[Subhandler-devel] Re: SVCD subtitles

[Julio S. <jsa...@us...>]

El jue, 16-10-2003 a las 23:57, Laurent Pinchart escribi=F3:

> What is PES ?

PES is a Packetized Elementary Stream, that is, the raw stream that is
multiplexed into a Program Stream.

> Do you mean that all the subtitles are multiplexed in a single elementary=
=20
> stream ?

Well, yes.  Basic MPEG is a number of elementary streams multiplexed
together.  However, the private stream PS1 (stream code 0xBD) has a
number of substreams.  The first byte in the payload tells which
substream it is. The standard says nothing about what those substreams
are: they are private extensions.  We know, however, that AC3 audio is
there and so are subtitles.  Substream codes starting with 0x00 to 0x03
are CVD subtitles.  Starting with 0x20 up to 0x3f (I think) are the DVD
subtitles.  But all four channels of SVCD subtitles share the same
substream 0x70 and have to be differentiated by an additional
sub-substream code.  So SVCD subtitles are multiplexed together in PS1
substream 0x70 that is multiplexed with other material to form the PS1
substream that is then multiplexed with video (stream code 0xE0), audio
(stream code 0xC0 and 0xC1) and other things to produce the program
stream that is actually stored on the CD.  DVDs are similar, except they
may have up to 8 audio channels (0xC0 to 0xC7 if MP, but AC3 sound is on
PS1, no idea about DTS).  Broadcast video is still more bizarre: it can
have several video channels (0xE0 to 0xEF) that share the same carrier.

So, conceptually, it is sort of a stream hierarchy that gets all
multiplexed together.  In practice, multiplexing hierarchically does not
make  sense, since further multiplexing may break the compromises chosen
by prior steps.  That's why submux is wrong and subtitle multiplexing
should be put in mplex.

> > And the PS1 format probably would have to be extended to signal the kin=
d
> > of subtitle so that the image can be reconstructed.
>=20
> What do you mean ?

It presumes the picture is DVD subtitle.  That cannot be input to mplex
unless it learns how to transcode those subtitles into SVCD or CVD,
precisely what we wanted to avoid.

> Do you have more information about MPEG2, especially an overview and the=20
> headers/packets format ?

I don't.  Most information returned by Google is either too general and
consumer-oriented or too video research oriented.

I suppose a good book would be best.  The standard is expensive and
probably hard to read.

The link I gave that is now commercial is good.  It used to have
detailed tables of the headers, explaining bit by bit of them.  I have
it in print from when it was a free resource.  If it is half as good as
it was back then, I can recommend it heartily.  For the web, we need a
good collection of links to free resources, but this link may be worth
the 5 dollars or so the guys asks for access.  I am tempted,,,

If you find worthwhile links, drop a note.

> Done :-)

Great, I am answering on the mailing list myself.  This way, it gets
archived automatically and others can correct our mistakes or
misunderstandings.

Regards,

Julio