[sdiy] Top Octave Generator (was Chinese MG-1s??!?)

[ChristianH]

and, since those micro controller chips are dirt cheap, you could
distribute the 13 channels to 3 or 4 ATtiny processors (there are 8 pin
DIP versions). Running at 16 MHz, it should be feasible to do some 4
counters and port output in a 2 MHz loop.

Chris



On Wed, 30 Apr 2008 16:43:16 +0100 Tom Wiltshire <tom at electricdruid.net>
wrote:

> You definitely get a squarewave output. I thought that was typical of  
> TOG chips?
> I've heard of some that produce a 30% duty cycle, whilst most are  
> 50%, but I've never
> heard of a triangle or sine Top Octave chip.
> 
> The division ratios vary a bit depending on the master clock  
> frequency, but here's a set for a 2MHz clock.
> 
> C low	÷478
> C#		÷451
> D		÷426
> D#		÷402
> E		÷379
> F		÷358
> F#		÷338
> G		÷319
> G#		÷301
> A		÷284
> A#		÷268
> B		÷253
> C high	÷239
> 
>   2 MHz divided by 478 results in a frequency of 4184 Hz which is a  
> C5 note
>   2 MHz divided by 239 results in a frequency of 8368 Hz which is a  
> C6 note (the highest one on the organ usually).
> 
> Obviously if you double the master clock frequency, you can also  
> double the division ratios and keep the note outputs the same. Since  
> you can then adjust not-quite-right ratios to odd numbers in-between,  
> higher master clocks will give you better accuracy.
> 
> The Old Crow code produces the tones of an octave, but shifts it down  
> a few octaves, which makes life much easier since it gives you more  
> time to calculate the outputs. I don't know the detail on other  
> processors (and maybe AVRs would be better for this job?) but the  
> basic PICs will only run to 20MHz, with a 5MHz instruction cycle. If  
> we ran the chip at 16MHz to give us a convenient 4MHz instruction  
> cycle, we'd still need to get data out every other cycle.
> I'm loathed to say it can't be done, since PIC hackers often just  
> take that as a challenge and then prove you wrong, but if anyone can  
> pull that off, it'd be an impressive effort.
> 
> If you were willing to accept some variation in duty-cycle between  
> outputs, that might make life easier. Given the heavy filtering in  
> most electronic organs, I doubt it'd be a huge issue.
> 
> It's definitely a tricky one, or someone would have managed already!
> 
> Regards,
> Tom
> 
> 
> 
> 
> 
> 
> 
> >
> > Am I wrong? What am I missing?
> >
> > On Fri, 07 Jul 2006, Bob Weigel posted a followup  to a thread titled
> > "Top Octave Synthesizer" that ended with the following statement:
> > "There are much cheaper solutions for sdiy projects so if there's no
> > particular reason to need *that* one I'd look into something like
> > that. -bob " (He was referencing the S50240 chip)
> >
> > What are the cheaper solutions he's speaking of? The one thing the TOG
> > seem great at it is keeping the system in global tune without a lot of
> > tricky sync work between subsystems.
> >
> > Tom