[sdiy] Divide down question

[Scott Nordlund]

It could be done with sawtooth waves too:  use a comparator to make a square wave, invert it, amplify the sawtooth by a factor of two, and mix the two.  Of course the comparator's threshold has to be adjusted properly, or there will be asymmetric pulses and sub-harmonics sneaking in...

Pulse waves would be more difficult.  The thing with sawtooth and triangle waves is that you get an instantaneous one-to-one mapping of input amplitude to however many octave-doubled outputs you want.  You obviously can't do this with square or pulse waves, since the amplitude isn't continuous.  You could use a phase locked loop with a frequency divider in the feedback loop (so the frequency can be multiplied by any arbitrary number), but this needs time to adjust to changes in frequency.  It won't be instantaneous like the sawtooth or triangle-derived, and it can have objectionable glitches and artifacts.  But you could potentially get all the octaves you want in one go, deriving them from the frequency divider in the feedback loop.

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> Date: Mon, 24 Aug 2009 19:37:36 +0100
> Subject: Re: [sdiy] Divide down question
> From: cheater00 at gmail.com
> To: synth-diy at dropmix.xs4all.nl
>
> Tom,
> extremely interesting! I should think about that for a while. This
> 'multiply up' as I would call it is especially cool. Any sound
> examples?
>
> Which makes me wonder: can you do that with pulse waves..?
>
> I think you could almost certainly do that with sawtooth waves.
>
> D.
>
> On Mon, Aug 24, 2009 at 5:53 PM, Tom Wiltshire wrote:
>>
>> 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.
>>
>>
>>
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