[sdiy] Simple MIDI-syncable LFO...?

[Tim Parkhurst]

> -----Original Message-----
> From: Blandon Ray [mailto:blandoon at gmail.com]
> Sent: Wednesday, October 13, 2004 11:37 AM
> To: Synth-DIY List
> Subject: [sdiy] Simple MIDI-syncable LFO...?
> 
> Hi everyone,
snip
> One idea I had was to start with John Blacet's MIDI-to-SYNC PIC chip,
> divide the 24PPQ pulse down, and find some way to convert the
> resulting square wave to sawtooth, triangle, etc. But I can't locate a
> circuit that does that reliably at very low frequencies... then again,
> it doesn't have to be perfect, just good enough to be useful as a
> modulation source.
> 
> Thanks
> Blandon
> 
/snip


I'm not sure how well this would work since I haven't ever tried it, but
maybe you could use the 24PPQ pulse to drive a binary counter and an R2R D/A
converter. If you just have the counter count up and reset, you get a saw
wave; have the counter count up and then back down and you get a triangle.
You would put a low-pass filter (a glide circuit, basically) on the output
to smooth the waveform. The trick would be having the glide adjusted to
properly smooth the waves over the full range of frequencies. Then again, it
might be nice to be able to manually adjust the glide to allow
'stairstepping' in certain cases, or 'over smoothing' in others. The other
trick would be getting the counter to count 24 steps and then reset (or 12
and then turn around). A little extra logic, but not too difficult. Oh, and
some logic to divide the 24PPQ pulse so that you could get the LFO to cycle
at rates other than once per quarter note (this would be way to fast in most
cases). John B shows this division circuitry in the ap notes for his MIDI
chip.

This link shows a guitar effects page that describes a similar 'digital LFO'
made from ring counters. This makes getting a triangle wave very simple,
although the saw wave goes away. Still, this might be the way to go, or at
least some good inspiration.

http://www.geofex.com/Article_Folders/LFOs/psuedorandom.htm

Any comments? Anyone out there built something like this? I think this might
be a neat way to do a quadrature LFO too (one clock, four counters & four
D/As, pretty cheap and easy). With a little fiddling, you could make the
phase between the outputs adjustable (albeit in resolution-limited steps,
but still might be fun). 

Whadaya all think?


Tim (over smoothed) Servo

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