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Think of a patch where you feed the output of one oscillator (the modulator) into the frequency control voltage input of a second oscillator (the carrier). As the waveform output of the modulator rises above zero volts, it is added to the normal pitch control voltage for the carrier, and the pitch of the carrier goes up. As the waveform output of the modulator goes below zero, it is subtracted from the normal pitch control voltage, and the pitch goes down. But what happens if the result of subtracting the modulator from the pitch control goes below zero volts?

In many oscillators, the carrier would slow down to a standstill, and wait there until the combined input voltage went above zero again. But in an oscillator that explicitly says it implements through-zero frequency modulation, the carrier will start playing backwards – in essence, a negative frequency. (The same applies to modulating the frequency of an LFO.) This generally produces a more pleasing result, and is a desirable characteristic for an oscillator. It’s relatively easy for a digital oscillator to implement through-zero FM; it’s trickier for an analog oscillator – but it’s becoming more common.

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