[mythtv-users] Sound a second behind video

Michael T. Dean mtdean at thirdcontact.com
Thu Mar 13 03:31:29 UTC 2008


On 03/12/2008 11:11 PM, Mark Hutchinson wrote:
> On 12-Mar-08, at 9:02 PM, Michael T. Dean wrote:
>> On 03/12/2008 07:58 PM, Mark Hutchinson wrote:
>>     
>>> mplayer is fine with the file.
>>> The Internal player audio is off.
>> It plays fine (and in sync) on my system.  Even in the 3rd (Life  
>> Cereal)
>> and 4th (Caltrate) commercials and the show itself where I could see  
>> the
>> faces of the people speaking.
...
>> I rebuilt the seektable with:
>>
>> mythtranscode --mpeg2 --buildindex --allkeys -c 9999 -s '2008-03-11
>> 16:00:00'
>>
>> (which is the proper way to build a seektable for an MPEG-2 recording)
>> and it played back perfectly, with no sync offset.
> As a standard video

I played it as a recording.

>  though, using Internal player, would there be a  
> seek table?
>   

There would be if you build one with the --video argument to 
mythtranscode (see the --help output).  (Note that --video is also valid 
for mythcommflag, but we've already established you don't want to use it 
to build a seektable for these MPEG-2 videos.)

> Does this still not indicate a bug?  I see this behavior too while  
> watching live TV on SD. Fine on HD though from the same firewire device.

Ideally ffmpeg code would be able to play it even without a seektable.  
However, with a good seektable, we have information that's extremely 
useful at playback time (allowing a +7 second skip to skip 7 seconds of 
video--whereas the same in a player without a seektable--such as xine or 
MPlayer--can trigger a -60second to +120 second skip, depending on the 
video stream and the exact time when you hit the skip button and how 
much information is known about the video around that point--i.e. have 
you already played the section once and skipped back--and ...).  And, I 
would guess it would also be useful in frame delivery (but I have no 
proof of that).

So, since you and others have found that ffplay doesn't work well (and I 
just verified--in ffplay sound played almost a second before the video 
in the commercials, but was in sync during the show), it may be a "bug" 
(or at least a type of stream not handled well by ffmpeg libraries).  
That doesn't mean that we can't play it back, though... :)

If you can prove that you're getting a bad seektable during recording, 
we have something to fix.  Otherwise, we're probably at the mercy of the 
ffmpeg devs.

Mike


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