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.MKV video with 6-channel AC3 audio downmix to Stereo AAC, Best way to downmix 6-channel AC3 to Stereo AAC
post Oct 26 2011, 13:38
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Hello, I've been trying to search the forum but having AC3 and AAC as my primary keyword, the forum search engine won't let me due to the character limitations. So I hope this hasn't been asked yet. If it is, may I ask a link to where I could direct myself to the right thread? Also I hope I'm asking in the right category too.

So here it is. I have 350 separate .MKV files which are separate "scenes" of a single film project. Each of these MKV files have 3 tracks. Video, Audio, and subtitle. Now I tried to use MKVtoolnix to append all these files together but it wasn't possible due to the audio tracks of some of the mkv files are of different format. So here's the breakdown of the audio file differences out of those 350 files.

20 files are 6-channel AC3 @ 384 kbps
20 files are stereo AC3 @ 384 kbps
310 files are stereo AAC @ 192 kbps.

Now, Video and subtitle doesn't seem to have any problems, but MKVtoolnix only pointed out the audio error only. So This is what I'm assuming would be the best way to do it.

1. Demux the 40 mkv file to separate the audio from the video (also the subtitles too).
2. Downmix the first 20 files into Stereo AC3.
3. Convert the 40 files into AAC. (keeping the stereo).
4. reencode the bitrate into 192 kbps.
5. Remux the audio with the video (with subtitles too).
6. Append all the remaining files together.

Do I get those steps correctly? Or is there any way that I can make the steps simpler and more efficient in terms of minimalizing quality loss? Maybe there are some steps that I can do simultaneously... As they say, single process is better than 2 separate processes since it has risks of losing some of the quality in separate processes. Or perhaps I got all the steps wrong and there's a right way to do it?

On the other hand, what tool can I use to achieve all that work in the most efficient and effective way? I don't mind command line since I tested out with eac3to myself at first, but I still don't know if the steps I want to do are the correct one. A GUI probably better for me. I also don't mind buying some commercial decoders/encoders myself too if I have to.

Any thoughts?
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post Oct 26 2011, 21:07
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I've never worked with MKV files...

Subtitles are always a problem, and the programs I'm suggesting will probably loose them. You'd probably have to handle the subtitles separately, and re-multiplex them after converting & joining the audio/video files. Subtitles are usually added at the end after the audio/video editing has been done and you've got a complete movie/program. So, most video editing programs (and conversion programs) will ignore subtitles when you import a A/V file. sad.gif

Typically if you want to join different audio/video formats (and optionally render to a new format) you'd use a video editor. I use Corel Video Studio and Womble for video editing, but I don't know if these will work with your MKV files. Lightworks is FREE, but I have not tried it yet and I don't know if it can edit/convert MKV files either.

Another option is to use a conversion program like SUPER (FREE) to convert all of the files to a common format before joining them. SUPER does support MKV, and it has GUI interface. SUPER can convert audio files, video files, or audio/video files. It can demultiplex the audio or video from an A/V stream or it can 'extract" the audio or video to a different format. But again, I don't think it handles subtitles.

This post has been edited by DVDdoug: Oct 26 2011, 21:12
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