Information about audio out of sync problems

[infotech]

A lot of posters on the board have audio out of sync ( including myself ) problems when cutting or editing mpg files. I spent some time on the subject and decided to post my results. I researched what many were using or reccommending for tools ( and looked at what I have on my system ) and decided to use the following:

For mpeg sources I used three different mpg encoders 1)Adobe Mpeg ( Adobe Premier 6.5), 2) Cyberlink ( PowerVCR). and 3) tmpgenc ( 2.521). All sources were DVD compatible ( mpeg-2 720x480 NTSC, 4:3, 3kbit rate)

For cutters/editors I tried 1) tmpgenc (merge & cut), 2) tmpg DVD author (edit), 3) Womble mpeg-vcr ( 2 cuts - trim cut - that is cut left cut right, save the result and, two, the one button gop cut) and 4) cyberlink video trimmer( PowerVCR).

For viewing the results for audio problems I used PowerDVD which I have found to be the most reliable for playback of files.

For error checking I used the GOP fixer scan in mpeg-VCR and whether or not DVD author (and tmpgenc ) would read the file.

The results are:

Adobe Mpeg - all cuts from all programs produced mpegs that played without any audio sync problems, were readable by DVD author, and GOP fixer showed no errors before or after. It does however take many hours of Adobe processing to produce a one hour mpg file ( I did not use the editing portion of Adobe for this discussion for this reason)

tmpgenc source file - Cuts from everthing except the Womble gop cut produced staisfactory results. The GOP cut produced a file that DVD author said was an "illegal video file" ( gee, does this mean I will be arrested for producing an unlawful product?) anyway a file that it would not read ('invalid' I would prefer to say). The Womble GOP fixer scan showed no errors on the sources or cuts. All cuts played properly with PowerDVD.

Cyberlink encoded source file. Only the cyberlink video trimmer produced files that were in sync and usable by DVD author ( unfortunatly the trimmer interface is so cumbersome it takes a long time and one tired mouse finger to produce the cuts and even then the cut can be plus or minus a sec. from the place you put it). The other four cut/edit methods produce files that were unacceptable. Both tmpgenc and DVD author produced results that were grossly out of sync ( like Gene Kelly and what's her name in 'Singing in the Rain' ). The mpeg-VCR trim cut results were just out of sync enough to be annoying and the GOP trim produced an in-sync audio but the file was invalid by DVD author and tmpgenc standards ( they would not even read them ).

Running the Womble GOP fixer scan on the cyberlink source (and any cuts ) showed large numbers of time errors (GOP and PTS). Fixing the files after they were cut did NOT fix any of the problems. HOWEVER. fixing the cyberlink encoded files BEFORE anything was done to them did work and the resulting files could be cut and edited with any of the techniques and all sound was in sync and all files were valid.

Conclusion: If you are having sound sync problems after cutting ( like removing commercials from captured files, or editing your home movies ) then check the mpg source files for GOP and PTS errors and fix them BEFORE you edit.

Question for anyone. Is there a freeware GOP fixer that works?

Hope this helps some forum members get better clips.

-Charles

[infotech]

Update:

My information above works fine for producing single clips from a cyberlink stream, however, I discovered that the sound drifts in the final result when you try to merge a number of clips back together either with a direct merge or a dvd author ( sometimes the clips refuse to merge at all). I tried merging with Womble, tmpgenc, cyberlink trimmer, and tmpgenc dvd author. The only thing that did play was to create a DVD with each clip on a separate track ( rather than each chapter).

Back to more research...... (sigh)

[infotech]

Another update:

I've been analyzing the Cyberlink video and audio streams ( looking at frames and headers and PTS data) and have determined that the video is actually encoded at 30.00 frames per second instead of 29.97 (even though the headers say 29.97 ). This difference is enough to produce a 2 second mis-sync by the end of an hour clip. The files play standalone because the PTS's (presentation time stamp) synchronize the streams. When you edit/cut a file the PTS's are thrown out by the editors (tmpenc and womble) and the sync is lost. I verified that this happens by writing a program to look at the streams before and after and by using a hex editor. The same thing happens when you de-multiplex a Cyberlink file and multiplex it back. I am making progress on a way to overcome this and will post here when it is ready.