[sdiy] Triangle wave octave generation (frequency doubling)

[Scott Nordlund]

This is somewhat off topic, but related to the "voltage-controlled integrator" style DCOs used in some Italian synths, like the SX1000...

What if one were to take two of these kinds of oscillators, and cross-couple them so that the integrator reset pulse from oscillator B also resets oscillator A?  This would be conceptually like oscillator sync, but different since oscillator A will still reset according to its own clock pulse.  Of course the integrator voltages could also be boosted to keep the output level from dropping, and possibly also introducing a clipping effect.  

I haven't tried this yet, but I have noticed that it would be fairly simple to add to my Siel DK 600 (diodes and CMOS switches, maybe a pot, I could even solder the wires to the test point pins).  Anyone tried it?

----------------------------------------
> Date: Wed, 27 May 2009 16:59:48 +0100
> From: neil.johnson97 at ntlworld.com
> To: tom at electricdruid.net; synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] Triangle wave octave generation (frequency doubling)
> CC:
>
> Hi,
>
> Tom Wiltshire wrote:
> [...snipped...]
>> Does anyone know any synth/organs that have used this triangle-
>> folding trick? I've often wondered why more organs didn't use it,
>> since they often filter the square waves from flip-flop dividers to
>> try and get sine waves - you'd get a better result if you started out
>> with less harmonics to begin with.
>
> I achieved something like this with a sawtooth in my modified Jen SX1000. In my case I watch the rising sawtooth with a comparator, and when it reaches a set level I force a reset. There's also a matching variable gain amplifier to compensate for the reduction in amplitude.
>
> The result is that I can increase the frequency by a nicely variable amount. For example, if the threshold is set to half the full amplitude, the frequency of the sawtooth is doubled. If set to 1/3, the frequency is x3, and so on. I found that by the time I'm at x10 it all gets a bit squawky.
>
> I guess if you *only* had an integrator you could multiply nicely by any number. In the SX1000 there is the master oscillator which provides a reset to the integrator at the desired frequency, so the effect is more like sync. I might try an experiment to disconnect the main oscillator reset...
>
> Schematics online if you're interested.
>
> Cheers,
> Neil
> --
> http://www.njohnson.co.uk
>
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