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Old Feb 6, 2013, 11:07 AM   #1
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MP4 videos AAC 5.1 -> AC3 (quickest audio encode?)


I have a number of .mp4 videos encoded with 5.1 (6 Channel) AAC audio. I would like to end up with an .mp4 with AC3 5.1 and 2 channel AAC encoded for Dolby Prologic II (such as the way Handbrake will do when encoding a video using AC3 passthough). I do not need to re-encode the video.

I stress that I don't want to reencode the video, only adding the AC3 5.1 track from the current AAC 5.1 track.

If I hit the "ATV3 Specs" alternative in Handbrake every movie takes about 1.5h to re-encode (it re-encodes everything).

Any ideas? Tried mkvtomp4 but it can only ac3 -> aac and not backwards. That's a shame because I love that program, and it's so fast.
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Old Feb 6, 2013, 02:51 PM   #2
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AC3/A52, unlike AAC/MPEG-4, is a proprietary codec of Dolby Laboratories. Therefore, third parties can't opensource the encoder they have to license it from Dolby. To the best of my knowledge, therefore, Dolby hasn't licensed any free AC3 5.1 channel encoders. You'd have to buy a licensed software encoder like SurCode ($495) or the Dolby DP569 multichannel encoder rack unit (about $3000).

Alternatively, you'd have to go back to the source, which I presume are DVD's you own legally, to re-transcode using AC3 passthru....
"Nature abhors a moron." - H.L. Mencken

Last edited by Avatar74; Feb 6, 2013 at 02:56 PM.
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Old Feb 7, 2013, 07:24 AM   #3
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Found a program called Xmedia Recode. It even has an Apple pre-set option, and should do everything that is needed(Stated above). However, itunes is unable to import the file whichever setting i choose.

Anyone of you recognize the problem? The output file is m4v, h264, aac + ac3 etc.
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