Outlaw ICBM: Am I being anal here?

Discussion in 'AV Receivers' started by Aaron E. Smith, Jun 7, 2003.

  1. Aaron E. Smith

    Aaron E. Smith Stunt Coordinator

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    Howdy,

    I've got a few other threads going right now, but I figured this required a seperate discussion. I'm a proud Magnepan owner and I'm in the middle of considering a pre/pro switch. One thing I'm thinking about doing is purchasing the Outlaw/Magnepan ICBM, but I really dislike the way the ICBM handles the rear channel(s). I know that pretty much everything short of Logic 7 uses the rear channels as a monoaural pair, but I don't like the y-adapter combining of these feeds and then splitting them back out to my amps. Am I being anal or is their solution sort of poor?
     
  2. RichardHOS

    RichardHOS Second Unit

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    I didn't like that either. I also didn't like the limited crossover point selections. I decided to build my own active crossover networks based on Rod Elliot's design and PCB's. The total cost will actually be much cheaper than an ICBM, with precisely the configuration I want. However, time and elbow grease is required. [​IMG]
     
  3. BruceD

    BruceD Screenwriter

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    Also, unless all speakers are exactly the same distance from the listening position, the ICBM will destroy the time alignment settings in the prepro if it is placed between the prepro and the amps.
     
  4. Aaron E. Smith

    Aaron E. Smith Stunt Coordinator

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    Good, I'm glad I'm not alone in feeling this way. :b

    On a more serious note, how would you work around this? Can you think of how to bypass this and make it all work?

    Richard, can you tell me more about the DIY way of doing this? I'm OK with a solder gun, but I'd need instructions and whatnot. Do you happen to have a pic of what you were able to do?

    Thanks again!
     
  5. Roger Dressler

    Roger Dressler Stunt Coordinator

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    >>I don't like the y-adapter combining of these feeds and then splitting them back out to my amps. Am I being anal or is their solution sort of poor?>Also, unless all speakers are exactly the same distance from the listening position, the ICBM will destroy the time alignment settings in the prepro if it is placed between the prepro and the amps.
     
  6. BruceD

    BruceD Screenwriter

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    Thanks Roger, for a clearer more detailed explanation of the effect of the ICBM on the bass frequencies in the signal chain.

    Since the Magnepan ICBM does it's front main crossover in the neighborhood of 200Hz, are bass cancellations from other channels even more likely?
     
  7. Roger Dressler

    Roger Dressler Stunt Coordinator

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    (Please allow me to correct my earlier post, while 100 Hz has a 10 ms period, the cancellation happens at 5 ms. So for 80 Hz the delays would need to be within less than 5 ms of each other, not 10.)

    It's all related with relative time delay. If there are no time offsets in the signals, it won't matter at all what crossover freq is used. To get a cancellation, the signal needs to be opposite phase, 180-deg, which is 1/2 the period of the frequency. A 200 Hz signal is a rather high crossover frequency. It has a 2.5 ms period for the 180-deg point, so if the delay difference in your C path wrt to the L/R system is +/-1ms or less you will avoid full cancellations. This is rather little range, I agree.

    These small time delay offsets seem rather severe, but keep in mind that if all the delays in L/C/R are almost the same, the delays can be rather longer between the fronts and the rears because there is much less coherence in bass signals in source material between fronts/backs than there is across the fronts.

    By the way, does the Magnepan version of ICBM have any special facility to first cross over the bass from C into L/R (at 200 Hz), then crossover the L/R at a lower frequency? This would help keep midbass signals in the fronts, and allow a little more lattitude in time delay offsets. Or is the unit identical with Outlaw's version, where the bass from every crossed-over channel goes to the same place? I couldn't find info on it at Magnepan's site.
     
  8. John Kotches

    John Kotches Cinematographer

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    Roger asked:

     
  9. RichardHOS

    RichardHOS Second Unit

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    Actually Roger, you can have some phase problems even if time delays are not introduced into the signal before the ICBM. This happens when you select different crossover points for the different channels, and sum to a common subwoofer.

    For instance, if you had the maggie version and crossed the center over at 200Hz, and the mains at 80Hz, and the surrounds at 120Hz, then the low frequency output for each of these channels will be phase shifted to various degrees relative to one another. When summing to a common sub, you will be summing signals that are not phase coherent.

    The only way to prevent this, AFAIK, is to either use the same crossover points for all channels, or use a 6dB/oct crossover slope that doesn't have a phase shift (at least, I don't think it does, though could be entirely mistaken about that).
     
  10. RichardHOS

    RichardHOS Second Unit

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    Aaron,

    Check out Rod's page here: http://sound.westhost.com/projects-3.htm

    Look at the project 9 page. He has PCB's available for sell for that project that would really make it easier, and when you purchase the board from him (they're like $12 or so), he provides very complete and detailed instructions on how to assemble the components needed.

    I have the PCB's, but won't assemble the boards until I'm in the new HT room and can take some measurements first, which will be early next year.
     
  11. Kevin C Brown

    Kevin C Brown Producer

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    I personally wouldn't worry about the different phase shifts induced by using different crossovers in that:

    1) Most subs only give you a 0 or 180 phase switch, so you are probabloy not in perfect phase anyway. (I.e., I personally would not buy a sub without a variably adjustable phase knob.)

    2) Even though the shift is different for different freqs, it is a *continuous* function such that if you match phase with the set in the middle of the freqs you chose, it should prove to be "close enough" for higher and lower freqs.

    3) Most speakers are not even in phase within themselves anyway. Only such makers as Vandersteen, Thiel, and Dunlavy among a very few others actually design and manufacture phase coherent, time aligned speakers.

    4) And if you think there *is* an effect, just use the same crossover setting for all channels. But I personally believe that you'd be passing up one of the big advantages of the ICBM over any player's inherent BM scheme. [​IMG]
     
  12. Aaron E. Smith

    Aaron E. Smith Stunt Coordinator

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    This is an interesting discussion; thank you all for the info.

    To sort of circle back to my original agitation for a moment: why is it that Outlaw chose to force you to combine the back signals? Also, I can't grasp how it is possible to maintain any differences in the right and left signals once they pass into the ICBM since it will feed out identical signals to both the right and left rears.

    I think I'm missing something here, am I?
     
  13. RichardHOS

    RichardHOS Second Unit

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    Outlaw probably finalized the ICBM design while 6.1 was still the "top dog" format, and they didn't plan ahead for possible 7.1 applications. The same reason we'll wonder in a couple of years why all these receivers have "only" 7 channels of amplification. [​IMG]
     
  14. John Kotches

    John Kotches Cinematographer

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    Aaron,

    What you're missing is that for DD-EX and DTS-ES titles, the back "channels" are monophonic, ie the same signal is fed to rear left and right.

    Since this is the case, it is only necessary to have 1 input/output channel.

    Regards,
     
  15. Aaron E. Smith

    Aaron E. Smith Stunt Coordinator

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    Yea, there's alway another piece of technology out there to plan for. I'm just glad that I have some patience to wait for all those new technological doo-dads.... [​IMG] Sometimes I've held off for whole days!


    But with L7 those rears aren't necessarily the same I believe. I've been assuming that Ultra 2 processing and some other modes like it (I think Cirrus Surround) also recreate rears that aren't necessarily monophonic. Is that right or have I presumed wrong?
     
  16. Lewis Besze

    Lewis Besze Producer

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    Cirrus is mono.
     

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