Phase offset modulation works by overlaying two instances of a periodic
waveform
In electronics, acoustics, and related fields, the waveform of a signal is the shape of its Graph of a function, graph as a function of time, independent of its time and Magnitude (mathematics), magnitude Scale (ratio), scales and of any dis ...
on top of each other. (In
software synthesis, the waveform is usually generated by using a
lookup table
In computer science, a lookup table (LUT) is an array data structure, array that replaces runtime (program lifecycle phase), runtime computation of a mathematical function (mathematics), function with a simpler array indexing operation, in a proc ...
.) The two instances of the waveform are kept slightly out of sync with each other, as one is 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
sawtooth (ramp) wave subtracted from another will create a
pulse wave, with the amount of offset (i.e. the difference between the two waveforms' starting points) dictating the
duty cycle
A duty cycle or power cycle is the fraction of one period in which a signal or system is active. Duty cycle is commonly expressed as a percentage or a ratio. A period is the time it takes for a signal to complete an on-and-off cycle. As a for ...
. If you slowly change the offset amount, you create
pulse-width modulation
Pulse-width modulation (PWM), also known as pulse-duration modulation (PDM) or pulse-length modulation (PLM), is any method of representing a signal as a rectangular wave with a varying duty cycle (and for some methods also a varying peri ...
.
Using this technique, not only can a ramp wave create pulsewidth modulation, but any other waveform can achieve a comparable effect.
Wave mechanics
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