Technical- Dolby Pro Logic IIx phase shift in LFE channel

Discussion in 'Playback Devices' started by Eric D Wong, Oct 22, 2004.

  1. Eric D Wong

    Eric D Wong Stunt Coordinator

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    This must be the audiophile side coming out of me but back in 2 channel days I ran a single sub running through a AudioControl crossover that had a Linkwitz-Riley filter network (24 db/oct slope, high and low pass, 6 dB down at the x'over freq).

    I was playing around with my Denon AVR-2805 tonight equipped with Pro Logic IIx music and DTS NEO:6 music and I noticed something disturbing when I was playing with some music. I listen to alot of techno/trance/dance/club style music (lo fi, yes but thats what I like).

    The system in question is- Polk LSi25 main towers, Klipsch RW-12 subwoofer (with continuously variable phase knob).

    Anyway, I was using the various modes (excluding the DSP modes, like theater, hall etc) that there was a noticeable difference in the bass response, even when going from "stereo" mode to "direct" mode. So I began to try to pin this down.

    First I figured since direct was the pure mode it was bypassing the channel delay stuff. I set all the speakers to "zero feet" away from me to eliminate that variable. I also set the front 2 speakers to "0 db" and the sub to "0 dB" on the receiver and adjusted the subwoofer via the subwoofer level incase the direct mode bypassed the internal attenuator. This made things better, at least it was now consistent between direct and stereo mode, although when using the surround music modes there was a different quality of bass.

    I then began playing with the phase knob on the rear of the Klipsch, I noticed in direct mode, there was most output from the sub at 0 degrees phase and in surround mode, the bass dropped off again and when I reveresed the phase once more it was "OK" again. Da heck, this sux no wonder my bass was never consistent.

    Is it inherent that dolby pro logic IIx music and the dts:neo6 reverses the phase of the subwoofer channel?
    Also I have no clue what kind of high pass the Denon employs in the main channels.

    I think I have a solution to this one. I'm going to run my AudioControl EQ through the main L and R's, set the subwoofer to "none" on the receiver and let the audiocontrol do the bass processing. That way the front 2 speakers and the sub will consistenly be the same, with a much better crossover.

    Another thing is the Polk LSi25's have built in subs in the towers (although I have them set to small to filter most the bass). Maybe next time around its time to use smaller speakers so I get less intereference but the phase shift phenomena is still there. I wish I could try out another receiver thats compareable to the 2805, see if it does the same thing.
     
  2. ChristopherDAC

    ChristopherDAC Producer

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    Dolby Surround and similar technologies are based around phase-shifts. Frankly I think your problem lies in the fact that you seem to be attempting to run both the LFE connector and the speaker cables to your sub, or do I misunderstand you? There is a 90 degree shift between the centre and the 2 mains, and another 90 degrees each way to the single surround, in ordinary Dolby Surround, that is the in-phase content goes to the centre and the antiphase to the surround, with the uncorrelated content in the mains. The Pro-logic and Pro-Logic II systems, and in fact all other matrix systems, are based on this technique. The LFE derived channel is created by decorrelation of the Low Frequency material so it doesn't cancel out. My advice? set your mains to LARGE and run only the derived LFE channel into the separate sub.
     
  3. Eric D Wong

    Eric D Wong Stunt Coordinator

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    I'm not running both speaker cable and RCA's to the sub. I have only one input to the sub and thats the RCA for the LFE.

    Ok I get what you're saying then. No wonder theres a difference in phase cancellation depending on if I use the direct/stereo mode or if I go to use Pro Logic IIx music.

    I'm going to try it with my external crossover and see what happens.

    Maybe 2 channel should really stay 2 channel and multi channel should stay multichannel....
     
  4. Brian L

    Brian L Cinematographer

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    Kevin C. Brown (frequent forum contributor) did quite a lot of testing on the issue of phase reversal of the subwoofer channel.

    If you can get his attention, perhaps he can link to his many posts on the subject.

    Bottom line (I think) is that it does happen, and there is no easy way around it other than to quantify it and set your subs phase switch accordingly....albeit that it is a big PITA to have to do so.

