[PD] Analyzing subharmonic frequencies accurately

katja katjavetter at gmail.com
Wed Oct 19 09:15:34 CEST 2011

On Wed, Oct 19, 2011 at 2:50 AM, Mathieu Bouchard <matju at artengine.ca> wrote:

> Why would the filtering be done by FFT ? Isn't a single [lop~] good enough ?

A single [lop~] is a mild filter, probably 6 dB per octave. FIR
filters are normally used to make sure the potentially aliasing
frequencies are really filtered out before up/downsampling, some 20
coefficients per octave up/down if I remember well. For 32 octaves
down, FIR would be expensive, so it could instead be done as a
convolution in frequency domain.

In frequency domain it is possible to almost brickwall-filter, it's
very effective and cheap. I've documented experiments with it:


That said, it is of course dependent on the input signal whether
radical filtering is really important before downsampling. I don't
know what is the case of the original poster, so maybe this is
theoretical digression.


More information about the Pd-list mailing list