AHG! (Arbitrary Harmony Generator)

[earthtonesaudio]

One quadrature triangle wave LFO, two PT2399 delay chips (delay time set by LFO) and some crossfade/mixer circuitry:

For one channel of the mixer, "listen" only to that PT2399 which is currently receiving a "rising ramp" signal.
For the other, only listen to the PT2399 which is currently receiving a "falling ramp" signal.
When one of the ramps gets close to the end of its travel, crossfade each channel so it listens to the other chip.

I think this would give you (at one LFO frequency) one octave up and one octave down.  At other LFO frequencies, you'd get other amounts of shift up/down. 

If the triangle wave is symmetric, the amount of up shift equals the amount of down.  If you change the symmetry, then up and down would be shifted by unequal amounts.


So this seems pretty awesome.  Anyone know of a quadrature triangle LFO which is both easily adjustable (i.e. single control for frequency, independent of amplitude) and has adjustable waveform symmetry?  Maybe one of the Molten Voltage or Electric Druid offerings does this, or could be adapted for quadrature output?

[Paul Marossy]

Interesting idea. I wonder what it would sound like!

[PRR]

Not sure what "harmony" you will get?

Quad tri:

The obvious method is two tracking integrators etc. Perhaps a double-frequency divider.

A non-obvious technique is to trigger off the slope and phlip the faze. It seems neat until you try to make it "perfect".

http://www.musicsynthesizer.com/Waveshapers/quadrature.html
http://mypeoplepc.com/members/scottnoanh/birthofasynth/id29.html

[earthtonesaudio]

The obvious method is two tracking integrators etc.

Of course! 

...Obvious now that you've said it anyway.  I was drawing a blank before.  This helps, thanks!

[R.G.]

google "barberpole phaser" "Shepard Function" "Shepard Tone" and

http://machines.hyperreal.org/manufacturers/Paia/images/shepard.txt

[ibodog]

A buddy of mine did a remix of a "techno" track I wrote about 10 years ago.   He used something to get the Shepard Tone and you can hear it towards the end of the track: http://formantfive.net/wp-content/uploads/2007/04/18-flying-ascension-mix.mp3

[PRR]

> ...Obvious now that you've said it

Well, maybe not so obviously obvious. I thought I had seen the idea, but now I think it came from fried insulation in my memory. One thought shorted to another.

The early ARP synth gave both triangle and ramp waves. While there are ways to get one from the other, their technique (IIRC now) was to integrate control current to make the triangle into +/-10V threshold flip-flop as the basic timing, and run the same current into a 2X cap switch-dumped with triwave neg tips to make the ramp.

OTOH, four controlled integrators in a loop can make very sweet sines in quadrature.

I pictured cascaded integrator triwave, but that was just brain-burp.

You run the one integrator as the classic triwave. Use a fast comparator to sense zero-cross, and integrate that. Presto, 90 degree triwaves. (The 180 and 270 are trivial inversions.)

The Shepard stuff which R.G. points to is fascinating, and has musical parallels to what you seem to want to do, but octature(?) may be over the top.

[earthtonesaudio]

The Shepard stuff which R.G. points to is fascinating, and has musical parallels to what you seem to want to do, but octature(?) may be over the top.

I was wondering about that.  From what I've read about Spin Semiconductor's FV-1 chip, it seems that they do their harmony generation using only two ramps.
Maybe they can get away with it because the ramps are so precisely controlled, I don't know.