[sdiy] Digital accumulator VCO core?

cheater cheater cheater00social at gmail.com
Tue Feb 16 02:07:20 CET 2021


The aliased step which you worry about is cancelled out by the
presented algorithm. Notice that in the final version the signal also
contains a naiive aliased step - but in the opposite direction of what
you're applying it to. The two cancel out and you're left with only
the band limited one.

On Mon, Feb 15, 2021 at 11:05 PM Brian Willoughby
<brianw at audiobanshee.com> wrote:
>
> I don't agree with this 5-step simplification. Once you create a naive raw impulse, the data already includes an infinite series of harmonics that are aliased and cannot be removed by filtering. Even two passes of a filter cannot remove the aliases that fall in between the in-band harmonics.
>
> In contrast, you can synthesize a band-limited impulse easily using the sin(x)/x formula. There's no need to low-pass filter a synthesized band-limited impulse. The windowed version just leaves out values of sin(x)/x for large values of abs(x)
>
> What I don't know about is the math to convert a standard band-limited impulse into a minimum-phase band-limited impulse. Anyone?
>
> Either way, a synthesized band-limited impulse can be converted to a band-limited step via integration, and this can be done in advance. I'm assuming that the sample rate of the converter is fixed, and thus the band-limited data need only be synthesized once.
>
> Brian Willoughby
>
>
> On Feb 15, 2021, at 04:06, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
> >> If you do a polyblep you correct the sample just before and just after the transition to "blend" the fractional-sample-position step into whole samples, but I don't get what the curve is supposed to be or how you design it.
> >
> > Ok, here goes...  (See attached JPG image.)
> >
> > 1. Start with a raw impulse (A).
> > 2. Pass this through a boxcar FIR filter to get a band-limited impulse (B).
> > 3. Pass this through the same boxcar filter again to get an even more band-limited impulse (C).
> > 4. Now integrate this band-limited impulse to get a band-limited step (D).
> > 5. Finally subtract a naive step from the band-limited step to get (E).
>
>
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