[sdiy] Waveform phases and hard sync, sawtooth vs. triangle?

[Mattias Rickardsson]

Hej Magnus,

On Sat, 23 Jun 2018 at 16:24, Magnus Danielson <magnus at rubidium.dyndns.org>
wrote:

>
> > is there any general consensus or preferred design choices regarding how
> > the phase of sawtooth waves and triangle waves should align with each
> > other and with oscillator sync?
>
> Ah, we havve been discussing this before, seems you can't stop thinking
> about it every once in a while


I only design VCOs once in a while, that's probably why the list isn't
flooded with these questions all the time. ;-)


> > The attack of a tri/sine-based drum sound or synth sound should probably
> > start from a zero crossing.
>
> You should test around to see if you hear any audioble differences from
> different phases at the start of attack. This is relevant for brick-wall
> attacks, where is smooth attacks can be expected to make it meaningless.
> To test it, you can either trigger the actual attack by the through-zero
> of the oscillator, and then delay the actual attack differently to see
> if it is meaningful. This will however cause up to a cycle of delay of
> the sound, which may or may not be acceptable, so a better approach for
> a real synth would be to sync the oscillator hard to the attack, and
> vary the delay between attack and sync to see how it sounds. Either way,
> you should be able to patch things fairly quickly.
>

No need to try those details - it's been practiced in drummachine design
for 35 years. The audible differences from different start phases are huge,
and waiting for up to a cycle in a 50 Hz bass drum is way too long for
proper timing. What the world needs is a synced triangle oscillator
starting at a zero-crossing.
Or put in other words:
All free-running and no sync makes the TR-909 a dull drummachine.  8-)

> This could be reached in a couple of ways:
> >
> > - Is it really desired (from a usability perspective, not a circuit
> > design perspective) to have the synced sawtooth restart from the
> > endpoint and not the midpoint or quarter points? Does it matter in terms
> > of direct sound or audio-rate modulation?
> > An alternative restart point for the sawtooth would facilitate a
> > triangle restart from a zero crossing without having to change the
> > traditional phase alignment of saw & tri.
>
> It is only meaningful in a few contexts:
>
> 1) Mixing of waveforms
> ...
> 2) Waveshaping
> ...
>

I'd say it also makes a difference when it comes to linear FM, and even
ordinary audio:

Synced sawtooth starting from its peak maximizes the DC offset in the
resulting wave, which is a bad feature if the wave is used as an FM
modulator, where it bends the pitch of the carrier. Or even when listening
to it, through an amplitude envelope, where it creates a thump.

Synced sawtooth from the mid-ramp zero crossing creates a DC offset in the
other direction.

Synced sawtooth from the quarter points (half the peak value) kind of
minimizes the DC offset for short sounds of unknown exact duration.

In general, short waveform snippets created both by sync and by random
start/stop times in general tend to have problems with almost
uncontrollable DC offset.

/mr
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