A concept in which a basic parameter of a sound producing or processing component can be taken down to zero and then into negative values, and the component will do something sensible with the transition. A basic example is that of the ring modulator, which can be regarded as a VCA capable of both positive and negative gain (see Four quadrant). When the ring modulator is in the negative gain range, it amplifies the input signal according to the absolute value of the gain, but the output is inverted.
Cynthia in 2007 introduced a VCO called the Zeroscillator. This has a Volts/Hz modulation input by which the resulting frequency can be driven into the negative. When the frequency value is negative, the output is at the absolute value of the frequency, but the output waveform "runs backwards"; it is the mirror image of the normal waveform. Transitioning the frequency value back and forth across the zero boundary results in a series of mirror-reflected partial waveforms, and doing this at audio frequencies produces a variety of unique timbres. There are now several through zero VCOs for modular synthesizers available.