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Listening to 2CH CD's with Surround Sound upmix via amplifier. How does it work?

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Lord British

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As someone who has no idea what goes into producing a CD, I'm sure the explanation is more involved than I'll ever understand, but I'm curious to know how the technology works. I can listen to certain 2CH albums upmixed by my Pioneer Elite SC-95 amp (Dolby Surround, DTS Neo:X) and it fascinates me how it "knows" to put the vocals in the center channel, and split other elements to the rears. Where does it get this information from? And why do some CD's work better than others in this regard?
 

jimfisheye

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There isn't any actual connection to the source multitrack elements in the mix. The upmix techniques aren't uncovering anything or accessing any source elements. It's literally starting with a finished stereo mix with no underlying data or anything cryptic. Pulling and tugging on it and using eq and phase tricks to try to separate elements in the mix and direct them to a surround speaker array.

Sometimes you can separate elements in a mix well with certain audio techniques and you can genuinely craft an upmix that has reason to exist. Preserving original elements and fidelity and expanding the content to a surround array.

Other times (and more often) it comes across more as playing with the mix in a childish way. You end up with random placement in the surround array and artifacts from the phase and eq trickery that reduce the fidelity vs the original. There may be an in-between here depending on the listening system. A stereo mix upmixed and mastered to 5.1 might deliver more content and detail on the surface on a small or medium surround system vs a small or medium stereo array. But a reference quality stereo array in a great listening space will still deliver the most fidelity and depth while the same reference level system for surround would reveal the damage from the upmix tinkering.

If you're interested in hearing what the artist delivered unaltered (for good or bad), an upmix is the polar opposite direction from that!

There's no "metadata" like information cryptically hidden in a stereo mix that gets uncovered in an upmix. An upmix is literally using a finished stereo mix as the source and pulling and tugging on that to create what you can get away with.


A simple example:
A lead vocal is often mixed dead center in stereo.
Subtracting one channel from the other in stereo leaves you with the difference. Subtracting that from the source leaves you with the mono content - elements that are identical in both stereo channels. Thus you can separate mono content out of a stereo mix. There's no 'metadata' like construct that "knows" this. You're deriving it from the stereo mix as presented.
 

J. PUPSTER

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I've been doing some up-mixing stereo from various sources lately via Surround Master or Penteo or SpecWeb, and it's like a box of chocolates (you never know what you're going to get.) Vocals land on the sides or even rears (one recent song I ended up rotating the 4 channels 90 degrees left to get the vocals into the front- that's post UM.)

Another thing I've noticed is these matrix type programs seem to like music that has been brick walled but that still have separation in their stereo mix (maybe just stronger signals to latch onto?). I believe much of it has to do with how they were recorded with certain types of mic's etc. A lot of the Chesky & Opus3 recordings and certain producers seem to do better than others; and also what the ultimate end music by theses and the artists intend to sound like (in stereo of course.)

As to your question of "how does it work" someone here like @chucky3042 is better suited to answering that - albeit very technical probably!
 

jimfisheye

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Some of those upmix plugins make quick work if you have to drop a song into a movie soundtrack and need it to sound 5.1 and need to work fast. Working an upmix with intention always gets better results though. (Like most things... It don't come easy!) Listen. Take apart what you can get away with.

Heh. I made a 5.1 upmix from a mono source a while ago. Actually turned out pretty well. Try that with one of those upmix plugins! :D
 

ssully

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Here is an old (though not as old as Dr. Greisinger's AES powerpoint deck above, which is from 2003) , but still one of the better short nontechnical summaries:

Both Dolby Pro Logic II, dts Neo 6 and Harman/Kardon's Logic 7 in fact do analyze the relative phase and level between the input signals and use the ambient out-of-phase information to achieve their surround effects. Although the operation is quite complex, the digital algorithms route the out of phase information to the surround channels and lock the in-phase information into the center (vocalists and instruments that are mixed with equal signals in the left and right channels), leaving left- and right-biased information to the main left and right front speakers untouched. Note too that DPLII, dts Neo 6 and Harman/Kardon's Logic 7 all have Cinema and Music modes.


Noted that Dr. Greisinger makes this surprising point:
•It is possible (but sometimes difficult) to make a discrete mix that is clearly superior to an automatic two-to-five conversion.
–Comparing the original discrete mix to the same mix after downmixing and upmixing can be quite revealing, as the down-mixed and up_mixed mix is often superior to the original.
–Analysis of the reasons for this observation can lead to better mixing technique..
This would be interesting to test now in 2020! (Downmix a discrete 5.1 mix to 2.0 --> upmix using DPL II or other upmixer --> compare the discrete vs synthetic 5.1 mixes)
 
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