[MPlayer-users] Bug: av sync, bframes, dwStart issues

[John Stebbins]

Corey Hickey wrote:
> John Stebbins wrote:
>   
>> Something that I stumbled across last night that hadn't occurred to me
>> before.  If you re-encode an avi file that already has audio delayed, you
>> will be incrementally delaying the audio even more.  E.g. you have
>> high quality archive of h.264 and you want to transcode to lower
>> resolution for a portable device.  To do this correctly, the user needs
>> to be aware that the audio will need to be manually adjusted
>> (e.g. -audio-delay -0.83) to get the proper sync. Note the value of
>> the manual adjustment depends on the amount of delay that was
>> inserted into the high quality archive copy and has nothing to do with
>> the new encoding format. So the user also must remember the original
>> settings used to encode their archive.
>>     
>
> You can also use -ignore-start, if dwStart hasn't been altered for other
> reasons.
>   
Yes, I thought of this shortly after hitting the "send" button. Like you
said, its not ideal, but since dwStart is so seldom used for anything
else, it works pretty well.
> Yes; lavc will do that too, depending on the vb_strategy parameter (see
> also b_sensitivity and brd_scale).
>
> What I was wondering, though, was whether the B-frame variation actually
> affected A/V sync within the file, as you had indicated. Have you
> observed this effect with any video players? I just ran a test with
> mplayer and there wasn't a problem.
>   
There I go making bad/untrue assumptions.  Thanks for testing.  I guess
the player has the video delay whether there are actually bframes
present or not. So now I have to wonder how the player knows that
bframes are suppose to be present.  And does a similar test with
b-pyramid enabled sync correctly as well. If this is the case, then the
player could compensate for the sync because it knows in advance that it
will be delaying video. I know this doesn't "fix" the problem for all
the other players that exhibit this same problem, but I still believe
this is something the player should be doing.