[PD] why overlaps affect frequency of [osc~]?

Alexandre Torres Porres porres at gmail.com
Tue Nov 26 19:33:17 CET 2013

```thanks master

well, I was betting it was that, and it actually brings me to more direct
questions I should have asked before. So here they go.

Simply put: How come and why does overlapping affect the sample rate?

> the same happens if you raise the "samplerate" via "upsampling".

I guess this is what's really confusing, cause I can't seem to get it.

If you do upsampling with block, for instance, you don't get this frequency
change issue with [osc~]. It'll run the same frequency, no problem... but
overlapping will do that somehow.

Why the hell the actual upsampling does not affect it?

And does this mean that overlap of 4 and upsampling of 4x makes it really
run at 16x the sample rate?

cheers

2013/11/26 IOhannes m zmölnig <zmoelnig at iem.at>

> On 2013-11-26 11:19, Alexandre Torres Porres wrote:
> > Howdy, ever tried to compute a hann window inside a subpatch where the
> FFT
> > is happening?
> >
> > And then if you're overlapping it by 4, do you see that only 1/4 of the
> > cycle from [osc~] came up?
> >
> > That means the [osc~] frequency was 1/4 what it should be...
> >
> > Now, why and how does it happen? I just have no clue at all.
> >
>
> that is mainly because the "sample rate" within the overlapped canvas is
> higher than the sample rate outside.
> (e.g. if you are doing an overlap of 2, you are in fact processing the
> double amount of data in the same time, so your overlapping "sample
> rate" is 88200 if the non-overlapping rate is 44100).
> the same happens if you raise the "samplerate" via "upsampling".
>
>
> [osc~] uses the sample-rate information (within the signal data
> structure) to calculate the phase of a cosine-table lookup.
>
> btw, there has been a long-standing feature request [1] to extend the
> signal-structure to hold both overlap factor and sample-rate, so
> signal-processing objects could handle upsampling and overlapping
> differently.
> (speaking of which, it would also be nice, if any dsp object could have
> a notion of the current overlap cycle)
>
>
> gfmasr
> IOhannes
>
>
> [1] https://sourceforge.net/p/pure-data/feature-requests/16/
>
>
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