[sdiy] Buchla 295 10-band comb filter topology

[Mike Bryant]

> A quick visit to Wikipedia says that PM does not alter the frequency

I think this shows the wisdom of consulting a proper text book.  PM can alter the frequency but at the rate of change of the phase modulation rather than at the rate of the modulation itself.  Thus it's a first derivative effect.

If the modulation is a sine wave, then PM is actually a cos wave and produces the same output signal.
________________________________
From: Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of brianw <brianw at audiobanshee.com>
Sent: 25 November 2022 23:42
To: Donald Tillman <don at till.com>
Cc: synth-diy mailing list <synth-diy at synth-diy.org>
Subject: Re: [sdiy] Buchla 295 10-band comb filter topology



On Nov 25, 2022, at 2:59 PM, Donald Tillman <don at till.com> wrote:
> On Nov 25, 2022, at 12:04 PM, brianw <brianw at audiobanshee.com> wrote:
>> Yes, people label the B295 as a "comb filter," but I consider that artistic wishful thinking. Note that the frequencies on the panel are basically pseudo-log.
>
> No not the people; it was Don Buchla who literally labeled it a "Comb Filter".  I'm pretty sure he meant it generically.

Ah, thanks for the correction. I've never seen a B295, and the photos online seemed to only say "comb filter" on the clones. I clearly didn't look closely enough.

I suppose "frequency response looks like a comb" is a fair description.


> 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?

A quick visit to Wikipedia says that PM does not alter the frequency, so I'm wondering how the DX-7 implemented that. I suppose the only way to tell would be to have a Chowning module where you could patch a DC CV into the PM input and verify that the frequency remains the same, no matter what DC value the CV input takes. Unfortunately, I don't know of any voltage-controlled Chowning-style FM Synthesis modules. That would be an interesting analog/digital hybrid.

My naive implementation of a single-operator FM on the Apple II (one modulator, one carrier) altered the frequency, so I guess I had no hope of ever building a Chowning voice (not considering the processing limits of a 1 MHz 6502). I suppose it's not too difficult to imagine a different code implementation that would not alter the frequency (unless the modulator is a ramp wave, rising or falling).

People also discuss the difference between linear FM and logarithmic FM.


> And Leo Fender famously swapped "Tremolo" and "Vibrato".\

That has bugged me for ages. I wonder if Leo can blame the psychoacoustic phenomenon where small changes in pitch are indistinguishable from small changes in amplitude. Then again, a whammy bar is hardly subtle.


> This field has rich history of goofing up the terminology.

Just like the fashion industry: naming fluorescent color dies as "neon." I mean, did they think they'd never have literal neon in clothing? Now that EL wire (electroluminescent wire) has been around for a decade or more, you can pretty much pull off "neon like" clothing. Of course, neon/plasma is not electroluminescence, ... and there's also the catch that neon is used to excite the fluorescent coating in a fluorescent tube.

Brian


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