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Is there a minimum cable length?

Discussion in 'Archived Threads 2001-2004' started by RobertCharlotte, Apr 15, 2002.

  1. RobertCharlotte

    RobertCharlotte Supporting Actor

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    I got it in my head somewhere along the line that some cables need to be a certain length. The word "attenuate" is connected with this vague memory; something about the signal needing to "settle down," I guess.

    Anyway, I'm pretty sure this is wrongheaded nonsense, but I thought I'd better check with folks who know better than I. I've got several places where my cable TV coax could be shortened to neaten up my rack, and this foolish half-notion is all that's keeping me from doing it.

    Can someone set me straight?
     
  2. Bob McElfresh

    Bob McElfresh Producer

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    AFAIK there is no minimum length for CATV coax. Attenuation is a fancy word for "reduction". Coax is usually graded by rolling out 100 feet and shoving higher and higer frequency signals in one end and measuring the attenuation at the other.

    For a Component Cable set, it's important to make all 3 cables the SAME length. But thats the only length rule I can remember.
     
  3. Saurav

    Saurav Cinematographer

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    Agreed. Never heard of signal needing to settle down. All cables attenuate, the longer the cable the more the attenuation. Usually, attenuation is a bad thing (unless you're intentionally trying to attenuate, in which case you should use a volume/signal control, not a long cable). So, in almost all cases that I can think of, shorter is better when it comes to cables.

    Just IMO, of course.
     
  4. RobertCharlotte

    RobertCharlotte Supporting Actor

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    Cool. That will significantly cut down on the number of loops of cables I have behind my equipment. The real headache, of course, is that my cable box sits right on top of my VCR, but right now I have about 3 feet of wire connecting them. I can have faith now that reducing that down to 8 inches or so won't do any harm.

    Thanks!
     
  5. Chu Gai

    Chu Gai Lead Actor

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    just not so short that you cause severe bends in the cable.
     
  6. Saurav

    Saurav Cinematographer

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    Or you have to throw a bunch of cables away because you have to move something if/when you get a new component.
     
  7. RobertCharlotte

    RobertCharlotte Supporting Actor

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    I don't expect severe bends to be a problem. The cable will need to make a U-turn, obviously, but I should be able to give it about a 4-inch radius to do it. Basically, the cable will start it's arc at the terminal on the cable box and end at the terminal on the VCR.
    And I don't worry too much about throwing away cable. I've got a spool of coax I bought when I moved into my current house and had to wire the entire upstairs for cable TV, so the only expense I ever have is occasionally having to go buy more F-connectors. [​IMG]
    Thanks, again!
     
  8. Andrew Pratt

    Andrew Pratt Producer

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    The only minimum I've ever heard about was that optical cables shold be no shorter then 1 meter ...not sure why though but I've yet to see any shorter then that so maybe there's something to it.
     
  9. Chris_Freeman

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    Any cable carrying a digital signal should be at least 1 meter. That includes optical cables like Andrew said. The reason for this has to do with interference, although I am not familiar with the technical reasoning behind it. Also Im not sure this would apply to coax. It is fairly cheap cable so I would try it and see if it degrades the signal any.
     
  10. Saurav

    Saurav Cinematographer

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    Hmm, that's interesting. I've never heard of any such requirement before.
     
  11. Chris_Freeman

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    I work in the IT dept of a large investment bank and I checked with one of our telecom techs and there is a technical reason for the 1 meter minimum. Please forgive me if this is not a great explanation, I understand the concept but not sure how to explain it. He said it has to do with feedback, when a high speed digital signal is transmitted over very short distances electrons traveling down the wire can actually be reflected back and cause interference, which degrades the signal.
     
  12. Yohan Pamudji

    Yohan Pamudji Second Unit

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    AFAIK there's a minimum length for coax for computer networking purposes, but I don't know if this applies to AV coax.
     
  13. Bob McElfresh

    Bob McElfresh Producer

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    Chris: thanks for that input. The "minimum length" sounds like a Catagory 5 computer networking issue. Similar to "no more than 4 ninety degree bends in the wire" type of thing.

    I DONT think this applies to AV coaxial cables. The reflection issue is solved by specifying the impedence of the cable and the input electronics so the signals dont "see" a different impedence which causes a reflection.

    Robert: did this answer your question?
     
  14. Chris_Freeman

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    Right Bob, it is primarily applied to Cat5 cable ,but the same pricipal should be true for any cable carrying a digital signal. Though Im not stating this as fact, just passing on info that I think is accurate. You may be right though in that it may not apply in this particular case.

    Especially since most AV do applications have a specified impedance like you stated.
     
  15. RobertCharlotte

    RobertCharlotte Supporting Actor

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