    BGL
     
  5. Seth_L

    Seth_L Screenwriter

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    It sounds like your sub isn't properly time aligned with the rest of your system. The distance controls in your reciever will allow you to get your sub much closer to being on time with the rest of your speakers. Your bass will sound much better and more "in phase" when properly set.
     
  6. Eric D Wong

    Eric D Wong Stunt Coordinator

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    I've tried using both the "correct" distance from the sub as well as setting everything to zero and also setting the front L,R and sub to the same distance. There was MORE of a deviation when the "correct" distance was used then when everything was set to zero.

    My theory was in "direct" mode the distance/delay settings get ignored. I'm not sure if this is really so though. I suppose a call to Denon is in order and hopefully a tech would be able to accurately answer that.

    Update, I tried the external crossover. The phenomena is still there but not as pronounced as it was before. Probably because of the better crossover characteristics. Maybe its time to pitch Dolby Pro Logic IIx music and save that just for "video" watching. Dont get me wrong I love Dolby Digital cause it accuately portays what the director intended, but using these matrix processing on 2 channel is bastardizing the sound. Its fun to play with at first thinking oh wow, I have sound coming from all 7 speakers, but then it looses its appeal.
     
  7. Seth_L

    Seth_L Screenwriter

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    The correct distance isn't set with a tape measure.

    Play a test tone 1 octave below your crossver frequency (so 40Hz tone with an 80Hz crossover frequency). Get out your SPL meter and look at the SPL. Start with the sub set to it's true measured distance and increase the distance until you find the peak volume. On my system that's at 19 feet, but my sub is about 6-7 feet away from me.
     
  8. Eric D Wong

    Eric D Wong Stunt Coordinator

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    Hmmm, now theres an idea I havent thought of. I have a real time analyzer so that job should be even easier. However I dont think its going to affect the phase shift pheneomena I'm experiencing. We'll see....
     
  9. Lewis Besze

    Lewis Besze Producer

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    Distance setting is actually phase manupilation,so it should affect the outcome.
    Also don't discount the fact that it could be the way Denon "doing things". Untill some other users come out and claim the same thing,we[you] simply can't discount that possibility.
     
  10. Jeremy Anderson

    Jeremy Anderson Screenwriter

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    Just a thought: Since DPL-II spreads the sound across the front three speakers, it may be that even though your mains are in phase with your sub, your center channel may cause cancellation. Try setting the center width control so that it uses only the mains (or set center to NONE) and see if you still notice a dropoff in bass. If you suddenly get the bass back, it may be that you need to play with your center channel's delay time a bit.
     
  11. Eric D Wong

    Eric D Wong Stunt Coordinator

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    Jeremy, thanks for the suggestions- yes if I turn the center channel to "off" the bass phenomena switching between PLII and direct goes away. Thank god. The effect is most pronounced if I switch the center to "large". I usually run it in small. The center speaker I got now probably cant drive enough air, its only got 2 5 1/4" drivers to it. I will try setting the delay setting as well.
     
  12. Jeremy Anderson

    Jeremy Anderson Screenwriter

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    Keep in mind that changing the delay can change the phase relationship of the speaker, so in certain cases it may be advantageous to switch the positive and negative wires going to the speaker. The only way you can tell is to run a phase test. Fortunately, they include this on every DVD with THX Optimode tests.

    For instance, run the phase test for the center channel and bump delay up and down a bit while it is playing. You can hear it go in and out of phase with the other speaker playing during the test.
     
  13. Eric D Wong

    Eric D Wong Stunt Coordinator

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    Ok I should mention that when I did this first test, it was when I was using the outboard crossover. I switched the system back again to using the receiver's inboard crossover and the same phenomena occurs with the phase shifting of the lFE channel. It even does it when I switch between direct mode and stereo mode. Stereo mode is weaker in the bass then direct unless I get up and flip the phase switch on the sub.

    I also dont know what the high pass slope of the receiver is, but its probably pretty shallow. I think I will use the external crossover from now on, its a much better alignment (linkwitz riley) and alot more phase consistent.


    PS other owners of Denon AVR2805 or 3805s, if you play some music with a pretty good bass line and switch between stereo and direct mode, can you hear the difference in the punchiness of the bass?
     

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