[music-dsp] wavetable and aliasing (Again)

Ethan Duni eduni at ucsd.edu
Tue Apr 29 17:23:00 EDT 2003


> A whole phase? That'd imply B's precise waveform wouldn't matter at all. I
> just can't bring myself to believe that is the case.

-This is the "classical" definition, also referred to as "hard sync." And
yeah, it doesn't matter what the waveform of B is. I believe this definition
originated in analog synths, where the oscillators originate as sawtooth
waveforms which are subsequently shaped into the other popular flavors.
These sawtooth waves are easily generated by feeding a current supply into a
capacitor, and which then feeds a comparator that dumps the cap when the
voltage gets to a certain level. So to sync oscillator A to oscillator B,
you just connect the comparator of oscillator B also to the cap in
oscillator A, so A's phase is reset whenever B's is. This all occurs before
B is shaped into whatever waveshape you've selected.

> One version I've
> heard of is, A syncs to B upon B's crossing zero. Is that really just a
> tale?

-There are probably 100 (subtly) different implementations of sync. This one
sounds reasonable for something like a sine wave or sawtooth, but what if B
has a whole mess of zero-crossings every period?

> Whatever the deal is, there is also the difference between hard an soft
> sync. As I've heard the tale told, "hard" means "reset to the beginning of
> the cycle" while "soft" means "flip polarity". Am I getting this anywhere
> in the vicinity of things Right and True?

-My understanding is that soft sync is like hard sync, except you only reset
the slave oscillator if it was "close" to resetting anyway. I think the idea
here is that you're using the sync to correct for a small difference in the
tuning of the oscillators, and you don't want to incurr the nasty distortion
of a hard sync if they happen to be far out of tune.

By the way, there are many aged masters of oscillator syncing on the
synth-diy list. Interestingly, in analog synth design, syncing often shows
up as a parasitic effect (as opposed to a feature)..

Ethan




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