[sdiy] Sawtooth vs. Triangle core VCOs

[Donald Tillman]

On Jan 24, 2013, at 3:09 PM, Paul McLean <keysndrums at gmail.com> wrote:

> I've been wondering about the pros and cons of these two approaches.
> Would folks be willing to comment and the advantages and disadvantages of both?

I'll offer a different perspective then most (because, hey, that's what I do)...

For waveshaping operations, a sawtooth core seems more mathematically correct as you have the waveform linearly going from the lowest to the highest value, and you're free to map that wherever you want in the waveshaping circuit.  'Sounds elegant, at first...

But there is the 0/360 degree discontinuity.

And in practice it's rare to see any especially interesting waveshaping of the sawtooth.  There's a whole class of waveforms where the 0/360 discontinuity is a problem, and dealing with that can be a difficult task for a waveshaper circuit.

(As an aside, I do admire the design elegance of the single transistor Moog saw-to-triangle converter, as it goes into a higher impedance state during the 0/360 degree discontinuity, so it's naturally less glitchy than a more complex circuit.)

A triangle core has the advantage that it doesn't have the 0/360 degree discontinuity.  So your triangle and sine waves have less glitching.  But more importantly, these are the low harmonic content waveforms, so you *really* don't want any glitching going on.  Similarly the high harmonic content sawtooth can be built from the triangle, and any glitch in that process won't be audible as the sawtooth is a high harmonic content waveform.

Also, you get your waveforms with fewer steps in the chain with a triangle core.  Here:

----------------

sawtooth core (high harmonic content)
    --> shape to triangle (low harmonic content)
        --> shape to sine (no harmonic content)

triangle core (low harmonic content)
    --> shape to sine (no harmonic content)
    --> shape to sawtooth (high harmonic content)

----------------

So, if you believe that it's easier to make harmonics than to remove glitches in waveforms where any glitches are especially audible, then I think a triangle core is a win-win in the waveshaping department.

  -- Don

--
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com