Basic question about how 5.1 tracks are played on 7.1 systems


I have an obscure question that I’m not able to find the answer.

I believe most of us take for granted that a TrueHD or DTS HD 5.1 track will automatically be decoded and playback as 7.1 on a system with back surrounds.

Eg some movies are still release in 5.1 track only, eg interstellar.

My question is how is it done, is it a matrixed surround back, or is it discrete data in the surround back channels?

Believe this has been the case since DD and DTS days.


i believe its done in the avr’s decoding chip to upmix it to 7.1 channels…
i dont think its “Discrete Data” at all…

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SoundWorks Collection: Dolby Presents Surrounded: 7.1 Cinema & Beyond

Old but gold clip on the advent of 7.1 soundmix.

My understanding is 5.1 is the most basic to create that enveloping sound effects. 7.1 or even Atmos are created simply because the listening space has expanded over the years in our “home”. For most Hollywood movies, studios will usually encode the bed layers in 5.1 discrete channels. As a result of DSP is getting better like Dolby Surround Upmixer (DSU) and DTS X Neural for instance are getting better in “extrapolating” the discrete channel info from Surrounds and “matrix” for the Surr-Backs. There is really no incentive to go for discrete 7.1. Do you still recall DTS used to release a very weird format called, DTS 6.1 Discrete where mono-channel was used to “fill in the audio gap” for sound coming from behind. This is all thanks to the craze over some of the more “atmospheric gaming titles” which rely on ambiance and stealth to win.

also got “DTS ES”…the movie title was,
“The Haunting”

Yes, that’s right. This is one of the best use of 7.1 surround back channels. It’s still hold up every well. For those who has never watch this, pls do yourself a favor and buy or download it. This truly showcase how a good sound mix if done right will perform way better than formats like Dolby Atmos and DTS X.

Used to have that DVD… now sadly i cant find it… i must have thrown it all out when i was clearing out all my “DVD’s”…
now i only keep “Bluray’s” n “4K UHD” Discs…

*One channel extension technique is the method by which MLP Lossless, Dolby TrueHD, and MPEG-2 LII deliver compatible downmixes for soundtracks with expanded channels. *

In these codecs, a 7.1-channel soundtrack is first downmixed to create a 5.1 mix, which is supplemented by a two-channel extension (which we’ll call “extension B”). The 5.1 mix is then further downmixed to a two-channel stereo mix, and another supplemental stream is created that carries the 3.1-channel “extension A.”

So the 7.1-channel program is delivered in three separate components: a twochannel mix, the 3.1-channel extension A, and the two-channel extension B.

The total payload is still 7.1 channels, with preconfigured subsets to create two-, 5.1-, and 7.1-channel presentations. If a listener desires a stereo presentation, the decoder plays only the two-channel downmix, thereby minimizing DSP resources for the simplest hardware products—a useful idea.

If a listener selects a 5.1 presentation, the decoder reconstructs it from the two-channel downmix plus the 3.1-channel extension A substream by means of rematrixing. If a listener wants a 7.1 presentation, the decoder reconstructs it by rematrixing the reconstructed 5.1-channel program with the final twochannel extension B substream.


If you listen to the Atmos demos on Youtube. They are only 5.1, but you hear height effects. So, somehow, the Atmos processor seems to be doing something…

That’s DSU (a form of DSP) at play here. Only then will height speakers be engaged. I find it kinda weird that YouTube still don’t playback Dolby Atmos even though DD+ already been used in most streaming content and YouTube is on a streaming platform.

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Wow thanks for the great response this question has generated. Appreciate the details.

My curiosity is because when we play 5.1 in a 7.1 layout, the receiver still shows True HD, not DSU.

The reason I’m asking the questions is because I feel the same thing should be done with front wide channels. It should be in the track, and should automatically decode to it when the speaker layout is detected.

Unfortunately, dolby only uses it for object rendering in Atmos. I like DTSX PRO approach of filling front wide, but Pro is still nowhere to be seen for most processors…

Some say yes, some say no…

Due to the way wides are currently being used, they are silent most of the time…

I have read may owner impressions of front wide layout - it makes a large difference to the front stage presentation . The fact that it can be active almost 80-90% of the time, while surround are rarely active, and atmos maybe 1% of the time…
It has huge potential to improve the HT experience.

Except for the fact front wide are probably the most difficult speaker to place unless in a dedicated room… But that’s a different conversation…

What receiver are you using? Yes, if going for native format, it will show Dolby TrueHD. If using any D&M Pre/Pro or AVR, it will only show number of speakers used. To engage DSU, my understanding is one will have to select the desired DSP processing to activate.

Yes agreed…wides definitely seem more worthwhile than even investing in height or in/on-ceiling speakers since Dolby Atmos has rarely been used. But as you said, wides require at least a reasonable width in the front stage to truly take advantage of it…It cannot be located too close to the mains as it will defeat the purpose.

My room doesn’t have the width to accommodate wides but I never really feel the need if you apply an appropriate amount of toe-in.

Hi Desray,

Due to the naming convention , most people think one needs a wider space to implement front wide speakers, like this layout.

But actually the above layout doesn’t have much separation between the LR and the wide channels.

Imo, a longer and narrow room benefits the most with front wide speakers. Because the room naturally constrains the width of the front stage.

Using wide speakers, and with appropriate acoustic treatment of the front sidewalls, one can make that wall acoustically disappear, and replace with the sound field of the wide speakers, to create the impression of a front stage space that is wider than the room.

This placement also helps bridge the large gap from the mains to the side surrounds in these narrower rooms.

I just read that DSU now up mixes to front wides after and update some time back… Hope to be able to test it soon!

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That’s an interesting take on the wides…to make this tangible, you definitely need a dedicated room for that, or else this will be the most anti-WAF layout one can get.

What amp/AVR you are using?

Got the AVM90. :grin:

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How do you find it? Do share your experience. I have been trying to share tips for advanced settings but didn’t have the time to start yet. If you have any tips to share with members here, do post here.


i should had hoot FACEOFF then, when i had the chance!!!

Damn… i had so many of those DTS ES Dvd’s before…
Sadly i threw it all away when “Bluray” format came out…

Thx bro @Wechnivag for sharing this insight.

I had the perception that rooms with narrow width have difficulty to do wides and it’s more suited for rooms with wider frontal space.

If this is the case, then wides is definitely v good to have. And more impact than SB or overheads as it will widen up the frontal sound and have a more seamless panning from front to back.

But how many % of movie titles support this?

I have just gotten a copy last week, they have released the new 4k edition. Will check this one out