Synth Education

When FM Feeds Back

When an oscillator uses its own output to bend its own pitch, a pure tone becomes a buzzsaw. One knob takes you from clean to screaming.

One Oscillator, Infinite Timbres

Turn the feedback knob up slowly. Watch the waveform change shape.

At zero, it's a pure sine wave. As you increase feedback, the oscillator starts using its own output to push its frequency around. That adds harmonics.

By the time you reach 0.3, it looks and sounds like a sawtooth. The feedback is generating all the harmonics a sawtooth has.

Keep going past 0.5 and the harmonics become unpredictable. The waveform gets jagged and complex. This is the beginning of chaos.

What's Happening Inside

The oscillator uses its own output to bend its own pitch, which adds harmonics. On every audio sample, it does this:

1. Read the previous output sample (a value between -1 and +1).
2. Multiply it by the feedback amount and the base frequency.
3. Add that to the base frequency. This is the instantaneous frequency for this sample.
4. Advance the phase by that amount. Output sin(phase).

With zero feedback, the frequency never changes. You get a pure sine. With moderate feedback, the output pulls the frequency up when the sine is positive and pushes it down when negative, creating the asymmetric shape of a sawtooth. With high feedback, the modulation is so strong that the frequency swings wildly each sample, and the waveform becomes chaotic.

Why It Matters

The Yamaha DX7 used exactly this trick. Operator 6 could feed back into itself, and that single feedback loop was how most DX7 patches got their sawtooth-like excitation source. Without it, FM synthesis would be limited to bell-like tones built from sine waves.

Feedback FM is efficient: one oscillator with one parameter produces a continuously variable spectrum, from pure sine through all-harmonic sawtooth to noise-like chaos. No wavetables, no filters, no extra oscillators needed.

References

• Wikipedia: FM Synthesis