That's just a matter of what name you give things.
I like to name things by their correct name, especially when describing technical things. This helps to better understand the technology behind our beloved hobby.
Bed Channels
Bed Channels is an Atmos concept that applies only in the
mixing process. As a mixing engineer you can use either Bed Channels or Objects… when You are mixing.
https://professionalsupport.dolby.com/s/article/What-is-a-bed?language=en_US#:~:text=A bed is a channel,for stereo or surround panning.
The mixer engineer can use “Beds” up to 7.1.2
TrueHD and Substreams
The final rendered consumer TrueHD file is structured as a series of
substreams, and can be seen in the chapter “2.3 External organization” of this document:
https://developer.dolby.com/globala.../dolbytruehdhighlevelbitstreamdescription.pdf
Dolby Atmos in a TrueHD file
The Master Dolby Atmos mix is “exported” to a final consumer file TrueHD, that use 4 substreams:
- Substreams 1,2,3 with pre-rendered 2.0 / 5.1 / 7.1 mch mixes that are “downmixed” from the Atmos Master when producing the TrueHD file at the production studio. This is done by the “export” Tool.
- A Substream 4 that contains Atmos Metadata, for the AVR/processor to “decode” and “render” the Atmos mix to the available speakers.
This is what I’ve found in trusted posts of some forums like AVSFORUM. Still looking for “official” Dolby description of this.
It still contains all the sounds and is still what the Atmos objects are subtracted from for Atmos playback. It is not an entire separate copy of the audio, if you think it is please provide some references (oh I forgot, Dolby don't document Atmos).
Yes, the substreams 5.1/7.1 in the TrueHD file contains all audio, that has been “downmixed” from the Master Atmos mix into 5.1 / 7.1
“it is not an entire separate copy…” What do you mean? If you “decode/extract” only that substream from the TrueHD file you do get an entire separate copy.
Actually I think inspclouseau was saying it's inherently a crap codec, at least that's how it read to me.
I agree that saying only that one thing is a “crap”, without adding any additional information or explanation can be confusing.
My understanding is the downmix can be performed on auto as you describe, but it can also be manually directed by the mixing engineer.
I think the mixing engineer has little to none control of the “exported” 5.1/7.1 substreams from the Master Atmos mix to the TrueHD file. The only control would be to “adapt” the original Atmos mix to find a compromise that could be sound not bad in the 5.1 substream. Something similar they do when they try to export to the “binaural stereo Atmos” for Headphones. Some good things that work OK for Full Atmos Speakers cannot work well in “virtualized binaural stereo”.
And it is possible (and indeed demonstrated with Dark Side of the Moon Atmos test tones) to have sounds come out in different places between the True HD stream and the Atmos render to a 5.0 system like mine.
Can you elaborate that? How do you play and what different places you get?
What your AVR show: AC3 or TrueHD?
I’ve been digging into that BD and found that the Dolby TrueHD Atmos “track” has an additional AC3 5.1 lossy stream (not exactly the TrueHD substream MLP lossless coded), that can be extracted using MKVToolNix.
That 5.1 AC3 file test tones plays like this:
5.1 channels as it should be
Rear Surrounds --> Side Surrounds
Left Top Front --> L, C, SL, SR(low volume)
Right Top Front --> R, C, SL(low volume), SR
Left Top Rear --> SL, SR (low volume)
Right Top Rear --> SL (low volume), SR
The automatic downmix on Dark Side of the Moon is not good, like the Atmos test tones there's a lot of wishy washy "in the middle" going on. The positioning is much more precise on my 5.0 system when I tell it to play the Atmos, so I'm getting a local render to 5.0 rather than the substream/bed whatever it is on the disc.
Can you elaborate that?
How do you play “Atmos” in your 5.0 system differently than when you get the substream? What system? What Options? What Your AVR display as signal?
