[mythtv] A/V Sync problem only on some PBS Hi-def shows

DaveD mythtv at guiplot.com
Thu Apr 17 07:46:30 UTC 2008

Michael T. Dean wrote:
> On 04/16/2008 12:12 AM, DaveD wrote:
>> Steve Wilson wrote:
>>> --- DaveD wrote:
>>>> I've posted this problem a couple of times to the
>>>> users list, so far no 
>>>> solutions.  I have bad (unwatchable) A/V sync on
>>>> some of our 720p HD PBS 
>>>> shows.  Some shows (Desert Speaks) always work, some
>>>> shows (e2) never 
>>>> work. ...
>>> I had that problem and it was only on the high-def PBS
>>> channel as the sound would wobble really bad and that
>>> was the only channel. I either turned on or off
>>> Aggressive Sound Card buffering under Setup -> General
>>> and the problem went away. I can't remember which way
>>> since I built a new system.
>> Thanks for the response.  I'm not sure I got the right setting, but I 
>> tried turning TV Settings / Playback / Extra Audio Buffering off and it 
>> broke ALL 720p playback, not just the problem shows.
> The one Dave mentioned:
> Aggressive Sound card Buffering
> If enabled, MythTV will pretend to have a smaller sound card buffer than
> is really present.  This may speed up seeking, but can also cause
> playback problems.
> should be off.  (Under General Settings, about page 2.)
> However, for almost all not-Myth-encoded video (hardware encoders and
> DVB/ATSC capture cards):
> Extra audio buffering
> Enable this setting if MythTV is playing "crackly" audio and you are
> using hardware encoding. This setting will have no effect on MPEG-4 or
> RTJPEG video. MythTV will keep extra audio data in its internal buffers
> to workaround this bug.
> should be on.  If you did your test of Extra audio buffering with
> Aggressive Sound card Buffering on, the results are meaningless.
> Mike
> _______________________________________________
Thanks for the clarification.  Aggressive Sound card Buffering was on.  
I turned it off.  It made no difference.  When extra audio buffering is 
on, some 720p shows play normally, others loose sync badly after 20-30 
seconds.  They recover, then loose it again, then recover, etc, etc,...  
With extra audio buffering off, no 720p shows play in a watchable 
manner; they start out instantly with sputtering sound.  Again, the 
state of Aggressive Sound card Buffering makes no difference.

New info: I tried "Use video as timebase".  This fixed the sync 
problem!  But now the voices go from normal to chipmunk to low, slow, 
back to normal as the video and audio speed up and slow down together!

I have been here before:  I have tried every combination of every 
setting I can find on two significantly different systems and nothing 
helps.  I have tried FC6, FC7 and Kubuntu package-based systems and I 
have tried building from source.  I can't help but I think it only shows 
up on Seattle Comcast-compressed 720p broadcasts.  I haven't tried OTA 
yet, but I have a tuner and just need to get it up high enough (and 
amplified) to try directly from the station to see if Comcast's extra 
compression is a factor.

I have copied a file that exhibits the playback problem to a web site:

This is a short section of a normally half hour show and is about 
742Mb.  I wonder if someone one the list would be willing to download it 
and play it with Myth to see if they can play it normally?  I have just 
started the upload so it probably wont be done for a couple of hours, or 
so, as my upload speed is kinda slow.  If this plays normally for 
someone else, I'll keep looking for why my systems can't play it right.  
On my systems, it starts to loose sync after about 35 seconds and always 
seems to be in the same place.  With the "Use video as timebase" turned 
on, I noticed the background music getting bad wow in about the same 
place, or slightly before.

Thanks for all the help.


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