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AC3 audio editor, What is the best way to crop an AC3 audio track?
jeffdgr8
post Aug 27 2008, 10:20
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I want to combine an HDTV video recording and an AC3 DVD audio track, but the audio track doesn't quite fit with the video. I need to crop off the beginning and end (MGM lion roaring) and add the proper amount of silence to make the track the correct length to fit the video before muxing them together. I've extracted the audio from the DVD and saved it as a .ac3 file. The only program I've gotten to open the AC3 file for editing is Nero Wave Editor (latest v4.3.4.0) and for some reason, it only opens the first 15:30 of the track. I've tried Sound Forge, Audacity, Audition, and Soundbooth and none of these programs will edit AC3 audio. Maybe some of these programs require a plugin to unlock this capability. Does anyone have any suggestions? I don't really need to do a lot of editing to the track. I'm not adding anything except for silence, just removing is all. So if it were possible to do this without having to decode and reencode the whole track, that would be best.
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j7n
post Aug 27 2008, 17:47
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AC-3 streams can be cut without re-encoding with the help of a hex editor. It's a long and tedious process.

Always cut whole AC-3 frames. Each frame starts with header 0B 77. This header occurs at regular intervals in a CBR file.

To see how many bytes you should remove use this formula:

bytes = bitrate[kBit/s] * 1000 * time[s] / 8.

So, a lion lasting 4.6 seconds ends at byte 258048 in a 448 kBit/s stream.

It is likely that your DVD has already defined a cell (chapter) for the lion clip. If this is so, just demultiplex the MPEG-2 Program Stream by cells, and you'll have the lion separated. Your DVD ripper migth allow to select chapters before ripping. If it does, this is the easiest way to get rid of chapter 0.

After this is done you'll have to determine the remaining video-audio delay, which is beyond the scope of this discussion.

If one of your video sources runs at 25 fps and the other at 23.976 fps, you will have to resample and re-encode the audio. One transcoded HDTV capture is not worth all this work IMO.
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