Fudging 6.1

Discussion in 'Archived Threads 2001-2004' started by Greg Bright, Sep 12, 2002.

  1. Greg Bright

    Greg Bright Second Unit

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    I seem to remember in the early days of surround sound that inexpensive devices were available that generically extracted a difference signal from the front left/right channels to produce a mono surround channel without using additional amplifier power. That in fact one could just insert a speaker between the negative speaker terminals of the front channels, add a small volume device, and, voila, instant surround sound. So...would it be possible to rig up a similar installation today to extract the center rear channel of DD EX? As I understand it the same sort of phase shift technology that drove Dolby Surround is used in EX as well. Any thoughts on this? This would be a really inexpensive way to get into 6.1. After all, who doesn't have a spare speaker laying around?
     
  2. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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    The surround center information would consist of the information that is common to both the surround left and surround right. So if you connected a speaker between the negative terminals of the surround left/right you would get the opposite of what you want.
    This method worked for extract the mono surround info from main signals of pro-logic souce because the mono surround info was the out-of-phase info from the main signals. But in this case the surround center signal is the in-phase info from the surround right/left.
    But home-brew 6.1 can be done if:
    1) Your 5.1 receiver has pre-amp outputs for the surround signals
    2) You have a spare pro-logic receiver
    Connect the front right, center and left speakers to the 5.1 receiver as you normally would. Take the surround right/left pre-amp signals from the 5.1 receiver and connect them to the right/left CD input on the pro-logic receiver. Wire the 3 surround speakers to the front right, center and left speaker outputs from the pro-logic receiver. Set the pro-logic receiver to 3-channel pro-logic mode (no surrounds). Now calibrate the volume setting of the pro-logic receiver to match the volume of the front speakers that are connected to the 5.1 receiver.
    This way the pro-logic receiver will extract the information that is common between the surround right and left signals and send it to the surround center speaker instead.
     
  3. Greg Bright

    Greg Bright Second Unit

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    Ryan:
    Thanks for your insightful response. I still want to believe that there is a way to retrieve the common signal from the L/R surrounds without the use of an additional DPL receiver. And I realize they can probably be obtained for pennies on the dollar.
     
  4. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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    "I still want to believe that there is a way to retrieve the common signal from the L/R surrounds without the use of an additional DPL receiver."

    Even if you can, it might mess up the balance of the signals. The common signal is already being sent to the surround right and left so if you send it to an additional speaker you'll be amplifying that signal. If you tried to turn down the surround right/left to compensate then a signal that is sent to just the right surround, for instance, would be too soft. With the DPL receiver the decoder actually redirects information from the right/left to the center.

    The "trick" worked with extracting surround info from main signals because the surround info was out-of-phase in the right and left main signals. It didn't matter that the mains were playing this signal since they would cancel each other's output. Not so with the surround center info being in-phase in the surround right/left signals.
     

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