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Old 13th January 2017, 09:39   #25501  |  Link
MrVideo
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Quote:
Originally Posted by Lathe View Post
Well, 2 things I notice right away:

1. when a Blu-ray is rendered into an MKV file and also happens to have a TrueHD audio track, the AC3 core is ALWAYS ripped out. What that means is that when you then go from the MKV back to a playable Blu-ray or a BDMV folder, all that remains is just an AC3 @640 track. This took me a while to learn when I kept encountering all these TrueHD audio tracks, and apparently like you, I also like to keep the full HD audio if possible.
You can't rip out the AC3 core and have AC3 5.1 to put back into a Blu-ray. If you rip out the AC3 core, you are left with only the TrueHD portion. While doing some digging, I found an old posting:
Quote:
There are 2 different TrueHD streams:

(1) TrueHD+AC3. This is a mixed format. There are both (alternating) TrueHD and AC3 frames in the stream.

(2) TrueHD. This stream only contains TrueHD frames and no AC3 frames.

All Blu-Ray TrueHD streams are (1). All HD DVD TrueHD streams are (2). eac3to can convert between both stream types. Converting from (1) to (2) is very easy: The AC3 frames are simply removed. Converting from (2) to (1) is more difficult: The TrueHD stream must be decoded and reencoded to AC3 and then the AC3 frames must be injected into the TrueHD stream in a specific way.

When muxing to MKV, the AC3 frames are usually removed, so MKV TrueHD streams are usually type (2).

There's no special metadata which tells anyone how to convert TrueHD to AC3, AFAIK.
So, if you have a MKV file that contains only AC3, because the TrueHD was thrown away, there is no way to get back the TrueHD.

As for mkvmerge's handling of TrueHD+AC3, I found the following:
Quote:
The Matroska specs state that one track must only contain data for one codec. Therefore mkvmerge cannot keep the AC-3 interleaved with the TrueHD part in a single track.

Starting with release 7.7.0, mkvmerge's solution is to present the AC-3 core as an additional track. This applies to TrueHD tracks read from any source file, be it from e.g. a raw TrueHD file, from a Matroska file or an MPEG transport stream. Before release 7.6.0, mkvmerge was silently discarding the AC-3 core.
Now comes the tricky part. If you've created a MKV file with a newer mkvmerge, so that you have both audio tracks, getting it back into the required Blu-ray spec is going to be tough. Why? Because the two audio tracks must be interleaved in a very special way. The first quote seems to imply that eac3to can be used to interleave the two streams into a single TrueHD+AC3 for Blu-ray use. I've never tried it.

Last edited by MrVideo; 13th January 2017 at 09:41.
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