What you are talking about here is “regular” audio-rate FM though (no phase modulation), and while it’s interesting on its own you won’t be able to achieve the glassy tones that you can get with proper through zero frequency modulation.

The DX family, Digitone, Preen, Opsix, VolcaFM, Akemie modules etc do TZFM. Most analog synths (apart from certain eurorack modules) only do regular exponential or linear FM. There is a big difference in the kind of tones you get from the two.

Sorry but I suck at describing the difference. Hopefully some of the smarter geezers on here will be able to explain it better. This is the best description I could find right now:

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