Why 75 ohms?

Discussion in 'Archived Threads 2001-2004' started by Brian Dobbs, Jul 25, 2001.

  1. Brian Dobbs

    Brian Dobbs Ambassador

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    I recently posted a message concerning the type of cable that I should use for the digital coax out of my DVD player. Lots of people said that I should make sure the cable has a 75 ohm impedance rating. I was told that RCA video cables do have this rating, but RCA audio cables do not. I went out and bought a coax 75 ohm 6 ft cord because that was recommended by so many, but no one has explained why the cable must be 75 ohms. Can anyone tell me why? thanks
     
  2. Craig W

    Craig W Second Unit

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    Electrical Theory:
    The inputs and outputs for the digital sources were designed for an impedance of 75 Ohms. If the cable does not match the designed impedance rating then signal reflection (ie. the signal could bounce back and fourth between the input and output) will occur meaning the part of the signal will continue to propogate on the line. But for these short runs it really does not matter too much. The reflections are typically an order of magitude less(1/10) of the actual transmitted signal and most decoders will simply ignore this noise.
    Impedance matching plays a huge role in long runs and it is a critical part of the design of large digital and analog systems.
    I have hooked up some systems using high grade coax digital interconnects and some using RCA Audio patch and I have not been able to hear a difference.
    [Edited last by Craig W on July 25, 2001 at 12:27 PM]
     
  3. Bob McElfresh

    Bob McElfresh Producer

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    The output impedence on the DVD player is designed to be 75 ohms. The Input impedence on the receiver is designed to be 75 ohms.
    The best way to transfer a signal between 2 devices is to use a cable that matches the output and input impedences.
    Let me try an analogy:
    The door to your house is the "output" device. The sidewalk in front of your house is the "input" device.
    You need to build a walkway between the two so people can flow from one to the other. (You need to use a cable so signals flow from the output to the input).
    If you build a walkway lower than the door and sidewalk, people could stub their toe when they reach the sidewalk. (The signal along a cable can bounce and reflect back and forth).
    If you build a walkway higher than the door, people could stub their toe leaving the door, and they have to duck-walk to avoid hitting their heads on the overhang & trees. (The signal voltage leaving the DVD player will see a higher-resistance and the voltage will have to drop to pass along it).
    The "best" walkway is one that matches the height of the doorway and the sidewalk.
    Does this help? (I'm glossing over issues because I dont know your background. Is this enough?)
     
  4. Brian Dobbs

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    Thanks for your replies! I understand a little better now.
     
  5. Chu Gai

    Chu Gai Lead Actor

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    That was a rather nice reply Bob and a good job of using an analogy.
     

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