Technical (THX) Surround EX Question:

Discussion in 'Archived Threads 2001-2004' started by Kevin C Brown, Nov 9, 2001.

  1. Kevin C Brown

    Kevin C Brown Producer

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    I thought I knew the answers to these, but maybe not:
    1) What does a (THX) Surround EX (or DTS-ES matrix) decoder do with the out of phase material sent to the rear L + R speakers?
    Or...
    2) Could it be, that for DVDs mastered for (THX) Surround EX/DTS-ES matrix, that there is specifically no appreciable out of phase material sent to the rears? We all know that the in-phase material ends up in the rear center channel.
    I'm asking because some people use (THX) Surround EX/DTS-ES matrix modes on "std" DD/DTS DVDs. Seems to me, that this would "throw away" the out-of-phase material that may accidentally be present there.
    I'm also asking in the context of a DIY 6.1 set up:
    If I apply *straight* Dolby Pro Logic decoding to the rears on any of (THX) Surround EX/DTS-ES matrix/DD/DTS material, the out of phase material is gone. But maybe it's not present in the (THX) Surround EX/DTS-ES matrix material to begin with! So you *shouldn't* use straight DPL decoding back there for "std" DD/DTS because you might be losing some info from the L + rears.
    But I recently came across an older DPL processor that specifically includes a "Dolby 3 Channel" mode. This mode specifically decodes Dolby Surround soundtracks (2 channel) to only the front 3 channels (which, on a DIY 6.1, would be the 3 rears). It specifically "folds" the out of phase material into the fronts (or in reality, the backs).
    Does that make sense?
    [​IMG]
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  2. NathanP

    NathanP Supporting Actor

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    Kevin, Try these threads.
    Here
    Here
    Good luck!
     
  3. Kevin C Brown

    Kevin C Brown Producer

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    I took a quick look, and those were both threads I have "cruised" in the past. Not there.
    Probably need an "expert" to answer this one.
    I know there have been "Dolby Labs" people before I have seen posting on HTF...
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  4. Kevin C Brown

    Kevin C Brown Producer

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  5. Joe Schwartz

    Joe Schwartz Second Unit

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    Here's my understanding of Dolby Surround EX decoding. You've got two discrete channels (surround left and surround right) from the 5.1 decoding, which are fed as inputs to a separate Dolby Pro Logic decoding stage. (As far as I know, it's still regular DPL, not DPL II.) This extracts four matrixed channels from the two discrete channels: left, right, center, and surround. The surround rear channel is essentially the "center" (in-phase) output of this DPL decoding. I think the "surround" (out-of-phase) output is thrown away.
    I've read in Widescreen Review that Star Wars Episode 2 will introduce a "height" channel, to be played from speakers mounted on the ceiling. I wouldn't be surprised if this channel is actually the "surround" (out-of-phase) output that's currently thrown away.
     
  6. Lewis Besze

    Lewis Besze Producer

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    Kevin,
    Yes the Dolby 3 is the best for,what you after.
    The Dolby 3 is basically the " "Phantom" mode for the surrounds.It was designed for people [back in those DPL times],who didn't have surround speakers,so when they have engaged the Dolby 3 mode, the processor,would "fold" the surround info[mostly out of phase,hey it's limited range mono surround],into the mains.
    So if you use that mode for you "home brew" EX then you achive ,what you wanted.
    This was mentioned back in '99 on the pages of S&V magazine [shortly after the Phantome Manace came out],by David Ranada.
    He also used the above mentioned set up.
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    "You Hungarians always disagree"
     
  7. Kevin C Brown

    Kevin C Brown Producer

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    Joe- Ahhh, that's one of my questions! [​IMG]
    If it is true that for (THX) Surround EX decoding, that the out-of-phase channel is "thrown away," maybe then it follows that DVDs mastered that way *intentially* don't include any out-of-phase material in the 2 rears.
    Lewis- I have actually found a way to do this with straight DPL back there: Y the output signals from the front R and rear R channels from the processor into 1 output into the *resulting* rear R channel, and same for L. That way, wouldn't lose the out-of-phase info.
    If there *isn't* any out-of-phase material for (THX) Surround EX/DTS-ES discs, doesn't matter in that nothing would be added.
    Something (neat) to think about!!
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  8. Joe Schwartz

