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Sync Issues

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Lincoln Waldron
Sync Issues
on Jan 8, 2009 at 4:25:31 pm

Hey there...
We’re having a headache in our production department with putting back excellent Live Audio to a live DVD. For our own records, we had a live event that we recorded from a camera switcher straight to DVD, but discovered something went wrong with the audio. We had a sound desk back up recording of the event, which we wanted to add to the video.
So we set about decrypting the DVDs with MacTheRipper, and transcoded the VOB files to Photo JPEG with MPEG STREAMCLIP. Once in the timeline, we discovered our audio back up didn’t match the time base of the video, meaning if you sync it at the beginning it drifts out of sync at the end, and vice versa.
Having considered all options from frame rates to audio sampling rates straight through the whole process, we still sat without a solution or an answer to the problem. The VOB files are themselves still in sync, but when you add the new sound, the need arises to have to time stretch it ‘shorter’, so as to match the video.
The theory now is that there’s significant enough data loss between decoding the DVDs and transcoding the VOB files, to the point where the information difference between the treated video files and the untreated audio is great enough to not match up in real time. Over 2 hours, there’s about 20 seconds difference.
Does anyone have any ideas on this, or solutions as it were, that might help us understand the issue? It’s obviously a stupidly long process of working, but it’s a once-off correction that just seems to behave very bizarre.

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Tim Ward
Re: Sync Issues
on Jan 9, 2009 at 8:30:58 pm

[Lincoln Waldron] "Over 2 hours, there’s about 20 seconds difference."

If the actual drift is more like 7 seconds, then you should adjust the audio by 0.1% to accommodate. That is a common drift, and you may also run into a pitch problem (of 0.1%), which may or may not be acceptable. In this case (0.1% drift), the audio would technically be recorded at 48.048kHz sampling rate, but read and play as 48.000kHz. There is 0.1% difference between them, and that also happens to be the same difference between 30.00 FPS NDF and 29.97 FPS DF. This is an issue that can arise when recording separate audio from the video without syncing the audio recorder with the video equipment, and/or by using inferior gear.

Your sync problem may be more complicated since you've involved a DVD recorder-conversion-demux-edit process, which throws more possible issues into the mix. If you didn't roll a back-up (or master) tape (DV, Beta, etc.) this time, you should do that every time you use the DVD recorder for cases like these.

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