Through-zero FM is a form of frequency modulation in which the carrier’s frequency can swing below zero Hz, which reverses the direction in which its waveform runs. The oscillator can follow a modulator through zero without breaking up.
Linear through-zero FM keeps the average pitch stable even at high modulation depths, unlike standard exponential FM. The result is cleaner sidebands, stable tuning, and more predictable timbres. It needs an oscillator designed to support it.
