[mythtv] [mythtv-commits] Ticket #2468: Mythtranscode removing audio and subtitles and audio tracks swapping places.

[Daniel Kristjansson]

On Wed, 2006-10-18 at 18:48 -0700, Geoffrey Hausheer wrote:
> As far as what is needed....
> DVB uses one of:
> ETSI EM 300 472 teletext (commonly DVB-TXT) which defines text in
> terms of text on a given line number (5 allowed).  This should be easy
> to convert into raw text
> DVB-VBI (sorry don't have full specs on this, but it is basically what
> is used in analog broadcasts, and also text based)  probably not used
> much if at all.
> ETSI EM 300 743 (commonly DVB-SUB) which can be transmitted as both
> text and images, in multiple color depths, and postioned arbitrarily
> on the screen.  The streams are tempral based and independant of
> audio/video.
> 
> DVD Subtitles are bitmap images sent through a seperate stream.  The
> images are conditionally overlayed on the original image.  The images
> can only have 4 colors (one of which must be the transparent one)
> 
> ATSC uses EIA-708 which is text-based, with downloadable fonts.  the
> data is stored in the picture rather than in a seperate stream.

Don't forget the ivtv (PVR-250) driver, which uses it's own format
for embedding EIA-608 captions.

> So it is a mess.  Converting to DVD standard would be possible for all
> of these formats, but requires creating new bitmap images, and quality
> would be hit-and-miss.  DVB-SUB would be easier to convert to, but is
> not compatible with MPEG2-PS containers in general.  Converting to any
> other format would be futile as it would require OCR.
Actually, the DVB teletext, DVB VBI, ATSC EIA-708, and ivtv formats
could be embedded in the video stream as user data. The DVB-SUB and
DVD Subtitles can't really be handled that way (you could emboss one
set of subtitles in the video permanently, but this would be ugly.)

> In any case, this is a huge amount of work.  If someone is really
> interested, I'd be happy to point them at the relevant specs where I
> know them, but there are many more interesting tasks to takle in the
> lossless transcoder, so this one isn't on my plate.
Yep.

For instance, enabling transcode to a TS format instead of a PS
format is a much more important enhancement and would make
implementing captions and additional language streams much more
doable. Not only could you bring over the captions, subtitles and
additional audio streams, they would also be in a standard format;
this means that it would require no changes in the MythTV video
playback code and they wouldn't require additional ffmpeg hooks.

-- Daniel