The VZ-1 takes a hybrid approach to its sound generation. Where the CZ line used Phase Distortion Synthesis and the Yamaha DX line used Phase Modulation (incorrectly advertised as FM), the VZ-1 uses Phase Distortion for ring modulation and waveform generation but true Frequency Modulation Synthesis to generate harmonics.
Is my VZ-1 broken? Have I discovered an implementation quirk that nobody on the internet has ever complained about? No and no. My VZ-1 is not broken and on closer inspection at least 1 person on the internet is aware of this problem, and they figured it out for me! Digital synthesis expert acreil got roped into a Gearspace thread about the VZ-1 back in 2021. The discussion starts off not being very interesting until acreil shows up, who then repeatedly gets contradicted by someone who loudly says things that are wrong, and then to put that all to rest acreil finds the relevant Casio patent for the VZ series and drops the bomb that the VZ's don't do phase modulation at all. They do wave shaping.
Excuse me what now? Yes, pedantically wave shaping is phase modulation with a carrier frequency of 0Hz, but why would you call that phase modulation?
The VZ-1 takes a hybrid approach to its sound generation. Where the CZ line used Phase Distortion Synthesis and the Yamaha DX line used Phase Modulation (incorrectly advertised as FM), the VZ-1 uses Phase Distortion for ring modulation and waveform generation but true Frequency Modulation Synthesis to generate harmonics.
https://www.vintagesynth.com/casio/vz-1
also see the former blogpost of the OP:
Casio is lying to us
Is my VZ-1 broken? Have I discovered an implementation quirk that nobody on the internet has ever complained about? No and no. My VZ-1 is not broken and on closer inspection at least 1 person on the internet is aware of this problem, and they figured it out for me! Digital synthesis expert acreil got roped into a Gearspace thread about the VZ-1 back in 2021. The discussion starts off not being very interesting until acreil shows up, who then repeatedly gets contradicted by someone who loudly says things that are wrong, and then to put that all to rest acreil finds the relevant Casio patent for the VZ series and drops the bomb that the VZ's don't do phase modulation at all. They do wave shaping.
Excuse me what now? Yes, pedantically wave shaping is phase modulation with a carrier frequency of 0Hz, but why would you call that phase modulation?
https://blog.jacobvosmaer.nl/0028-mysterious-vz-1/