Synth Education

The Edge Where Tone Becomes Noise

There is a feedback level where the sound starts breaking apart. Just below that edge is where the richest textures live. The threshold shifts with pitch.

Finding the Edge

FM feedback turns a sine wave into something richer by feeding its own output back into its phase. At low feedback you get a clean, slightly buzzy tone. As you push higher, the waveform starts to fold over on itself, doubling its period, then doubling again. At some point the pattern stops repeating and you get noise.

That transition point is not gradual. It is a sharp boundary. Just below it, the sound is dense and alive. Just above it, the sound falls apart into hiss. The sweet spot is right at the edge.

Use the auto-sweep button below. It slowly crawls the feedback knob from zero to full over ten seconds. Listen for the moment the tone cracks. Watch the pattern display: you will see the waveform split and multiply before it dissolves.

The Threshold Moves

Try different notes after the sweep. You will notice the cracking point happens at a different feedback level for each pitch. Higher notes break apart sooner. Lower notes hold together longer. This is not a bug. The feedback loop is sensitive to how many samples fit in one cycle, and that changes with frequency.

This means if you want to use feedback FM in a patch, you cannot just set one feedback value and forget it. You need to understand where the edge sits for the notes you are playing.

What You Heard

The pattern display shows something called a bifurcation. At low feedback the output traces one clean loop. As feedback rises, that loop splits into two, then four, then eight. At some point the splits happen so fast that the pattern fills up with dots. That is chaos.

The math behind this is the same as the logistic map, population dynamics, and dozens of other systems that transition from order to chaos through period-doubling. FM feedback is just one more example. But it is one you can hear.