[vlc-devel] Subtitles synchronization : identified a bug cause that breaks audio output under linux / need help

Pascal Thomet pthomet at gmail.com
Sat Apr 27 18:52:52 CEST 2013

Correct my last message (I made a mistake in my first test).
When using interleaved subtitles one can only apply a delay of a few
seconds (and the call to setJitter is effectively necessary).

On Sat, Apr 27, 2013 at 4:23 PM, Pascal Thomet <pthomet at gmail.com> wrote:
> Thanks Rémi for your analysis!
> I understand that when subtitles and video are interleaved there are
> certainly buffering issues.
> However, my tests (explained below) show that subtitles syncing seems to be
> robust enough, but audio syncing is broken at the time being.
> Here is what I tested :
> - subtitle sync can be quite robust, even with interleaved subtitles : I
> tried to open a movie with interleaved subtitles and to aply a delay of
> +/-500 seconds to the subtitles. It works like a charm (when I comment out
> the call to SetJitter). It also works with separate subtitles.
> - Audio sync is broken at the time being : I can cancel out the sound easily
> like this ;
>      open a video
>      select the menu "tools/track synchronisation"
>      enter "3" into the "Audio track synchronization" field (ie add 3
> seconds delay to the audio)
> Note: SetJitter is not the culprit here, since the audio will disappear
> whenever the call to SetJitter is commented out or not.
> I think that the problem with audio sync is beyond my competencies (much to
> rooted in the core of the source).
> One more remark concerning interleaved subtitles : whenever someone wants to
> correct the subtitle delay, it is (almost certainly) because he obtained the
> subtitle from a different source. If an interleaved subtitle is out of sync,
> it probably means that the whole file is crap anyway. So, may be it would be
> possible to have something like that ?
>     es_out_SetDelay( p_input->p->p_es_out_display, AUDIO_ES, i_audio_delay
> );
>     es_out_SetDelay( p_input->p->p_es_out_display, SPU_ES, i_spu_delay );
>     ////imaginary function is_interleaved_subtitle(...) would to still have
> be implementted
>     if (is_interleaved_subtitle(...))
>     es_out_SetJitter( p_input->p->p_es_out, i_pts_delay, 0, i_cr_average );
> However this would have to wait until the situation with audio sync is
> fixed.
> Regards,
> Pascal Thomet
> On Sat, Apr 27, 2013 at 2:32 PM, Rémi Denis-Courmont <remi at remlab.net>
> wrote:
>> Le samedi 27 avril 2013 15:02:45, Pascal Thomet a écrit :
>> > But what I do not understand is : why would applying a simple subtitle
>> > delay require the jitter to be corrected?
>> Indeed, the inter-arrival delay variance, a.k.a. jitter, should not be
>> affected
>> by manual change of inter-track delay. But then again, it does affect the
>> overall buffering requirements.
>> This may not be a problem for a separate subtitle track file, but it is
>> obvious
>> when tracks come interleaved from the same file. Suddenly audio or text
>> data is
>> going to arrive much earlier or much latter than video data. That is why
>> the
>> pts_delay value is partly dependent on the subtitle and audio delay. And
>> thus
>> the SetJitter() call.
>> --
>> Rémi Denis-Courmont
>> http://www.remlab.net/
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