[FFmpeg-devel] [RFC] Lowpass filter

Alexander E. Patrakov patrakov
Tue Aug 26 05:02:55 CEST 2008

Kostya wrote:

> Since I'm no filter expert, I choosed the easiest IIR filter - i.e.
> Butterworth. Who knows, implementation may be changed in the future.

Then, as I said, this "unspecified filter" is practically useless. Besides,
if we leave this "unspecified filter" in, code will accumulate that ignores
the fact that it is unspecified and assumes Butterworth (and will break if
we later switch to Bessel).

> And I still believe delay = f(order).

It is a function of the filter type, order, cutoff frequency and signal
frequency, and even isn't a rational number of samples in the general case.
E.g., consider a Butterworth filter of order 4 with a cutoff frequency
0.125 fs, and pass a sine wave of frequency 0.1 fs through it. The delay
(as defined by zero crossings) will be 7.28 samples, but it is 6.57 samples
for very low signal frequencies such as 0.001 fs. Please, play more with
this filter and gnuplot.

Alexander E. Patrakov

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