[sdiy] Triangle wave octave generation (frequency doubling)

Neil Johnson neil.johnson97 at ntlworld.com
Wed May 27 17:59:48 CEST 2009


Tom Wiltshire wrote: 
> 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.


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