Will an Ordinary RCA Cable (<75 Ohm) Degrade a Digital Bit-Stream as Digital Coax ?

Discussion in 'AV Receivers' started by Arthur S, Sep 26, 2005.

  1. Arthur S

    Arthur S Cinematographer

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    I've been advised that using an ordinary RCA patchcord as a "Coaxial Digital" connector is not good because it will not be 75 Ohms.

    How many Ohms would a cheap 6 foot RCA patchcord be, and how would it degrade the digital bit-stream?

    Thanks
     
  2. John Garcia

    John Garcia Executive Producer

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    Use a composite video cable, they are 75 Ohms and are basically interchangeable for coaxial digital. In a pinch, I've done it many times, even with the "in the box" type cables and it has always worked. Audio patch cables may or may not be 75 Ohm. I've seen people use audio patch cables and experience random dropouts.
     
  3. Chu Gai

    Chu Gai Lead Actor

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    John's quite right. Whether you can get away with non 75-ohm cable is really a hit or miss proposition and depends greatly on the length of the cable. Sometime back, I posted where someone experimented with using a coathanger. If interested, I'll try an dig up the link.
    There's no way of telling what the impedance of the patch cord you have is. The impedance is actually better stated as the characteristic impedance and that value depends upon the thickness of the wire, the thickness of the insulating material, the type of material and so forth. Not all insulating materials are suitable for use because their electrical properties vary signficantly with frequency. That's why you'll see 75-ohm cables, or other impedances, made with such materials as polyethylene, foamed polyethylene, teflon, etc. You won't see them made with PVC because the electrical properties of PVC at high frequencies don't permit the manufacture of a cable with a well defined characteristic impedance.
    Nothing wrong with using the wrong cable if it's a short length. Either you'll have problems or you won't. The good thing is that the remedy is very inexpensive.
     
  4. Arthur S

    Arthur S Cinematographer

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    I have been doing what John suggested, using a composite video cable that is part of my years long collection of various cables that came with all the equiment I've bought.

    I just tend to be skeptical when it comes to spending much on wires of any kind.

    Reflections are suppposed to be the culprit in any dropouts that can occur.

    After reading about the supposed superiority of coaxial digital vs. optical digital cables, I'm not convinced that the 75 ohm issue is any more valid.

    Chu

    I remember reading about the coathang, 0 dropouts. That is one thing that makes me skeptical.
     
  5. Jacob C

    Jacob C Second Unit

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    I have had problems with a friends system that uses a regular audio patch cable. Use 75 ohm. It not really that much money to buy a decent cable. You don't have to buy a crazy special "audiophile" cable.
     
  6. Arthur S

    Arthur S Cinematographer

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    thanks Jacob

    I am using one of those yellow ended cables that came with something I bought. Just for the hell of it, I might switch to an ordinary RCA patchcord and see if I suffer any dropouts.

    FWIW

    Cables and wires have about the highest profit margin of all the equipment you might buy. The "audiophile" guideline is 10% of your budget for cables and wires. My guideline is more like 1%.
     
  7. Ralph B

    Ralph B Supporting Actor

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    I used a 6ft 75phm cable with no problems at all.


    I would love to see that thread on the coathanger!!!!!! [​IMG]
     
  8. JeremyErwin

    JeremyErwin Producer

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  9. Ralph B

    Ralph B Supporting Actor

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    very interesting, thanks.


    seems to me although interesting those guys need to relax on that debate....lol


    in the end it worked and was just an experiment. very funny.
     

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