On Wed, Oct 19, 2011 at 2:50 AM, Mathieu Bouchard matju@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:
http://www.katjaas.nl/FFTwindow/FFTwindow&filtering.html
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.
Katja