[sdiy] Phase cancellation on waveforms other than sine?

John Ames commodorejohn at gmail.com
Thu Apr 5 06:33:31 CEST 2018

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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