Phase modulation (PM) is a technique similar to frequency modulation (FM) that is used in digital synthesizers. In PM, instead of modulating the frequency of an oscillator, the phase of an oscillator is modulated by another oscillator. When the modulating oscillator is at a high point, it causes the phase of the carrier oscillator to shift forward, resulting in a higher pitch. Conversely, when the modulating oscillator is at a low point, it causes the phase of the carrier oscillator to shift backward, resulting in a lower pitch.

One reason why PM is used instead of real FM in most digital synthesizers is that PM is computationally more efficient. FM synthesis requires complex algorithms that involve exponential functions and can be computationally expensive. In contrast, PM can be implemented with simple multiplication and addition operations, making it much faster and easier to implement in hardware and software.

Another reason why PM is preferred over FM is that it allows for more control over the harmonics and timbre of the synthesized sound. In FM, the number and amplitude of the harmonics that are generated can be difficult to predict and control. In PM, however, the phase of the carrier oscillator can be shifted by precise amounts, allowing for greater control over the harmonics and the resulting timbre of the sound.

In addition, PM is more forgiving in terms of tuning and stability. In FM, the oscillators must be precisely tuned and stable to produce a clean and stable sound. In PM, small tuning errors or instability in the oscillators can produce interesting and desirable harmonics and modulation effects.

Finally, the popularity of the Yamaha DX7 and its use of PM played a significant role in the widespread adoption of PM over FM in digital synthesizers. The DX7 was a groundbreaking instrument that popularized digital synthesis in the 1980s, and its use of PM paved the way for other digital synthesizers that used similar techniques. As a result, PM became the de facto standard for digital synthesis and is still widely used today.
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