[sdiy] MXR Pitch Transposer Schem/Circuit

Tom Wiltshire tom at electricdruid.net
Mon Mar 11 12:09:29 CET 2013


On 11 Mar 2013, at 10:42, Steve Ridley <spr at spridley.freeserve.co.uk> wrote:

> 
> 
>>> I think something like this could be done with BBDs but the
>>> artifacts of the modulated clock would surely be "interesting"!
> 
> I vaguely remember seeing a circuit (or app note) for this. It used two
> BBDs and was intended for dictaphones - so they could play stuff back
> slow enough to type but keep the pitch fairly constant. I guess quality
> wasn't critical.
> 
> Steve

That was what I'd thought of doing too. I've done some experiments recently using the new NCO peripheral on the enhanced midrange PICs to generate a BBD clock, which works very nicely. You can wrap up a VC digital LFO and a BBD clock chip all onto one 8-pin DIP. This discussion reminded me of that.

If there was more than one NCO available (big if - I don't know if there's a PIC with two NCOs), you could organise the chip to provide the ramp-modulated clocks, and probably the crossfade signals too. Turning all that into software makes the whole thing much more manageable. It'd be a PIC, maybe a DAC (for the crossfade signals? Or use PWM?) a couple of BBDs and a LM13700 (two VCAs for the crossfading). Four or maybe five ICs.

Using modern technology to get a utterly obsolete and lo-fi pitch shifting technique!! Isn't that what it's for?!

Tom




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