[FFmpeg-devel] [RFC PATCH 1/3] aacdec: always skip the first 2048 samples if there's no side data

[Thierry Foucu]

On Tue, Sep 12, 2023 at 9:25 AM Lynne <dev at lynne.ee> wrote:

> Sep 12, 2023, 09:43 by andreas.rheinhardt at outlook.com:
>
> > Lynne:
> >
> >> For some reason, this was never set, which meant all **raw** AAC in ADTS
> >> streams, except faac, had extra samples at the start.
> >>
> >> Despite this being a standard MDCT-based codec with a frame size of
> 1024,
> >> hence a delay of 1024 samples at the start, all major encoders,
> excluding
> >> faac and FFmpeg, use 2048 samples of padding.
> >>
> >> The FFmpeg encoder will be modified to also output 2048 samples of
> padding
> >> at the start, to make it in line with other encoders.
> >>
> >
> > Does this also have actual advantages besides "being in line with other
> > encoders"?
> >
>
> Not really. I don't have an opinion on this. 1024 is the natural
> delay of the codec, so maybe it would be best to leave it at that.
>
>
> Note:
Not all encoders add 2048. Another version of the Fraunhofer encoder will
add only 1600 samples

and for HE-AAC of the same encoder will add 3200 samples.
Should we not then have an option to set it ?


> >> Yes, this leaves FATE pretty sad. Will fix it with the real version of
> the patch.
> >>
> >
> > Didn't we once guess the number of skip samples like this, only for this
> > guesswork to be removed intentionally? (This is not a rhetorical
> > question; I thought it to be true, but I see that there is still code
> > for faac in decode_fill(); maybe I misremember.)
> >
>
> I don't remember something like that. The faac workaround dates back
> from 2012 (bfe735b5824c7d10ba42932a17d786db50e3b2d4), and it's only for
> faac.
> It's less of a guess, as most encoders to use the FIL extension to signal
> themselves.
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-- 

Thierry Foucu