Term for frequency modulation as often implemented on analog synthesizers. A common voice architecture for analog synths is to have two VCOs which are both available as audio signal sources, but it is also possible for one VCO to serve as a frequency modulation source for the other. When implemented in this way, the result is often referred to as "cross modulation". Such an implementation makes FM possible without needing a separate oscillator to serve as the modulation source.
Although it was not the first synth to implement such an architecture, the term was first widely used with the Sequential Circuits Prophet-5, introduced in 1978.