Octave Down and Detune Idea

edited July 2009 in WTPA1 Mods
I have an idea, for generating octave down and/or a detuned outputs.

Taking each bit before it hits the R2R ladder, buffering and splitting it into say three (that's just for keeping the additional parts count down - sky's the limit theoretically) separate 'voices', and phase shifting (subtracting X amount from the rising edge side of the bit when swung high, and adding it to the falling edge) each bit, and dividing it after that to make an octave down. After this processing, the 'dry' bits and each set of the adjusted ones would come to their own R2R, with levels out for each, all to be summed at the output op amp ultimately.

The octave down is easy enough - for that I'd use four CD4520 dual counters per 'voice' (a la this schematic "http://www.milton.arachsys.com/nj71/img/Jen/SX1000/sch/osc_subosc.gif" - a 4520 is like two 4024s), though the phase shift I'm scratching my head on. I need the modulation frequency to be referenced to each individual bit, right? And at that, all those in tempo to a master detune offset? And which kind of gate would do that add/subtract work, or do I need some additional S&H/latch circuitry?

I reckon this could be done within the firmware with an output multiplexer/S&H scheme, and I suppose a clock multiplier (X # of channels) to keep the multiplexer ahead of the game when you drop the clock rate down, but my coding skills are nonexistent for the time being :X (not to say I won't try tackling it, someday!)


  • After some thought, I realize what is needed more than shifting the phase is adding to the width of the pulse - albeit, for both the hi and lo portions. Ideally, this widening would be arbitrary in length (it would get subaudible after a while though), and would envelope detuning by a matter of cents to tuning down a matter of semitones to one or more octaves down.

    There remains the problem of how this would be done, and I'm tempted to considering tacking on an another 8-bit PIC just for this alone. I could pull off the multiplexing outputs I mentioned too, and probably tune/octave up as well. Seven detuned saws screaming like a VIrus sound good to anyone (even an 8-bit saw is good enough!)

    Which leads to a newer question, where to begin figuring that out, or is there a better way for any of this?
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