[FFmpeg-devel] [PATCH 3/4] mpegts: add fix_teletext_pts mpegts demuxer option

[Michael Niedermayer]

On Tue, Jul 30, 2013 at 10:12:40PM +0200, Marton Balint wrote:
> 
> 
> On Tue, 30 Jul 2013, Michael Niedermayer wrote:
> 
> >On Sat, Jul 27, 2013 at 09:14:46PM +0200, Marton Balint wrote:
> >>The option (if set) overrides teletext packet pts with the PCR of a program
> >>which the teletext stream is part of that is not discarded. Using the same
> >>teletext PID for multiple programs is possible, therefore we need some kind of
> >>heuristics to know which program PCR we should synchronize to. Using the first
> >>non-discarded PCR pid among the programs of the teletext stream seemed like a
> >>good choice.
> >>
> >>Based on a patch by Reimar Döffinger.
> >>http://lists.ffmpeg.org/pipermail/ffmpeg-devel/2012-September/131610.html
> >>
> >>Signed-off-by: Marton Balint <cus at passwd.hu>
> >>---
> >> libavformat/mpegts.c  | 46 ++++++++++++++++++++++++++++++++++++++++++++--
> >> libavformat/version.h |  2 +-
> >> 2 files changed, 45 insertions(+), 3 deletions(-)
> >>
> >>diff --git a/libavformat/mpegts.c b/libavformat/mpegts.c
> >>index c7c957f..5e2af02 100644
> >>--- a/libavformat/mpegts.c
> >>+++ b/libavformat/mpegts.c
> >>@@ -106,6 +106,9 @@ struct MpegTSContext {
> >>     /** compute exact PCR for each transport stream packet   */
> >>     int mpeg2ts_compute_pcr;
> >>
> >>+    /** fix dvb teletext pts                                 */
> >>+    int fix_teletext_pts;
> >>+
> >>     int64_t cur_pcr;    /**< used to estimate the exact PCR  */
> >>     int pcr_incr;       /**< used to estimate the exact PCR  */
> >>
> >>@@ -131,7 +134,7 @@ struct MpegTSContext {
> >>     int current_pid;
> >> };
> >>
> >>-static const AVOption options[] = {
> >>+static const AVOption mpegtsraw_options[] = {
> >>     {"compute_pcr", "Compute exact PCR for each transport stream packet.", offsetof(MpegTSContext, mpeg2ts_compute_pcr), AV_OPT_TYPE_INT,
> >>      {.i64 = 0}, 0, 1, AV_OPT_FLAG_DECODING_PARAM },
> >>     { NULL },
> >
> >>@@ -140,7 +143,20 @@ static const AVOption options[] = {
> >> static const AVClass mpegtsraw_class = {
> >>     .class_name = "mpegtsraw demuxer",
> >>     .item_name  = av_default_item_name,
> >>-    .option     = options,
> >>+    .option     = mpegtsraw_options,
> >>+    .version    = LIBAVUTIL_VERSION_INT,
> >>+};
> >>+
> >>+static const AVOption mpegts_options[] = {
> >>+    {"fix_teletext_pts", "Try to fix pts values of dvb teletext streams.", offsetof(MpegTSContext, fix_teletext_pts), AV_OPT_TYPE_INT,
> >>+     {.i64 = 0}, 0, 1, AV_OPT_FLAG_DECODING_PARAM },
> >>+    { NULL },
> >>+};
> >
> >i dont know how commonly this is needed but the defaults should be so
> >that the demuxer works in as many cases as possibe
> >requireing the user to know and override defaults is a big problem
> >as normal users wont know where to look if something doesnt work
> 
> I'd say the fix is needed quite often, here are two possible
> scenarios which usually cause out of sync pts-es:
>  - using the same teletext PID for multiple programs
>  - adding teletext service to a HD channel by using the teletext PID
> of an SD channel - this is common because as far as I know there is
> no widely used standardised method of transferring teletext in a
> HD-SDI signal, so the teletext is typically generated by the SD
> encoder and not the HD...
> 
> Since most decoders have no problem with invalid teletext packet pts
> values, broadcasters don't care about it, and use it shamelessly...
> 
> On the other hand, guessing which program we should synchornize the
> teletext to will always be a guess, so this is a hack in the end of
> the day. That is why I disabled it by default, I kind of disliked
> the idea that the mpegts demuxer will return some guessed PTS values
> by default, instead of the actual ones in the transport stream...
> 
> Anyway, I can be convinced to change the default to 1, and spare the
> majority of the users the google search of finding the magic option
> to fix their teletext subtitles.
> 
> Or another option can be considered: keep it disabled by default in
> the API, but set it by default in the ff* tools.

ive no preferrace, my preferrace is to make what is most convenient
to the end user the default but i dont know which of the 3 options
that is

[...]

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