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

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

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.


>
>> 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
>>
>> The real ebay dictionary, page 1
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>> "In good condition" - "Can be repaird by experienced expert"
>> "As is" - "You wouldnt want it even if you were payed for it, if you knew
>> ..."
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>
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