[mythtv] HDTV: Multiple audio streams

[John P. Poet]

Daniel,

Have you made any progress with this?

I am should have my taxes figured out by Sunday, and should be able to
resume looking into this.  Just wondering if you have investigated any more.


John


On Tue, 24 Feb 2004, Jason Hoos wrote:

> There may be sufficient information in the PMT itself to determine which
> stream to take.  As you say, there is some extra information associated with
> each stream in the PMT.  On the streams I have, the PMTs at least seem to
> include the ISO 639 language code, which RewritePMT currently completely
> ignores.  Currently (well, last I looked at it anyways) RewritePMT just
> grabs the PID of the first audio stream it finds; my guess is that the
> station you're having trouble with shuffles the order that the streams are
> listed in in the PMT every now and then.  I don't have any stations around
> here that I know of that have any overdub or vision-impaired audio streams,
> so I don't know what information those streams would be labeled with.
>
> I don't know how reliable this information is.  The one channel I looked at
> had the audio stream labeled as being Spanish, even though my ears told me
> it was English.  I'm not sure if the PMT was just flat-out wrong, or if the
> stream could have been in Spanish normally and just not for this show.  The
> video stream was labeled as English, so who knows.  I'm thinking the PMT for
> this station might actually be wrong, since I'm pretty sure this station
> broadcasts Spanish on a different subchannel entirely.  On the other hand,
> this station only has one audio stream per subchannel, so perhaps if
> RewritePMT looked for the stream with the right language and other
> descriptors, and failing that just took the first one it found, things might
> work right most of the time...
>
> Jason
>
> P.S. If you need some information on PMT structure let me know.
>
>
> > -----Original Message-----
> > From: Daniel Thor Kristjansson [mailto:danielk at cat.nyu.edu]
> > Sent: Monday, February 23, 2004 10:18 PM
> > To: Jason Hoos
> > Subject: RE: Multiple audio streams
> >
> >
> >
> > Well I'm looking for an example now. I can't seem to find one when I
> > need it; but the Simpsons seem to be the main offender. Basically I
> > would get something like the stream for the blind or Spanish. I think
> > these are all AC3 streams, from my reading of the standard this is all
> > that is allowed with ATSC. The standard does define some labeling for
> > the different stream types under the AC-3 audio service
> > descriptor 0x81
> > which can be in either the PMT or EIT tables. Each AC3 stream seams to
> > be pointed to in the VCT table. Then the AC3 audio service descriptor
> > has some pertinent information.
> > From http://www.atsc.org/standards/a_52a.pdf (p 122-123)
> >  bsmod    -- ?? I think this may be the type of stream, such as stream
> >              for the blind.
> >  full_svc -- 1 bit flag for whether this is complete or needs to be
> >              mixed with another audio service
> > if (bsmod<2) {
> >  mainid  -- 3 bit field telling you which particular main service this
> >             belongs to.
> > } else {
> >  asvcflags -- 8 bit flag field associating this with one or
> > more of the
> >               eight main services
> > }
> >  langcod/langcod2 -- language
> >  text[]   -- description of service
> >  ISO_639_language_code -- 24 bit ISO 639 language code
> >
> > But this seems to be supplimentary to the PID tables, right?
> > I think it
> > describes one of the eight main audio services described by the
> > "service_location descriptor".
> >
> > The service_location descriptor might have (a_65b p 116)
> >  PID_audio_1   AC-3 audio    Spanish
> >  PID_audio_2   AC-3 audio    English
> >  PID_audio_3   AC-3 audio    English
> >  PID_audio_4   AC-3 audio    French
> >  PID_video     MPEG-2 video  No lang.
> >
> > I'm assuming they mean in the VCT table (p 74). Apparently this is in
> > the PSIP table to "minimize the time required for tuning" but
> > PAT and PMT
> > information is required for MPEG-2 compliance (p 120). But
> > then they go
> > on to say the PMT should always be processed because the PSIP will not
> > cover cases of multiple audio tracks for the same language,
> > which leads
> > me to think that PMT is always the more correct thing to look at.
> >
> > Conclusion...
> > First, we can to look at this VCT table for all the streams that match
> > our target language, but we need to look at the PMT AC-3 table to find
> > the bsmod service in our language that we want (i.e. bsmod for the
> > sighted, for the blind, the "emergency" channel, effects, dialog,
> > dialog for the hearing impaired, etc.)
> >
> > Does this make sense?
> >
> > -- Daniel
> >
>