[FFmpeg-devel] [RFC] Lowpass filter

Alexander E. Patrakov patrakov
Mon Aug 25 18:43:03 CEST 2008

Kostya wrote:

> Here's my lowpass filter made more generic, so it will create
> lowpass filter for any even order with any cutoff frequency.
> I submit this in a plain form since it's easier to review
> this way (there's not too much left from the old implementation).

1) a general remark on the interface specification that can be addressed by
just adding comments:

There are many types of IIR filters: Bessel, Butterworth, Chebyshev and so
on. They all differ by their characteristics. Characteristics such as the
transition band and the group delay must be known for applications (e.g.,
by saying "this is a Butterworth lowpass filter" as opposed to just "a
lowpass filter"). E.g., one may want to use your filter to "clean up" the
LFE channel before downsampling it in the codec, and in such situation
there is a strict requirement that the other channels are delayed by
exactly the necessary amount so that, while encoding a low-frequency sine
wave, the zero crossings of the filtered LFE channel and the delayed
non-LFE channels match.

2) you use int16_t for samples. Why not also create a floating-point

Alexander E. Patrakov

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