[sdiy] Buchla 295 10-band comb filter topology

[Donald Tillman]

> On Nov 25, 2022, at 3:42 PM, brianw <brianw at audiobanshee.com> wrote:
> On Nov 25, 2022, at 2:59 PM, Donald Tillman <don at till.com> wrote:
> 
>> Just like "FM Synthesis" is really phase modulation, but the term is used generically.
> 
> This has always baffled me. I understand the difference between frequency and phase, but my math skills aren't great enough to answer the question: Is there any way to phase modulate without frequency modulation? ... or the converse: Is there any way to frequency modulate without phase modulation?

It baffled Chowning, too... in his famous paper he describes "frequency-modulated wave" in the first equation, but it's really an equation for phase modulation.

In practice, the difference is day and night.  Phase modulation is much more musical.  And feedback around a frequency modulation VCO has the potential can get stuck at 0 Hz.

> A quick visit to Wikipedia says that PM does not alter the frequency, so I'm wondering how the DX-7 implemented that.

You can scope the output of a DX-7, and it's pretty obvious.

The big deal about the DX-7 was that they found a way to completely avoid real-time multiplies.  The entire implementation is single-cycle add and ROM lookup operations.  And those are so easy and quick they were able to implement 16 notes * 6 operators => 96 phase modulation oscillators in 1983 technology.  For $2500.00.

  -- Don
--
Donald Tillman, Palo Alto, California
https://www.till.com