    Joe Schwartz Second Unit

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  9. Joe Schwartz

    Joe Schwartz Second Unit

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    Okay, I gave this a try with the pod race scene of The Phantom Menace, and it definitely does contain out-of-phase material in the surround left/right channels.
    Here's how I tested it. My DVD player has analog outputs for each of the 5.1 discrete channels, so I connected the surround left and surround right DVD outputs to the analog inputs of my receiver. Then I switched the receiver to play those analog inputs, using Dolby Pro Logic decoding. This means I was playing the surround left/right channels through my front left/right speakers, the surround rear channel through my front center speaker, and the out-of-phase "throw-away" channel through my surround left/right speakers.
    It was fascinating to watch the pod race scene with this setup -- it really illustrated how carefully the sound was spread among the matrixed surround channels.
    Next, I played the scene again, but this time I disconnected my three front speakers and listened only to the surround speakers. In other words, I only heard the out-of-phase "throw-away" channel. There was definitely a lot of sound coming from those speakers. You can try this experiment yourself to verify it -- all you need is a DPL decoder and a DVD player with 5.1 analog outputs.
    So, now I'm questioning whether a true Surround EX decoder really does throw away that channel. I can't think of any easy way to test that, and I don't have an EX decoder handy. (Not yet, at least.) If you're putting together a DIY EX decoder, then you're right that Dolby 3 decoding would preserve some sound that strict DPL would lose. The only question is whether you're supposed to lose that sound, and I have a hunch that Dolby Labs won't be giving us any answers. [​IMG]
    [Edited last by Joe Schwartz on November 11, 2001 at 02:05 PM]
     
  10. Lewis Besze

    Lewis Besze Producer

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    Joe,
    DPL's surround is nothing else but, L-R[the difference] put out, out of phase,so neturally there is always out of phase info there.Naturally the discrete rears of DD/DTS contains such info as well,and shouldn't get lost during EX decoding.
    However the home brew EX that only use DPL for the rear center,is losing such info during decoding,unless it is Dolby 3,or something similar that "folds" the surrounds into the L R,or use the method Kevin described above.
    ------------------
    "You Hungarians always disagree"
     
  11. GregK

    GregK Screenwriter

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    There will almost always be content in the "surround"
    out of a home brew EX/DPL decoder with complex stereo
    surround sound fields. Even if phasing is closely
    monitored, DPL is still a matrix decoder, and matrix
    decoders can and do "crash" to a limited degree with
    complex material. For example, play Van Halen's rock
    and roll classic tune "You Really Got Me" in standard
    DPL. The guitar is in the the left channel while the drums
    are in the right, (or vise versa) and the vocals are dead
    center. The vocal reverb 'may' have some out of phase
    info, but in any case, you will hear drums and guitar in
    the surrounds. Why? ..The three front channels are all
    called to deliver positioning simultaneously, so the MP
    matrix 'crashes'. To the best of my knowledge with Dolby
    EX, audio mixers are not monitoring the unused EX
    "surround" output, so occasional bleed-through wouldn't
    be noticed or deemed important to the audio mixer.
     
  12. NathanP

    NathanP Supporting Actor

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    To throw away or not to throw away.. that is the question.
    [Edited last by NathanP on November 11, 2001 at 04:22 PM]
     
  13. Roger Dressler

    Roger Dressler Stunt Coordinator

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    >>If you're putting together a DIY EX decoder, then you're right that Dolby 3 decoding would preserve some sound that strict DPL would lose. The only question is whether you're supposed to lose that sound, and I have a hunch that Dolby Labs won't be giving us any answers.
     
  14. Kevin C Brown

    Kevin C Brown Producer

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    But Roger, from Dolby Labs, what does an official (THX) Surround EX decoder do with that out-of-phase material? [​IMG]
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  15. Kevin C Brown

    Kevin C Brown Producer

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    Joe- Great test, by the way. (I had to re-read it about 3 times to make sure I understood what you were doing!) That removes my 2nd question...
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  16. Shad R

    Shad R Supporting Actor

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    A height channel??? on the ceiling?? Wooohoo!!!!! That would be kinda cool! Also, I read in some HIFI magezine that Dolby is working on a 10.2 surround sound format. BRING IT!
     

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