DVB subtitles

[Chris Reed-2]

If anyone's interested, I've made some good progress getting subtitles
working with DVB subtitles (from MythTV recordings).  I've got the
graphic decoding and CLUT decoding working.  I'm trying to make it
display the subtitles at the right time, but it's a few seconds off, and
currently polls for subtitle display, same mechanism as SPU.

My current code changes have broken SPU subtitles, but I can probably
make that work again.

One thing to note is that I copied the PTS extraction code from
parse_spu_frame, but found it was incorrect (some of the bit shifts were
off by one).

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[stuart-20]

Chris Reed wrote:

> If anyone's interested, I've made some good progress getting subtitles
> working with DVB subtitles (from MythTV recordings).  I've got the
> graphic decoding and CLUT decoding working.  I'm trying to make it
> display the subtitles at the right time, but it's a few seconds off, and
> currently polls for subtitle display, same mechanism as SPU.
>
> My current code changes have broken SPU subtitles, but I can probably
> make that work again.
>
> One thing to note is that I copied the PTS extraction code from
> parse_spu_frame, but found it was incorrect (some of the bit shifts were
> off by one).

Wow, thanks for your efforts Chris.  I know people have been talking
about subtitles off and on for ages.

10K foot questions:
-How are we doing on target memory space?
-Are DVB subtitles from ATSC broadcasts?**

**If so what are you doing about transcoding the video down to
StandardDefinition/Stereo for playback on a mvpmc box?  I would have
thought transcoding would have wiped the subtitle information.

...thanks


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[MVallevand]

On Thu, Nov 20, 2008 at 1:56 AM, Chris Reed <cr212@...> wrote:
> If anyone's interested, I've made some good progress getting subtitles
> working with DVB subtitles (from MythTV recordings).  I've got the
> graphic decoding and CLUT decoding working.  I'm trying to make it
> display the subtitles at the right time, but it's a few seconds off, and
> currently polls for subtitle display, same mechanism as SPU.

Chris I know several people have been asking for this so thank you.
This is second to AC3 on my list of requests.  Of course people will
immediately want teletext subtitles.

Martin

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[Simon Hyde]

On Thu, 20 Nov 2008, Chris Reed wrote:

> If anyone's interested, I've made some good progress getting subtitles
> working with DVB subtitles (from MythTV recordings).  I've got the
> graphic decoding and CLUT decoding working.  I'm trying to make it
> display the subtitles at the right time, but it's a few seconds off, and
> currently polls for subtitle display, same mechanism as SPU.

Ah...well...I think your best bet here is to use the video event queue
which I added when adding the WSS/aspect ratio handling code. I designed
it so that it could easily be used to display subtitles.

Basically you need to store your bitmap and any other information in
a malloc()ed buffer in memory somewhere, then do a:

vid_event_add(pts, VID_EVENT_SUBTITLES, (void *)pBitmap);

The "pts" you use here is just the 32 least significant bits of the actual
(33-bit) pts.

Then modify video_events_start in src/video.c to handle, decode and
display anything with type VID_EVENT_SUBTITLES as this new bitmap.



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[Simon Hyde]

On Thu, 20 Nov 2008, stuart wrote:

>
>
> Chris Reed wrote:
>> If anyone's interested, I've made some good progress getting subtitles
>> working with DVB subtitles (from MythTV recordings).  I've got the
>> graphic decoding and CLUT decoding working.  I'm trying to make it
>> display the subtitles at the right time, but it's a few seconds off, and
>> currently polls for subtitle display, same mechanism as SPU.
>>
>> My current code changes have broken SPU subtitles, but I can probably
>> make that work again.
>>
>> One thing to note is that I copied the PTS extraction code from
>> parse_spu_frame, but found it was incorrect (some of the bit shifts were
>> off by one).
>
> Wow, thanks for your efforts Chris.  I know people have been talking
> about subtitles off and on for ages.
>
> 10K foot questions:
> -How are we doing on target memory space?
> -Are DVB subtitles from ATSC broadcasts?**

Nope...DVB is the european/worldwide standard for digital TV broadcast.
ATSC is North America's bash at trying to do it themselves because
following other people's standards would be too easy (*).

I suspect (although I don't know) that Closed Captioning on ATSC
broadcasts is probably just a stream of text in ASCII. DVB subtitles on
the other hand are sent as bitmaps containing the texts, allowing the
broadcaster to select an easily read font, and to send text in any
character set without the box first having to have support for it.

DVB subtitling is a lot closer to DVD subtitling (which also uses bitmaps,
encoded in a different format) than it is to ATSC Closed
Captions/DVB-Teletext.




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[JP Vossen]

> Date: Thu, 20 Nov 2008 06:56:57 +0000
> From: Chris Reed <cr212@...>
>
> If anyone's interested, I've made some good progress getting subtitles
> working with DVB subtitles (from MythTV recordings).  I've got the
> graphic decoding and CLUT decoding working.  I'm trying to make it
> display the subtitles at the right time, but it's a few seconds off, and
> currently polls for subtitle display, same mechanism as SPU.
>
> My current code changes have broken SPU subtitles, but I can probably
> make that work again.
>
> One thing to note is that I copied the PTS extraction code from
> parse_spu_frame, but found it was incorrect (some of the bit shifts were
> off by one).

[...]

> DVB is the european/worldwide standard for digital TV broadcast.
> ATSC is North America's bash at trying to do it themselves because
> following other people's standards would be too easy (*).
>
> I suspect (although I don't know) that Closed Captioning on ATSC
> broadcasts is probably just a stream of text in ASCII. DVB subtitles on
> the other hand are sent as bitmaps containing the texts, allowing the
> broadcaster to select an easily read font, and to send text in any
> character set without the box first having to have support for it.
>
> DVB subtitling is a lot closer to DVD subtitling (which also uses bitmaps,
> encoded in a different format) than it is to ATSC Closed
> Captions/DVB-Teletext.

Lack of mvpmc CC support has been a major bummer for me.  Unfortunately,
I'm in the US, so it sounds like your work is not going to help.  It is
also my understanding that ATSC is just an ASCII stream with time
markers; at least there are various tools that can dump that (e.g.,
http://ccextractor.sourceforge.net/).  There are some threads in the ML
archives on the topic:
http://sourceforge.net/search/?ml_name=mvpmc-devel&type_of_search=mlists&group_id=103474&words=caption

Later,
JP
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