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

[Paul B Mahol]

On Fri, Jan 6, 2023 at 6:25 PM Michael Niedermayer <michael at 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 at 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 at 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.
>
> also what actually is the optimal frequency response of this filter ?
> with a 22khz cutoff, a 22.1khz sine should be silence is that
> really subjectively better than a 21.9khz sine ?
> Iam not sure about this. Has someone done actual hearing tests with
> actual real audio? the sinc filter originates from the idea of lossless
> reconstruction of frequencies below nyquist if iam not mistaken, but humans
> are not trying to losslessly restore a block of frequencies. A human
> listener
> generally wants to enjoy listening to some media. Has someone looked into
> what is actually best for that real use case ?
> This question matters because with it we can tune the filter parameters to
> target humans.
>
> But lets push the doubts about choosing resampling purely based on
> frequency
> analysis away.
> swresample has several parameters with which you can tune this:
> we have a filter_size, if thats bigger you should get closer to the ideal
> sinc. Theres phase_shift which may reduce the (i assume) unhearable
> aliasing.
> And cutoff which should allow to tune the (i assume) hearable
> reflection/attenuation tradeoff also theres filter_type to allow you to
> tune the
> window function.
>
> If there are issues reported by people using their ears, please provide
> more
> details, iam interrested in these cases.
>
>
> >
> >
> > > This change should be limited to the case that benefits, this would
> force
> > > this
> > > even without resampling in some cases.
> > >
> >
> > It is forced only if sample rates between input and output differs.
>
> If iam not mistaken it affects rematrixing without resampling too
>

How so?
I really doubt that this patch do that.


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