[sdiy] Phase cancellation on waveforms other than sine?

[John Ames]

Hmm, yeah, I do need to actually listen to it...mostly just been
looking at a graph, but that can be deceptive sometimes.

On 4/4/18, Ian Fritz <ijfritz at comcast.net> wrote:
> 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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>
>