Please Help Me Understand Downmixing

[volcanolotus]

I have a soundbar but I'm about ready to take the plunge and get a receiver. I'm trying to decide if I want to go 2.0, 2.1, 3.1 or 5.1, and I'm having a hard time figuring out what I'd actually get with different audio tracks at different configurations.

Setup is LG B7, Panasonic UB820, current model Chromecast (4K/HDR), and a laptop (less concerned about that for sound as it's pretty much only used for the kids to play games occasionally). I also used to have a laserdisc player and am strongly considering getting another. This is cinema-only (does not need to do double duty for music or anything like that). I lean more towards dialogue-heavy films (Criterion et al), but I assume a mono or stereo mix sounds essentially the same no matter which kind of system you're using. I just want the action movies to sound great when I do watch them

I think the easiest way to ask is to write out what I think happens in each scenario and hopefully people can confirm, or correct me where I'm wrong. (I'll also try to keep this post up to date, for posterity, since I haven't found this info in a single place anywhere I've looked.)

DVD/LD DTS/DD to 2.x/3.1: 5.1 mix with info on the missing channels missing (i.e., no downmixing)
DTS-MA/TrueHD (any) to 2.x: Downmixed to 2.0
DTS-MA/TrueHD 5.1 to 3.1: 5.1 mix with info on the missing channels missing (i.e., no downmixing)
DTS-MA/TrueHD 7.1 to 3.1: Downmixed to 5.1 with info on the missing channels missing
DTS-MA/TrueHD 7.1 to 5.1: Downmixed to 5.1
Atmos/DTS:X: Same behavior as 7.1

Further, I'm under the impression that 2.1 isn't really a thing and that the receiver just directs the appropriate frequencies to the sub from the 2.0 mix - is that correct?

Thanks in advance for any help here!

[oddbox83]

They will be down mixed - you can’t just ignore the rear channels in a 5.1 for example as you’d ruin the audio mix, so all the channels are “folded down” for stereo for example. Usually this is fine but very rarely you’ll come across a disc that has issues folded down.

Atmos/DTS X is a bit different. Those extra sounds are in the core 7.1 mix, and only receivers compatible will be able to unpick the metadata and place them above. So even though it’s a bit different you still don’t lose actual sound just the discrete channels.

[volcanolotus]

Originally Posted by oddbox83

They will be down mixed - you can’t just ignore the rear channels in a 5.1 for example as you’d ruin the audio mix, so all the channels are “folded down” for stereo for example.

That's sort of what I was wondering - is this true even for the old, lossy codecs? From what I had read, the reason the DTS Laserdiscs of stuff like Jurassic Park are still desired today because they have the original theatrical sound mix (or something close to it) as opposed to a trend that started with DVD of putting all the "important" information in the front channels since DVD was a more mainstream format and customers might not have understood that downmixing was not happening. I also feel like I've played DTS DVDs in the past where they asked me whether I wanted DTS or stereo on the main menu and I got an "are you REALLY sure" type message when selecting DTS. So that part I felt pretty confident about - but maybe I'm wrong!

The type of "folding" for 3.1 was something else I'd wondered about. At one point I had read something that suggested the downmixes go 7.1 -> 5.1 -> 2.0 (with Atmos/DTS:X just being metadata in the 7.1 mix as you mention). If there's no such thing as a 3.1 mix, do 3.1 systems get the 2.0 mix with a phantom center speaker? Or do they get the 5.1 mix with the rear info dropped?

Originally Posted by oddbox83

Usually this is fine but very rarely you’ll come across a disc that has issues folded down.

That's interesting! Is there a list of those somewhere?

[oddbox83]

Even the lossy codecs do, yes.

If a 3.1 soundbar is fully compatible with the codecs it is receiving, it will also downmix to a true 3.1 sound, folding down the rear channels. If it’s not compatible (many don’t do DTS) then it’s just to 2.0. But still a fold down, not a deletion.

Usually a mix will be fully checked to ensure it folds down properly, but sometimes it can fold down out of phase (Doctor Who Day if the Doctor 3D version is one I can recall) or overpower dialogue.

[Grinningcat_o3]

Piggy backing off this thread, if I have a Sonos Arc with no DTS support, what exactly is happening to my audio when I'm listening? Because I still get DTS playback on my blu rays.