Through-zero FM

Through-zero FM is frequency modulation in which the carrier's frequency is allowed to pass through zero and come out negative, continuing the phase in reverse rather than stopping.

Because the carrier keeps moving linearly through zero, the timbre stays musical across the whole modulation depth and retraces the same sonic path on either side — which ordinary FM, whose carrier stalls when its frequency reaches zero, cannot do. Tables of clean, harmonically simple frames such as plain sines are the usual carriers, since every carrier harmonic multiplies into a thicket of sidebands.