Phase offset modulation

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Phase offset modulation works by taking a lookup table and overlaying two instances of the waveform on top of each other. One is slightly out of sync with the other one, making it further ahead or further behind in its cycle. The values of both of the waveforms are either multiplied together, or the value of one is subtracted from the other.

This generates an entirely new waveform with a drastically different shape. For example, one ramp wave subtracted from another will create a pulse wave, with the amount of offset (ie the difference between the two waveforms' starting points) dictating the duty cycle. If you slowly change the offset amount, you have pulse-width modulation.

Using this technique, not only can a ramp wave create pulsewidth modulation, but any other waveform can achieve a comparable effect.