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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 27 2011, 06:57
Post #2

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Posts: 4
Joined: 26-October 11
Member No.: 94707

Thanks for the suggestions.
But I did use a video converter that supports joining. That one is actually a lot easier to do since I just use all 350 mkv files as an input set it to output as one single mkv file and the application just joins the file beautifully. The problem with that is the software renders the whole movie from the start until finish. The total length is over 8 hours. And it took me 3 days because the software renders frame by frame and reencoding them into a common format.

In the end, it joined successfully but there was a massive loss of quality from both the video and the audio. I know because I compare the scenes with the original source and there were blocks everywhere and parts of the video that's suppose to be sharp becomes blurry. (Even though the technical data seems to be identical resolution, unified bitrate and sample frequency, etc.)

When I use eac3to and mkvtoolnix, I can join and demux at the raw level, without reencoding ever occuring. That resulted in a very clean identical quality output. But that still gives me the problem. Mkvmerge won't let me append the 350 mkv files together because of the AC3 and AAC format differences in some of the videos.

Since the number of files that uses AC3 is a lot less than the AAC, that's why I assume it's better to demux the video from the audio and convert the 384 AC3 into 192 AAC. But I don't know what tool to use to let me do just that. Also have to downmix the AC3 as well.
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