[sdiy] Buchla 295 10-band comb filter topology

[Ian Fritz]

Many years ago on this list I asked why a distinction was made between PM and FM, as they are equivalent, phase being proportional to frequency (at fixed frequency, of course).

In response, Juergen Haible pointed out the difference only becomes relevant when considering what happens to the signal when you go up and down the keyboard. For FM the waveform stays the same. For PM it does not. This behavior can easily be seen in the standard textbook equations going back to early radio theory.

The book by Chowning and Bristol, “FM theory and applications” deals extensively with the DX7. It is specially noted more than once that the DX7 waveform does not change with keyboard pitch. So from this fact and the book title I conclude the DX7 does indeed produce FM signals.

So why is there this apparent disagreement? Does the DX7 waveform change with keyboard pitch or not? I’ve asked this question before but never got an answer. I do not have a DX7, but the waveforms of the plugin emulations I’ve seen are constant.

I’ve looked at a Yamaha patent and seen how the computational engine works. It does, of course, calculate in terms of changes in phase. But that does not necessarily mean that what is calculated is a standard PM signal, does it? 

Ian