[mythtv] [dvb] subtitling

Stuart Auchterlonie stuarta at squashedfrog.net
Thu Apr 28 15:20:52 UTC 2005

On Thu, Apr 28, 2005 at 04:27:42PM +0200, Jesper Sörensen wrote:
> Stuart Auchterlonie wrote:
> >If it is true that all of the types mentioned above (0x05 - 0x6A)
> >are in fact subtypes of the PES Private Stream, then things in
> >SIParser are due for a bit of housekeeping. Can anyone verify this
> >for me, so then I can get started on a patch :-)
> > 
> >
> There is no such thing as a "subtype" in the PMT. There are often 
> "descriptors" though, used to provide different kinds of metadata for 
> the streams (check the MPEG and DVB docs for more info).

By subtype I mean a byte in the Private Stream (0x06) which describes
it's content.

> Different stream types are used in different parts of the world. E.g. 
> I'm getting AC3 streams with stream type 6 and an AC3 descriptor, but in 
> the US they seem to have AC3 sound on type 0x81 (IIRC) and no 
> descriptor. I think subtitles are normally on type 6 but I don't think 
> they *have* to.

The last part is what I'm trying to get at. Do things like subtitles
HAVE to be on type 6 streams. Why? See below

> I don't understand what the problem is and how you intend to fix it?

The problem with SIParser as I see it is that the (as you call it)
descriptor byte, is ALWAYS checked, regardless off the stream type.

So there is a theoritical possibility of incorrectly re categorizing
a packet because the 5th byte after the stream type happens to match
one of the following... 0x05, 0x56, 0x59, 0x6A. 

(See switch (descriptor_tag) after switch (e.Orig_Type) in
SIParser::ParsePMT to see what I'm talking about...)


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