SV: Re: [sdiy] Simulating SW?

[René Schmitz]

Hi Magnus and all,

The cause for these problems is that often the FFT algorithms are just
used with transform lengths which are a power of 2 (since that is the
easiest, but infact the FFT algo isn't limited to that). And if the
actual sample length is different from the transform length they merely
padd with zeros (ugh!), or as you suggest, use windowing, both of which
create artefacts. Resultant spectra are that of a sinewave with
an inserted piece of silence after each wave. Or broadened spectral
side lobes in case of the windowing function.
A broadened spectral lobe at the fundamental would already be considered
THD, if one just subtracts the ideal line spectrum from the result, so
you should thrive for a line spectrum.
For the best results your sample length should be equal to your
transform length and also a whole number multiple of periods of your
signal should fit in there. The easiest interpretation is of course when
you have a single period. Then the resulting spectrum is that of your
signal continued smoothly in time toward +-inf, then you don't need
windowing.

Cheers,
  René


-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159