[PD] Analyzing subharmonic frequencies accurately

katja katjavetter at gmail.com
Tue Oct 18 14:35:03 CEST 2011

On Tue, Oct 18, 2011 at 11:20 AM, Roman Haefeli <reduzent at gmail.com> wrote:

> If you're interested only in the lower frequencies anyway, you can
> drastically lower the sample rate in order to save some CPU cycles when
> doing the FFT.

Right. A 2^17 point FFT is on average not expensive at all, but the
problem is that all computations (in the order of 17 * 2^17) are done
at once every 3 seconds, that's just too much and it causes buffer
underrun in the realtime dataflow.

Depending on input signal, drastically lowering the sample rate may
also require drastic filtering to prevent aliases, which could
otherwise spoil analysis results. Such filtering can be done in
frequency domain, using FFT again. Be sure to use 4 times overlap and
Hann windowing before FFT and after IFFT.


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