[MPlayer-dev-eng] A/V sync problem with independent demuxers

[Arpi]

Hi,

> I have found that MPlayer's internal A/V synchronization mechanism often 
> does not work well for situations (such as RTP streaming) in which the 
> audio and video demuxers are independent (i.e., are each fed from a 
> different input stream, rather than both being tied to a single input 
> stream).  In this case, the packets read for each demuxer (using 
> "demux_fill_buffer()") are given the correct "pts" (presentation 
> timestamp), but the rest of MPlayer often (but not always) fails to 
> maintain A/V synchronization.

disagree

> I have seen two different situations occur.  The first situation occurs 
> when the user presses the space bar to pause the playout.  At this point 
> the audio and video playout both stop immediately, as they should.  But 
> after the user presses the space bar again to resume playing, video gets 
> several seconds ahead of audio (i.e., "A-V:" is negative).  The length of 
> the gap seems to correlate to the duration of the pause.  Thus, it seems 
> that MPlayer's internal buffering is not handling pauses correctly.  One 

wtf?
mplayer is not threaded. if your demuxer keep reading one steram under
pause, it's your problem

> thing I noticed is that immediately after pressing the space bar to resume 
> playback after the pause, MPlayer issues an unusual number of reads to the 
> audio demuxer, before it resumes reading from the video demuxer.

of course. since audio card buffers up 0.3-1.5 sec (avg. values for avg.
soundcard) so the audio card's buffer has to be filled before we continue
playback. (the audio buffer is a FIFO-like thing, unless you fill it enough,
it won't start play at the right time)

btw it's depend on -ao used. some ao drivers can do real pause, ie. keep
audio buffer contents but stop playback imemdiately, so it doens't have to
re-fill buffer after unpause. some others can't do that, so the card plays
teh rest of data from buffer _after_ pause, and after unpause the buffer has
to be filled again.

> I have seen this problem happen when audio and video are MPEG-1 or 2 
> elementary streams; I have not yet confirmed whether or not the same 
> problem occurs with other codecs (but I suspect they probably do).

it's all demuxer problem, it doens't (shoudl not) happen when playback
normal files or use demuxers written by me.

> Could someone who's familiar with MPlayer's A/V synchronization mechanism 
> take a look at how it deals with independent audio/video demuxers?

the a-v core doesn't care if your steram sare independent or interleaved -
it just requires accurate PTS values for both streams and all is right.

the whole a-v works this way: (VERY simplified:)

while(1){
  if(a_pts-a_buffer_delay<v_pts)
    play_audio_chunk
  else
    play_video_frame
}

a_buffer_delay is usually 0.3..1.5 seconds, depending on card, driver,
samplerate, filters, channels, format, sometimes audio codec too.


A'rpi / Astral & ESP-team

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Developer of MPlayer, the Movie Player for Linux - http://www.MPlayerHQ.hu
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