How do 7.1 channels work?

Discussion in 'Beginners, General Questions' started by Derek B, Jun 24, 2003.

  1. Derek B

    Derek B Stunt Coordinator

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    Where do the extra 2 channels come from in a 7.1 channel receiver. All movies are recorded in 5.1.
    Thanks,
    Derek
     
  2. Ed Moxley

    Ed Moxley Cinematographer

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    Hey Derek,
    I'll let someone else explain the technical stuff, 'cause I can't. [​IMG]
    But......not all movies are recorded in JUST 5.1. Example: Lord Of The Rings - Extended version is dts, 6.1 discreet. O Brother, where art thou? is Neo 6 dts.
    I think the 7.1 is for the near future. I don't know of any movies in 7.1 yet. These other guys know more about this, than I do.
     
  3. JamesHl

    JamesHl Supporting Actor

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    7.1 basically uses proprietary processing (lexicon's 7.1 that i forget what it's called, matrix 7.1, etc.) to create two rear channels of sound from the surround channels. Having never experienced it myself I don't have a personal opinion, but most people say that a well set up system sounds pretty darn good.

    Oh, and Neo 6 DTS is DTS's version of pro logic, Ed. Which would mean that it was just like a dolby surround recording. O Brother Where Art Thou? is 5.1 DTS, I think. It's packed in a box on my floor so I can't look right now.
     
  4. ChrisWiggles

    ChrisWiggles Producer

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    *MOST* movies are recorded in 5.1 (on DVD of course). Many older movies are just stereo, or mono too.

    Dolby Digital EX and DTS ES matrix is a processing system applied across the rear two channels of a 5.1 soundtrack to extract and create a matrixed 6th main channel that goes behind you. Soundtracks that are mixed with this in mind (still recorded in 5.1), are often called 6.1 (to people's confusion because it is really just an extended 5.1 system). DTS ES also comes in a discrete variety (a more complicated system and explanation that i will omit), that IS a TRUE 6.1 recording. The 6th channel is discrete and stored as a separate channel on the disc, yet to make it backwards compatible with a 5.1 system and processor, that information is ALSO included across the 2 surround channels, and so there is some processing necessary to remove that from those channels, so the discrete 6th channel can be utilized.

    When people talk of 7.1, most of the time they are talking about "6.1", except they are using two rear centers instead of just one. These two speakers are often wired together off the same amp, or more ideally on separate amps, but they are playing the exact same sound. This is because if you use one rear back centered perfectly behind your head, effects that pan hard to that channel can seem to be coming from in front of you, which is not at all wanted. To eliminate this reversal effect, people use two rear centers, thus you have a 7.1 speaker system with 6.1 channels of audio (which is most often being created from just 5.1 channels on the DVD as explained in the matrix formats of DD EX and DTS ES-matrix above).

    James mentioned the other formats that have 7.1, the main one being Lexicon's Logic 7, which I'm not to familiar with, except that it has its very strong supporters. More commonly though, high end receivers have proprietary processing that creates sligght differences between the two rear centers, so that it really is more of a 7.1 format than just 6.1 with two speakers instead of one.
     
  5. LeeH

    LeeH Stunt Coordinator

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    Dolby Digital EX (and I believe DTS-ES also, but not sure) uses matrix encoding. Matrix encoding was used first by dolby pro logic to create the surround channels. All it is is anything that is encoded out-of-phase in the two surround channels is sent the rear channels. The only difference between 6.1 and 7.1 is one speaker because the two rear channels in 7.1 are mono (they produce the same sound). Correct me if I'm wrong.

    You posted right before me Chris.
     
  6. Michael Reuben

    Michael Reuben Studio Mogul

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  7. ChrisWiggles

    ChrisWiggles Producer

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    Well, I am pretty sure phase is not involved in extracting a matrixed rear center for DD EX and DTS ES-matrix. Sound effects with the same volume (and I'm assuming it has to be in phase for it to be picked out) is removed and sent to the 6th channel. Pro logic did this, in phase sounds of the same volume were sent to the center, out of phase sounds were stripped and sent to the surround.
     
  8. Jeremy Tebo

    Jeremy Tebo Stunt Coordinator

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    This is a bit off the topic, but if an SACD is played on a 6.1 or 7.1 setup, are the rear speakers not used at all? Or would the reciever create the extra channels for that too? What do you guys prefer for the rear channels on a 6.1 or 7.1 setup - dipoles, etc.?
     
  9. LeeH

    LeeH Stunt Coordinator

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    Michael, I over looked that. Logic 7 is something that I haven't been able to hear yet, but look forward to it.

    Chris, I may be wrong. But, from the articles and posts I've read on the Internet (and its been a lot) they have all said that the surround channels use the matrix encoding that pro-logic used to produce "the rear" channels. Your explanation seems to make more sense because then you would have a full frequency channel. I'm probably wrong (and I hope I am because it would make more sense).
     
  10. ChrisWiggles

    ChrisWiggles Producer

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    I'm no authority on precisely how the rear center is created, but it was my understanding based on posts here that made it very clear that phase shifts were not utilized, just as it is for extracting the center channel in pro logic. It's the easiest and most logical way to do it. Using preamp outs, if you just put a pro-logic decoder across those two channels, set to 3-channel mode (just front three) you accomplish the same effect. Plus, soundtracks that are not purposefully mixed for EX don't have to include that phase shift for it to work right. As long as the sound pans across, and it is the same level as it passes that center point, it will pan through that rear center. I too could be wrong, but it seems most logical don't you think?
     
  11. JamesHl

    JamesHl Supporting Actor

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    Rear center on non discrete is matrixed. It's like using pro logic to create a center in the front on stereo material, since the surround speakers are full bandwidth.

    Basically, Chris's explanation is right.

    Jeremy- to my understanding, some SACD is multichannel, and some is not. The multichannel will use multiple speakers, and stereo will use two.
     

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