[FFmpeg-devel] [PATCH] swresample/resample: improve bessel function accuracy

[Ganesh Ajjanagadde]

On Mon, Nov 2, 2015 at 6:49 PM, Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
> On Mon, Nov 2, 2015 at 6:37 PM, wm4 <nfxjfg at googlemail.com> wrote:
>> On Mon,  2 Nov 2015 14:49:54 -0500
>> Ganesh Ajjanagadde <gajjanagadde at gmail.com> wrote:
>>
>>> This improves accuracy for the bessel function, and this in turn should
>>> improve the quality of the Kaiser window.
>>
>>
>> "Should"? Does it or does it not? If you don't know, why the patch?
>
> It improves the window in the sense of mathematically matching the
> Kaiser window expression due to the improved bessel function accuracy.
> That claim I have verified and placed in the commit message with
> evidence.
>
> What that translates into in terms of resampling accuracy, I don't
> know. Normally, such things do help reduce the noise floor, but I
> don't know an easy way to test via FATE or associated tools. I put
> that caveat in the bottom lines of the patch.

Turns out the init speed is definitely improved as well (~20% boost
with default settings).
I did a cheap trick of calling build filter 1000 times to get a large
number of runs.
Results (x86-64, Haswell, GNU/Linux):

test: fate-swr-resample-dblp-44100-2626

new:
995894468 decicycles in build_filter(loop 1000),     256 runs,      0 skips
1029719302 decicycles in build_filter(loop 1000),     512 runs,      0 skips
984101131 decicycles in build_filter(loop 1000),    1024 runs,      0 skips

old:
1250020763 decicycles in build_filter(loop 1000),     256 runs,      0 skips
1246353282 decicycles in build_filter(loop 1000),     512 runs,      0 skips
1220017565 decicycles in build_filter(loop 1000),    1024 runs,      0 skips

Thus, this patch has both things going for it luckily. Will leave to
the maintainer (Michael I believe) to give details of accuracy
benefits as translated to the actual resampling if easily testable,
and I will add the perf numbers to the message.

>
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