Atmos vs 5.1

[Mr. Afternoon]

If it happened the way you describe, you wouldn't be able to rip the stereo or 5.1 substreams. But you can. There is no after the fact manipulation of those streams once they are on the disc. MakeMKV dosent have that kind of capability. One stream does not build off another. Except for the 7.1 Atmos stream.

What I wrote is what Dolby describes in their high-level bitstream paper for TrueHD. Remember, on a Blu-ray disc, if there is a dedicated stereo and 5.1 mix, they are encoded as separate TrueHD streams, and each TrueHD stream has its substreams.

 

[SeeMoreDigital]

What I wrote is what Dolby describes in their high-level bitstream paper for TrueHD. Remember, on a Blu-ray disc, if there is a dedicated stereo and 5.1 mix, they are encoded as separate TrueHD streams, and each TrueHD stream has its substreams.

The fact that Dolby has to use the term 'MLP' (Meridian Lossless Packing) in their documentation peeves me greatly!

 

[Sal1950]

The fact that Dolby has to use the term 'MLP' (Meridian Lossless Packing) in their documentation peeves me greatly!

Yea that sucks, but Meridian did design the lossless compression codec.
OTOH it could be worse, good thing they don't have to put MQA in there.

 

[inspclouseau]

I just read two technical papers from Dolby on TrueHD then promptly deleted them. lol. Made my head hurt.
To say I understood half of it would be an overstatement. But no where I could find anything about mixdown. They probably have a boatload of technical papers, so IDK.

Mu first really good laugh with my coffee this morning~

 

[Owen Smith]

For TrueHD, what it (supposedly) does is that it stores a stereo downmix in the first substream. Then it stores data to correct and transform that downmix to 5.1 in the second substream. If the original mix is 7.1, then it stores data in to correct and transform that downmix to 7.1 in the third substream.

That's not what I read, where did you get this information? TrueHD is just MLP with metadata, and MLP never had mutliple mixes. TrueHD is just 7.1 is my understanding, and if you have a 5.1 system the rears are mixed together, if you have stereo only then that is all mixed down on the fly.

 

[Owen Smith]

The way is having or setting the AVR to "only" understand that AC3 substream. And with the Player Blu-ray menu select the "Atmos" track.

Use coax or optical from the player to the AVR, then you should get the AC3 substream. This is a method of forcing the issue.

 

[Mr. Afternoon]

That's not what I read, where did you get this information? TrueHD is just MLP with metadata, and MLP never had mutliple mixes. TrueHD is just 7.1 is my understanding, and if you have a 5.1 system the rears are mixed together, if you have stereo only then that is all mixed down on the fly.

Dolby documentation implies this is how MLP works...and online there's some confusion...I'm a little confused myself to be honest. I'm not saying MLP has multiple mixes, I'm saying it's got a weird way of storing channels, which may lead people to believe it's storing multiple mixes when it is not. I do know that TrueHD is MLP with additional metadata, as MLP already has inherent metadata for downmixing settings. However, skimming the code for the open-source decoder in FFMPEG for TrueHD, it does appear that this weird "substream" approach to storing the data is actually what Dolby is doing with TrueHD.

 

[himey]

I'm not saying MLP has multiple mixes, I'm saying it's got a weird way of storing channels, which may lead people to believe it's storing multiple mixes when it is not.

I have heard it described that way too.

 

[Owen Smith]

What I wrote is what Dolby describes in their high-level bitstream paper for TrueHD. Remember, on a Blu-ray disc, if there is a dedicated stereo and 5.1 mix, they are encoded as separate TrueHD streams, and each TrueHD stream has its substreams.

That's a good document, but it only describes what is possible in the TrueHD stream not how it is actually used and the rules an Atmos decoder must use to select which substream to play. For example, you clearly can have a stereo substream, a 5.1 substream, and a 7.1 substream but nowhere does it say which ones a TrueHD stream must have or how a decoder must choose which to decode.

Edit: I can see why they'd put a 5.1 AC3 substream in, so that old players and/or old AVRs that don't handle TrueHD have something they can play even over coax or optical. But I cannot see why they'd put a stereo MLP substream in as well as the 7.1, the lossless stereo stream would consume a lot of bandwidth (admittedly not necessarily a problem on Blu Ray) and any TrueHD decoder can downmix 7.1 to stereo anyway. What it might be is the AC3 substream is sometimes 5.1 and sometimes stereo (which may or may not be matrix encoded).

 

[LuvMyQuad]

What I wrote is what Dolby describes in their high-level bitstream paper for TrueHD. Remember, on a Blu-ray disc, if there is a dedicated stereo and 5.1 mix, they are encoded as separate TrueHD streams, and each TrueHD stream has its substreams.

I can't say I fully understand all of this either. But everything I did see would apply to what goes on prior to authoring the bluray disc. There is just no mechanism there to explain how you can rip a complete 5.1 stream with no manipulation of the the other streams.

 

[Mr. Afternoon]

Edit: I can see why they'd put a 5.1 AC3 substream in, so that old players and/or old AVRs that don't handle TrueHD have something they can play even over coax or optical. But I cannot see why they'd put a stereo MLP substream in as well as the 7.1, the lossless stereo stream would consume a lot of bandwidth (admittedly not necessarily a problem on Blu Ray) and any TrueHD decoder can downmix 7.1 to stereo anyway. What it might be is the AC3 substream is sometimes 5.1 and sometimes stereo (which may or may not be matrix encoded).

From what I understand the substreams build off of each other. They are not individual mixes, but each substream is correction data to morph the previous substream into a new channel layout.

So if you have an Atmos file and a 5.1 setup with a non-Atmos TrueHD decoder, the decoder would go like this.

Read the first substream with stereo audio. Do I support 5.1? Yes. Read the second substream which contains correction data and use that correction data to morph the stereo substream to 5.1. Do I support 7.1? No. Ignore the third substream. There is a fourth substream (Atmos) I do not understand. Ignore that.