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[Blaze]

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[HertzDevil]

See this. However, the real FM frequency is not identical to a multiple of the FM rate due to the way the frequency modulation unit of the FDS works, so the table of FM rates that sound non-dissonant is different for every modulation shape table and modulation depth, and there is not much you can do without trial-and-error (partly because even the same note admits overtones in both the note and the FM rate parameter).

[Blaze]

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[HertzDevil]

The frequency modulation goes at the same rate as the FDS wave itself; if the modulation table never overflows/underflows, such as the default sine table, the FM frequency is simply the same as the wave frequency, which can be obtained from the register display. Otherwise, the FM frequency will be slightly different depending on the FM depth.

Almost all my FDS works use the channel for FM synthesis.

[Blaze]

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[Xyz]

Mod rate (I0xJxy) = frequency. Unless were talking about mod depth (Hxy)
Mod Depth on FDS is similar to the Modulator slider on vrc7. It defines the intensity of the vibrato and hence, the intensity of the modulation. You could make a crude mod wave ADSR by changing the Mod Depth at high speeds

[HertzDevil]

Truly relative modulation rate is possible if you are willing to write that implementation. Notice how the modulation rate ($4086 / $4087) is updated according to the frequency ($4082 / $4083) with a multiplication analogous to the 5B Jxy auto-envelope in 0.5.0 beta; it would cost a few more hundred CPU cycles to do it on every tick.

[Blaze]

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[HertzDevil]

You can; here is an example of the four modulator-to-carrier ratios 1:4, 1:2, 3:4, 1:1.

[Blaze]

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