[mythtv] 8902 HD transcode => MPEG4 --> a/v out of sync

Jesse Crews jcrews at gridlox.net
Thu Feb 9 02:52:16 UTC 2006


I've been having an issue with transcoding anything recorded from a 
particular ATSC transponder (NBC affiiliate, OTA). The output always 
shows the video somewhere around .5 seconds earlier than the audio. 
Maybe even an entire second.

Anyway, I'd like to try to figure this one out, but I'm not too sure as 
to how to go about isolating the area that has to be enhanced/fixed. 
All other local transponders have good transcode output.

I did run mythtranscode with mpeg2fix process messages enabled, and 
nothing came out. NVP did throw at me an "Guessing at a/v sync values" 
message, which, according to the comment, is indicitive of something 
terribly, terribly wrong in the source file. In this case, it looks 
like the NBC station is transmitting a funky stream. The a/v sync is 
wrong early in the output, so I'm sure something is breaking at a 
setup/initialization stage.

When I play the file back via mythfrontend, it also throws the guess 
message, but the A/V sync is correct. What could be different in the 
way that NVP calls are being utiized by mythfrontend vs. mpeg2fix in 
mythtranscode? There are some comments in there about how some 
operations being done "might not be a good idea."

Should I add some messages to print aPTS, vPTS, PCR (really, whatever 
the PCR is equivalent to in a TS), and try to see where drift is being 

There does appear to be some logic to figure out if the above values 
are wrong, and I recieved no such messages.

Thanks for any advice. Regards,
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