[sdiy] Divide down question

[Tom Wiltshire]

One thing that you can do to get pulse waves is to start with a  
standard square-wave divide-down scheme, but then run pairs of square  
waves at octaves into AND gates. If you feed a square at frequency f  
and one at 2f into an AND, you get a 25% pulse wave out the far end.  
You could use 4f and have 12.5% if you prefer. Given that you've  
already got all the octaves you need, this seems like the easiest way  
to get divided-down/up pulse waves.

T.

On 25 Aug 2009, at 09:55, Scott Nordlund wrote:

>
> 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.
>
> ----------------------------------------
>> 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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