[sdiy] Interesting article on top octave generators.

[Tom Wiltshire]

I think the tonewheel "n/m" thing is actually quite a bit harder to do. The obvious way would be an NCO, since that gives you the fractional divider structure, the n/m, but then you have a problem with jitter, which is not something the gearwheels in the original generator have to deal with. With rotating gears, you can multiply a frequency *up* or *down* by whatever tooth-ratio you like, within reason. Since in that situation you might be after sinewaves instead of squares, perhaps some kind of pulse-density-modulated sine output would be ok. Yes, there'd be some small amount of jitter, but after the PDM is filtered, it might well be inaudible.
It's actually a situation where DSP with a fairly low sample rate is more than acceptable, since we're only producing sinewaves and the highest sine frequency in the Hammond organ isn't that high - just under 6KHz. Hence a fairly modest DSP running at even a 20KHz sample rate could generate the sines with no aliasing.

The classic divide-down TOG can be done without jitter since it's always a whole number of master clock pulses. With the original chips running at about 2MHz, there's enough time to calculate the outputs on a basic processor going somewhat faster (20MHz, 32MHz, whatever).

I completely agree that RPi2040 is massive overkill even for a 12-note TOG, let alone six. It's been done on an AVR, so there's really no need for such heavy firepower!


> On 27 Dec 2024, at 14:06, Roman via Synth-diy <synth-diy at synth-diy.org> wrote:
> 
> Following that route - why not make TOG replacement which produses exact frequencies of tonewheels, or better yet, switchable frequencies of every historical or another weird scales?
> Frankly, using RP2040 for 6 tone TOG is not so impressive to make video about it. Microcontroller based TOG replacements are available for decades now. 
> 
> Roman 
> 
> 
> ---- Użytkownik Tom Wiltshire napisał ----
> 
> .
> > 
> > 
> > I agree that it does seem a bit crazy to replace some basic logic with a whole uP, but the arguments for doing it are pretty convincing. Doing twelve 9-bit dividers in hardware is going to be a lot of chips, whereas the uP is only one. That makes it much easier to fit inside whatever it is you're fixing. And it's cheaper - a boardful of simple logic costs *more* than a processor these days because of the economies of scale. It *is* crazy to use a million transistors when 10,000 would do, but when they all come on one chip and costs pennies, it starts to make a lot of sense.
> > 
> > 
> > It terms of the technological limitations, all of the uP-based divider solutions I've seen are pretty much exact clones and just as limited as the original chips! They certainly keep the original division ratios and consequent frequency error.
> > 
> > 
> > There are a few original divider chips out there, but the people that have them want $30 a chip or worse for them. $1 processor wins over that every time.
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