Through-Zero FM

[Michael B. Irwin]

Linear through-zero FM is the essence of the very popular Yamaha DX
synthesizers which everyone is familuar with. The capability to go
through zero (to reverse oscillator phase) makes it possible to produce
spectra with inharmonic partials (clangorous sounds) as well as to
produce the more typical spectra with harmonic overtones. One neat thing
about it is that the spectrum evolves  in a different manner than you
would get by sweeping the cutoff freq of a VCF in a normal synthesizer
setup. Supposedly this more closely mimics the behavior of acoustic
instruments. This type of FM is best suited to digital implementation
due to the freq stability and phase stability requirements for
reproducible (and controllable) sounds. It would be neat to make an
analog module that does this, consisting of 2 linear through-zero VCO's,
2 VCA's and 2 envelope generators.