Okay, since you do seem interested, I will try to explain with more detail:
When the frequency of an oscillator stays constant, its wavelength stays constant.
With phase modulation synthesis, both carrier and modulator stay at the same frequency always. Their wavelengths always match, they always stay synchronized.
With frequency modulation, the frequency of the modulator is constant, but the frequency of the carrier is always changing. The wavelength of the carrier is much more complicated in this case, it is an integral related to the changing frequency over time.
So, with a perfect sine wave modulator, the "ideal" mathematical version, the wavelength of the carrier adds back up to where it started; there's equal time spent in lower frequency and higher frequency and it balances out.
With an integer approximation of a sine wave modulator, they only approximately balance. It's similar to how
Euler integration step by step of a physical trajectory will have an error compared to the ideal continuous integral.
What happens when the wavelength of the modulator does not match the wavelength of the carrier? Its phase shifts a little bit each wavelength. How much it shifts depends on how much error. The amount of error is a bit chaotic with these kinds of systems, which is what I mean when I say every different note is going to have a different speed of phase shifting.
Anyhow, if the phase shifts, the timbre of the sound changes. If the phase is shifting slowly, you get a slowly changing sound. If it shifts quickly, you get a quickly changing sound.
There do exist ways to make integer approximations where the errors will balance (and even systems that don't tend to have a few "lucky" notes that are stable), but it requires careful design with this in mind. You're not likely going to get something that does that in the SNES or FDS.
Does this explain what the problem is and how it occurs?
Bregalad wrote:
rainwarrior wrote:
(On FDS it's really bad, but on these SNES examples it seems much slower, less of a consistency problem.)
On what basis do you make this claim ? Please show demos attempting to do that and explain what the problem is. The previously mentioned demo by KungFuKirby is actually very impressive and generate sounds close to what the Genesis can do on the SNES.
It was quite a long time ago that I did test of this with the FDS and I did not document them. Sorry. You can easily operate the FDS modulator with Famitracker, but you'd need to use a real FDS to get an accurate test.
RushJet1 tried making music this way. His goal wasn't to make perfect FM synthesis, it was just to use high frequency modulators to make interesting sounds, and he did make some music with it. He was dissatisfied with how different it sounded on FDS and Famitracker though, so I think he kind of gave up.
In the demo by KungFuFurby you can hear the phasing problem constantly. You can also here it in psycopathicteen's demos in this thread. I alerady commented on this (i.e. the phasing is slower, i think cause SNES has a lot more precision for the modulator).
Whether the phasing is objectionable, musically, is a matter of taste. I was just trying to point out that it's an inherent an uncontrollable part of the sound when you do this on such a system.