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

IOhannes m zmölnig zmoelnig at iem.at
Tue Nov 26 15:32:08 CET 2013

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
(speaking of which, it would also be nice, if any dsp object could have
a notion of the current overlap cycle)


[1] https://sourceforge.net/p/pure-data/feature-requests/16/

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