output with a constant beat.

In a nutshell, the patent covers a design that lets two oscillators be synced with a Hz difference that is constant across the range of the keyboard. If you tune (detune) the oscillators to be 6 Hz apart in pitch, the design would keep the oscillators 6 Hz appart no matter what pitches you play.
Abstract: 

Prior art synthesizers often select a small interval d to detune. For example, if the nominal oscillator frequency was 1000 Hz, then applying a detune parameter of 10 cent would result in a detuned oscillator frequency of 1006 Hz. However, at the next octave, the nominal frequency would be 2000 Hz with the detuned oscillator frequency of 2012 Hz. Accordingly, a detuned oscillator has a frequency deviation which is proportional to its nominal frequency. Hence, when mixing detuned oscillators, the resulting signal has a beat frequency which varies with the pitch and doubles with each octave.

An aim of the present invention is to provide a music synthesizer whereby sounds are generated with a constant optimum beat across a large range of tones.The present invention relates to a music synthesizer comprising a first and second oscillator and a signal generator which applies a constant beat parameter to a pitch signal to derive the input for said second oscillator, thereby enabling a synthesizer output to be generated at a substantially constant beat.