MOTM

P*IA sold a kit back in the 80s that generated 8 triangle and 8 
sawtooth waves that were phase locked 45 degrees apart. I still have 
one (somewhere). It's conceptually simple to generate these waves 
digitally. Going from memory, the circuit worked like this...

A sawtooth wave is just a counter going up (or down) that rolls over. 
A triangle wave is created by XOR-ing the most significant bit of the 
sawtooth wave with the other bits. To get the phase offsets, just add 
a constant to each sawtooth. The P*IA kit used an 8 bit counter, so 
the offsets for each sawtooth were: 0x00, 0x20, 0x40 ... 0xE0.

The biggest limitation I found was with the 8 bit resolution of the 
waves. There was quite a bit of zipper noise, particularly when the 
VCAs were closing down (I was using SSM-2020 chips for VCAs). I'm 
sure somebody could build one today with 12 bit (or better) 
resolution. Perhaps a future MOTM module?

At 8:40 AM -0500 3/6/01, Microtonal wrote:
>Shepard tones require 8 or more phase locked triangle and sawtooth waves
>driving 8 oscillators and amplifiers.  MOTM and no other modular that I know
>of has phase locked LFOs, though Doepfer has a dedicated Shepard Function
>generator, model A-191.  The magazine Polyphony (now Electronic Musician)
>had a design and maybe even a kit ages ago to create the phase locked
>control voltages, but you still needed 8 VCOs and VCAs to complete the
>effect.
>
>The effect is not limited to infinite pitch effects, you can use if for
>infinite phasing and other effects as well.
>
>John Loffink
>microtonal@...
>
>>
>>  Does anyone know how to patch existing MOTM modules to create the
>>  Shepard Function "Barber Pole" effect? Or any other non-repeating
>  > patches? I like listening to my synths play themselves...
>>
-- 
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Scott Juskiw                                            scott@...