[FFmpeg-user] DTSs & PTSs in MPEG2-TS streams

[Paul B Mahol]

On Thu, Feb 1, 2024 at 10:09 PM Devin Heitmueller <
devin.heitmueller at ltnglobal.com> wrote:

> On Thu, Feb 1, 2024 at 1:55 PM Mark Filipak <markfilipak.imdb at gmail.com>
> wrote:
> >
> > Hey Devin,
> >
> > On 01/02/2024 09.54, Devin Heitmueller wrote:
> > > Hi Mark,
> > >
> > > On Thu, Feb 1, 2024 at 1:15 AM Mark Filipak <
> markfilipak.imdb at gmail.com> wrote:
> > >>
> > >> FFmpeg assigns DTSs & PTSs to P-frames and B-frames in MPEG2-TS
> streams. How does it determine those
> > >> DTSs & PTSs?
> > >>
> > >> Since DTSs & PTSs don't actually exist for P-/B-frames in MPEG2-TS
> streams, FFmpeg must have a way
> > >> that's unknown to me.
> > >
> > > P/B frames can certainly have PTS values.
> >
> > Well, they _can_ as in they're not prohibited, but I can't recall ever
> seeing a P-/B-frame in
> > MPEG2-TS supplying a DTS or a PTS. I've parsed quite a few MPEG2-TSs.
>
> If you want to share a sample with me privately, I can take a look and
> possibly offer you a better explanation.
>
> P/B frames generally have PTS values.  B frames *must* have both PTS
> and DTS since they are different values.  If you're not seeing them
> then either your sample is broken or your parser is.
>
> > >  In fact, you have to have
> > > PTS values in B frames since they differ from the DTS (since with B
> > > frames the presentation order differs from the decoding order).
> >
> > I don't see DTS or PTS in the PES header. What I do see is
> 'PES_extension_flag'=='0' and
> > 'PTS_DTS_flags'=='00', so no PES ext and no dts or pts. That's why I
> asked my question.
>
> The PES extension flag is mutually exclusive of whether the
> PTS_DTS_flags field is set.  It's very common for there to
> PTS_DTS_flags to be non-zero while PES_extension_flag is false.
>
> > Every once in awhile I'll see 'PTS_DTS_flags'=='10' but that's unusual.
> Mind you, this is in
> > MPEG2-TSs. I don't how to parse anything else.
>
> That shouldn't be unusual.  It means there is a PTS but no DTS in that
> PES.  In fact, specifying the PTS without the DTS is generally what
> you're supposed to do if they are the same value (which is the norm
> for audio streams and video streams without B frames).
>
> > > There are potentially cases with some streams where the PTS and/or DTS
> > > is not specified for every frame,
> >
> > I'd say "usually", not "potentially".
> >
> > > in which case libavformat will
> > > interpolate those values based on the frame duration and the prior
> > > packet.
> >
> > Of course. But why? They're not needed. Once the TS packets are put in
> correct sequence via SCR, the
> > PS packets transported by those TS packets are in presentation order.
> The process to reorder the PS
> > in order to decode B-frames and then put the decoded pictures back into
> the original order is
> > straight forward. It's not "tricky" at all.
>
> Um, I'm not sure what to say here.  The DTS is what defines the order
> that the packets are announced to the decoder.  The PTS defines the
> order in which they are presented.  The SCR will constantly increment.
> I think perhaps you have a fundamental misunderstanding of how PTS/DTS
> works.
>
> > It has become "tricky". That's what I'm trying to investigate, but I
> can't read 'C' code.
> >
> > >> I'm seeing some strange things. They can't be as strange as they look
> because they look disastrous.
> > >> At issue is the cause of non-monotonous DTSs and obviously bogus DTSs
> for P-frames such as
> > >> '-9223372036854775808'.
> > >
> > > 9223372036854775808 is a magic number.  It corresponds to the #define
> > > for AV_NOPTS_VALUE.  If you see that value it means that the packet
> > > did not have a PTS or DTS value assigned to it and thus you may have
> > > to take special measures.
> >
> > Oh, dear. I guessed that.
> >
> > Magic number. Fixup. There's a million fixups. I don't see why they're
> needed. That's why my
> > preliminary finding is that FFmpeg is creating its own faults through
> fixups.
>
> This is essentially how all demuxers/parsers work.  The PES packet in
> the stream doesn't contain a PTS or DTS, the API tells you that, and
> thus the application has to use a heuristic to assign a value.  Some
> implementations may do the interpolation for you, but libavformat
> basically tells you there is no PTS and leaves it to the application
> to decide what to do.
>

libavformat implementation is very low level.


>
> Devin
>
> --
> Devin Heitmueller, Senior Software Engineer
> LTN Global Communications
> o: +1 (301) 363-1001
> w: https://ltnglobal.com  e: devin.heitmueller at ltnglobal.com
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