[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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