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Old 1 May 2005, 10:52 PM   #1
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Default AC3 Resynching Issues

Hi there

I've recently had some problem with my dvd backups. Since I got a large hard drive, I decided to start doing some 2CD backups of my dvd files. I later decided that there was no point having the movie as two separate CD (700MB) files, as I was never going to write them to cds anyway, so used Virtualdubmod to merge the streams into one file. The problem is that while some of the files merged perfectly, others go out of synch when the files go past the half way mark, where the second cd files started.

Fortunately I noticed this before I deleted the original backups and loaded them up in avi-mux gui to check the files. It appears that some of the encoded files have a short delay before the AC3 audio starts, which is screwing up all my re-encoding attempts to put them back as one file for each 2CD set.

I've tried:

- remuxing CD2 using avi-mux gui and then joining in virtualdubmod, but this results in a slightly different frame rate in the CD2 file (29.976004 rather than 29.976 typically, which is enough to stop virtualdubmod joining them)
- remuxing both CD files using avi-mux gui and joining them in virtualdubmod (exact same audio desynchronisation)
- joining the two files using eovideo (exact same audio problem)

I don't know what else to try honestly. I don't know how I managed to mess up the audio, but I don't particularly fancy having to re-rip the files again. Is there an easy way to visualise why the AC3 audio stream is messing up?
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Old 2 May 2005, 03:41 AM   #2
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"I later decided that there was no point having the movie as two separate CD (700MB) files"

"I don't particularly fancy having to re-rip the files again."

Despite your statement, re-ripping the DVDs that you own is probably the quickest - and definitely the most accurate - way of accomplishing your goal of a properly synchronized single file backup...
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Old 2 May 2005, 04:16 AM   #3
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Unfortunately I already know this, but for future reference I'd like to know how to fix this kind of error. There must be some kind of problem with the AC3 synchronisation in the CD2 files that disagrees with the CD1 files. Both play perfectly on their own, but when put together the CD1 half works fine, but the CD2 part has the audio playing slightly earlier than expected, maybe by a fraction of a second.

Is there a program I could load these up in outside of AVI-Mux GUI which could give me an accurate visual representation of when the AC3 stream in a CD2 file is starting? I assume that they way I've created the various rips has somehow created slight delays before my CD2 files start, so I need to create a delay in the middle of an avi file, or at least create a short period of silence at a designated point in the AC3 audio.

Any help that prevents me having to spend several days re-ripping files will be greatly appreciated.
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Old 2 May 2005, 04:19 AM   #4
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Try simply adding a delay of 1200 milleseconds to the second file BEFORE joining the two...
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Old 2 May 2005, 08:45 AM   #5
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Actually, the audio is only slightly fast by a fraction of a second when the files are merged, which makes no sense really. I'm no good at lip synching, which is why I was so happy when I found AVI-MUX GUI in the first place, but setting the delay AVI-MUX GUI tells me the files already have in Virtudubmod and remuxing the CD2 files doesn't synch it up exactly, so it must be reading the files wrong. Either that or the audio has been messed up somehow between the splitting.

Argh, I don't see why this is only happening with some of my files and not with others. It makes absolutely no sense whatsoever!!!
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