[sdiy] Divide down question

[Tom Wiltshire]

On 24 Aug 2009, at 16:00, cheater cheater wrote:

> Hey guys,
> in a divide-down architecture, every key has a divide-down 'level'
> which then gets used in the same note but an octave lower.
>
> Does any amplification happen at each stage?

Not really, or at least not in the analog sense. The division is done  
by flip-flops, e.g. digital circuits, which means the output is the  
same level as the input, so no amplification is required.

As an aside, I once considered doing a "divide up" scheme, using a  
triangle wave master oscillator at a low frequency fed to a series of  
precision rectifiers. This provides triangles in octaves going up,  
and does require gain, since the rectifiers cut the signal level in  
half. I hoped that the reduced harmonic content of triangles when  
compared with square waves would give me a smoother tone. I built the  
circuit and proved the concept, but never did anything with it.

> If I press multiple keys from the same 'class' (e.g. C1, C2, C3) will
> any 'amplitude stealing' happen?

No. Essentially all the tones are fed to a big mixer. Imagine a  
simple inverting op-amp mixer with lots of 100K input resistors and  
100K feedback resistor for unity gain. Now imagine what happens if  
you feed the same tone into two of the input resistors. Those two  
resistors are effectively paralleled, which will halve their joint  
value and cause the signal fed to them to double in level - exactly  
what is required.

> Where can I read more about divide-down synths? I am not yet good
> enough to read the schematics just like that; I tried googling around
> but couldn't find much of substance.

You might have better luck looking at some combo organ sites, since  
they're mostly done this way.

Good luck!
T.