Re: [FFmpeg-devel] [PATCH] libswresample: avoid s16p internal processing format

[Paul B Mahol <onemda@gmail.com>]

On 1/8/23, Michael Niedermayer <michael@niedermayer.cc> wrote:
> On Fri, Jan 06, 2023 at 07:04:59PM +0100, Paul B Mahol wrote:
>> On Fri, Jan 6, 2023 at 7:01 PM Paul B Mahol <onemda@gmail.com> wrote:
>>
>> >
>> >
>> > On Fri, Jan 6, 2023 at 6:25 PM Michael Niedermayer
>> > <michael@niedermayer.cc>
>> > wrote:
>> >
>> >> On Thu, Jan 05, 2023 at 11:08:25PM +0100, Paul B Mahol wrote:
>> >> > On Thu, Jan 5, 2023 at 9:53 PM Michael Niedermayer <
>> >> michael@niedermayer.cc>
>> >> > wrote:
>> >> >
>> >> > > On Thu, Jan 05, 2023 at 01:44:10PM +0100, Paul B Mahol wrote:
>> >> > > > Patch attached.
>> >> > >
>> >> > > >  swresample.c |    3 ++-
>> >> > > >  1 file changed, 2 insertions(+), 1 deletion(-)
>> >> > > > eee7a0685b44aa867562138a2e2437ecb8844612
>> >> > > 0001-libswresample-swresample-avoid-s16p-internal-transfe.patch
>> >> > > > From 9c4cd60e2dd41cf98d693c8251f4cfade0807073 Mon Sep 17
>> >> > > > 00:00:00
>> >> 2001
>> >> > > > From: Paul B Mahol <onemda@gmail.com>
>> >> > > > Date: Thu, 5 Jan 2023 13:40:12 +0100
>> >> > > > Subject: [PATCH] libswresample/swresample: avoid s16p internal
>> >> transfer
>> >> > > format
>> >> > > >
>> >> > > > Instead use float one by default for sample rate conversions.
>> >> > > > The s16p internal transfer format produces visible and hearable
>> >> > > > quantization artifacts.
>> >> > >
>> >> > > When does this occur and why?
>> >> > >
>> >> >
>> >> > It occurs always. Just compare output with 16bit and
>> >> > int32/float/double.
>> >> > Look at other people report on internet.
>> >> > Look at src.infinitewave.ca
>> >>
>> >> src.infinitewave.ca uses 32bit none of what it shows should touch the
>> >> codepath
>> >> you change.
>> >>
>> >> if we look at src.infinitewave.ca for swr we see 2 types of artifacts
>> >> 1. Aliassing which is at maybe -120db with the actual signal at 0db
>> >>    i would like to see some evidence that a human can hear this
>> >>
>> >
>> > For s16p<->s16p it is much lower, around -78dB thus this patch.
>> >
>> > Also for others and reports for swr its is lower than exact -120dB
>> >
>> >
>> > 2. Reflection and attenuation at the transition frequency
>> >> With linear filters there is a tradeof between attenuation of the
>> >> passband, reflection of frequencies beyond, latency and so on
>> >> You can have a perfect sharp cutoff with no attenuation and no
>> >> refelection
>> >> that requires a infinitly long filter. And while this looks best in
>> >> this
>> >> frequency plot, does it actually sound best ? If you can hear -120db
>> >> signals you surely would then also hear the ringing long before a
>> >> gunshot
>> >> from such long filter.
>> >>
>> >
>> One can always change linear FIR to be min phase FIR kernel.
>
> I certainly would welcome a wider range of filters in swr, if you want to
> add
> any low delay sinc approximation or in fact i would welcome any filter
> you want to add.

There is that afdelaysrc filter patch on ML to add FIR coefficient
generation for fractional delay audio filter that can be also used as
a interpolation FIR filter. And to me it seems better at same number
of taps than already used/available ones in soxr and swr.

Also I have done prototype of resampling filter using afir filter via
custom filters in filtergraph and it operates at similar speeds like
soxr (in these very non optimized approach) and providing better/wider
frequency output at highest band.

>
> thx
>
> [...]
> --
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
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