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Understanding Phase Modulation
Modular sampling rewards understanding. Rossum ships not just the instrument but the ideas behind it. Here Dave Rossum explains the technique at the heart of the Assimil8or.
Phase modulation is a new kind of audio cross-modulation for sampled sounds. It can produce rich and varied timbres and textures, as well as wild distortions and grating noises.
The word "new" is probably not accurate. Modular synthesizers have used frequency modulation (FM) since their inception. Low-frequency FM produces nice vibrato; audio-rate FM creates interesting timbres. But exponential FM alters the perceived fundamental frequency of an oscillator; linear FM is required to alter the timbre while staying on pitch.
In the early 1970s, John Chowning discovered, using digital oscillators, that linear FM through the zero point produced very diverse and pleasing timbres. Yamaha further developed this technology, but while they continued to call their implementation "FM," they were actually using phase modulation.
Let's look at some oscilloscope photos. If I frequency-modulate a sine wave with another sine wave, the 'scope shows the effect on the waveform: the modulator on the bottom in blue, the carrier in yellow on top.
A big problem with through-zero linear FM is that as the modulator's pitch rises, it has less effect on the carrier's timbre. The solution, as Yamaha discovered, was to modulate the carrier's phase rather than its frequency, directly changing the location in the carrier's waveform instead of the rate at which it moves through that waveform.
As a final example, here's the waveform when a sawtooth is modulated by a violin sample. The frequency and sharp edges of the sawtooth are maintained, but the complex timbre of the violin is added.
"When first planning Assimil8or, I thought phase modulation might be a neat feature. I was really surprised to find almost nobody had taken advantage of phase-modulated sampled sounds. My first few experiments were really exciting, and I hope you have fun exploring this capability." (Dave Rossum)