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

[Tom Wiltshire]

On 30 Apr 2008, at 16:08, Tom Corbitt wrote:

> What's the best way to recreate a TOG?
>
> Looking at past answers, the general trend seems to be towards using
> micros like pics to handle the dividing. I've seem the "old crow"
> code, but I've yet to see a post where anyone indicated that they'd
> taken this idea to completion and used it successfully to replace a
> common top octave chip.  It would seem to me that this approach would
> result in a square wave out, that you'd still need to filter back into
> a sine before using it musically.

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
>
>
>
>
> On Wed, Apr 30, 2008 at 9:10 AM, Metrophage <c0r3dump23 at yahoo.com>  
> wrote:
>>
>>  --- anthony <aankrom at bluemarble.net> wrote:
>>
>>>  The filter is what makes the MG-1 golden.
>>
>>  What makes the filter golden is patching a VCO to modulate the top
>>  octave chip, modulating the VCF cutoff with the LFO, and patching  
>> the
>>  envelope to modulate the VCF resonance. Whee!
>>
>>
>>
>>        
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