Can I safely use a video cable in place of

Discussion in 'Playback Devices' started by Gerard Martin, Mar 8, 2004.

  1. Gerard Martin

    Gerard Martin Second Unit

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    A digital coax cable for a CD to receiver connection. Additionally would there be any audio degradation by doing so. Your earliest reply sincerely appreciated.
     
  2. Craig Robertson

    Craig Robertson Supporting Actor

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    no, unless the cable is of such a poor design or faulty.
     
  3. Kevin C Brown

    Kevin C Brown Producer

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    Yes, both are 75 ohm and shielded coax cable. Or, should be. [​IMG]

    But be careful. Those yellow ended cheap-o RCA video cables won't work. Only component video cable will work.
     
  4. ChrisWiggles

    ChrisWiggles Producer

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    They should if they are video cable. A digital coax need only be 75ohm, which video cables should be. I've seen some 50ohm video cables for certain things, but regulard video cables can be used as digital coax, or analog audio, or whatever most of the time. Unless you get dropouts, it should be working fine.
     
  5. Kevin C Brown

    Kevin C Brown Producer

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    I don't think those cheap-o yellow ones are 75 ohm or coaxial. (They are definitely too cheap and skimpy to be coaxial.) Composite video feed, not component. Different animal. And, the typical plastic molded connectors definitely aren't 75 ohm rated.

    Yeah, typical audio interconnects are around 50 - 55 ohms.


    Now, I remember a thread a while back that put forth, do you really *need* 75 ohm, coax, shielded cable for a digital coax line? *That* is a different question. [​IMG]


    I remember an experiment that someone said a friend of his did, with believe it or not, chicken coop fence wire. He said he had pretty sophisticated measurements that said it just didn't matter. Jitter, dropped packets, checksum error counting, the whole nine yards. But what at least the manufacturers of DVD players and receivers (pre/pros) recommend, is 75 ohm, coax, shielded cable. [​IMG] (I am just now remembering yet another experiment with a coat hanger. Same conclusion...)
     
  6. ChrisWiggles

    ChrisWiggles Producer

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    Yeah, I wouldn't use patch cables, god knows what they are.

    But you seem to imply that composite video cable cant be used while component can. Composite video cable (it's just video cable) should do fine. All component video cable is, is 3 video cables sold or bundled together is all. Composite cabling is just one cable.... It's all the same stuff.

    digital coax is just one of the same cable, RGB is just 3-5 of the same cables, etc etc
     
  7. David C Lin

    David C Lin Stunt Coordinator

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    I have no problem using my video coax cable for digital audio for sending Dolby Digital and DTS from my DVD Player to receiver. No problem at all.
     
  8. Gerard Martin

    Gerard Martin Second Unit

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    Tried a RCA video cable as a substitute for coax, WOW, terrible sound, that lasted about 10 minutes before I replaced it with RCA audio cables for the analog connection.
     
  9. Craig Robertson

    Craig Robertson Supporting Actor

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    you're changing a lot more than than just cables when you make that change, so it's not just cable.
     
  10. MuneebM

    MuneebM Supporting Actor

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    That's like comparing apples and oranges! And I'd think the RCA cable for a digital audio connection would be better than the RCA cable pair for an analog connection...
     
  11. Bob McElfresh

    Bob McElfresh Producer

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    Yes, and even a wire coat-hanger has been used.

    Here is why a single wire works: you have no shield.

    A single, bare wire will correctly transfer the digital signals a short distance - but in a "clean" environment.

    The back of your rack is a very "dirty" environment with AC power wires, speaker wires and other signals running around. I believe the "Chicken Wire" and "Coat Hanger" experiements were all done in a laboratory - likely with the DVD player facing one way and the receiver/test equipment facing the other way so the wire made a straight line between the two.

    To protect the line-level signals from a "dirty" environment, you wrap a layer of foil/mesh wire around the center wire to create a shield. You tie the end of the shield to a zero-volt reference and this acts like a 'drain' for electrical noise.

    Coaxial cable is built to do exactly this: center wire protected by a mesh/foil shield.

    But something new happens when you run 2 wires in parallel with each other and send a changing signal down one wire, but not the other. The signal "sees" the zero volts and "sees" the input electronics at the end of the cable. It decides to take the path of least-resistance which could be the shield or the input electronics.

    To solve this, a convention was made: The video input connector and electronics appear as a 75 ohm impedence. All video cables for these connections are made with coax designed to be 75 ohms. Now that the connector and cable are "Impedence Matched" the signal flows to the electronics with the most efficiency/least amount of problems.

    The people who came up with the SPDIF covering the coaxial-digital cable connection adopted the same convention: the input is 75 ohm impedence and the proper cable should be a video cable.

    Single wire coat hangers/chicken wire work because:

    - They dont have parallel wires to cause impedence issues
    - They are done in a lab and not in a dirty, real-world environment
    - They are trusting to luck that both the DVD player and reciever have a close value of 0.00 volts. This can be done with a separate ground-wire between the 2 devices.

    Many people believe the following statement, but hopefully you all now know it's NOT true:

    "Because a coat-hanger or chicken-wire have been used it means that any RCA cable can be used for the coaxial-digital connection."
     
  12. Stephen Tu

    Stephen Tu Screenwriter

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    It is pretty much true though. Even though you may have a "dirty" environment, digital signals are quite robust against noise. It's one of the main reasons behind digital transmission vs. analog in the first place; you have to mangle the signal quite badly before you get errors on the other end, and at that point it will be quite obvious with sound dropouts; you don't get subtle degradations & distortions.

    You'd prefer to use a cheap coax video cable, but in reality any RCA cable is likely to work perfectly.
     
  13. ernie.bin

    ernie.bin Stunt Coordinator

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    i was changing my setup a bit and needed a coax cable for my dvd player to receiver. I just used a cheap sony stereo interconnect (I mean really cheap) as a standby until I could hit the shop for something nicer, but it worked fine.

    hit radioshack the next day and picked up a 3ft composite video cable because it was 1/2 the price of the "digital audio" cable despite the packaging not being able to claim any difference in their construction.

    still works fine. zero dropouts, nothing out of the ordinary compared to my old optical cables either.
     

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