[sdiy] Phase cancellation on waveforms other than sine?

[Ian Fritz]

I just set that up to listen to and watch ... just two identical VCOs with slightly different frequencies. As the waves approach cancellation their sum (of course) approaches a square wave, but a very weak one.  So, yes, the timbre is being modulated along with with the amplitude.  But the change in timbre is really not objectional, at least to my ear.  This kind of  symmetrical clipping gives changes in the relative amplitudes of the harmonics but not in their relative phases.  It's similar to PWM based on a Tri source vs PWM based on a Saw source.  The second sounds much "phasier" than the first. So give it a listen -- you may like it!

Ian

> On Apr 4, 2018, at 10:33 PM, John Ames <commodorejohn at gmail.com> wrote:
> 
> I'm mostly posting this here because as far as I can tell there isn't
> a dedicated DK Synergy mailing list, but the question is, I think,
> generally applicable. Anyway, I've been studying the theory of
> operation for the Synergy based on the available documents, and one
> thing I quite like is the trick of doing amplitude modulation for the
> oscillators by phase cancellation, so that the entire thing can be
> done with the sine lookup table already present and
> addition/subtraction, with no need for a multiplier at all. A very
> elegant solution!
> 
> However, one thing that confuses the bejeebers out of me is that the
> Synergy has not just sine waves for the oscillators, but triangle as
> well. And I can't for the life of me understand how that works. Two
> sine waves of the same frequency, shifted progressively out of phase
> and added, result in a third sine wave of the same frequency that
> gradually decreases in amplitude until at 180 degrees out of phase
> it's completely silent. Two triangle waves shifted given the same
> treatment, on the other hand, result in a triangle wave with the peaks
> clipped at a progressively lower amplitude so that it kind of
> transitions from a triangle to a "hexagon" shape to a very, very wide
> pulse wave before disappearing. I thought that was an error on my part
> when I saw it in a brief software test, but trying the same thing in
> Audacity gives the same result.
> 
> So, the question: am I missing something here? If not, how can the
> Synergy achieve the appropriate results for its triangle waveforms? I
> need to finish poring over the technical documentation again, but I'm
> kinda stumped trying to understand this.
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