can i use a cheap coaxial cable for video?

Discussion in 'Beginners, General Questions' started by Myo K, Aug 17, 2003.

  1. Myo K

    Myo K Stunt Coordinator

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    i bopught this 4 dollar cable that i believe its said it may be used for coaxial connections of audio , i was wondeirng if i can use it as a video composite cable?

    i use it as a video cable and it seems to work fine, but id like to know if there will be any possible damage that may result?

    can i use it for a sub cable too?

    i currently use a cheapo generica patch cables to hook up the sub.
     
  2. Bob McElfresh

    Bob McElfresh Producer

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    The rules are:

    - Video cables should be made with 75 ohm coax
    - The connectors for video cables should be as close to 75 ohms as possible. These are usually colored Yellow to help distinguish them from audio cables.

    - Audio cables can be made with any of the common available coax impedences: 50, 75, 110, 300.

    - Audio is much less sensitive to the impedence of the connector than video.

    Subwoofer frequencies, lowest of the low, are the least sensitive to the cable. (But subwoofers seem to like a snug-fitting plug).
     
  3. Myo K

    Myo K Stunt Coordinator

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    bob, what would happen if i were to use a cable that did not meet the 75 impedence??

    are they like speakers similar to impedence mismatch?

    or will it just suffer in the visual calirty department, with no potential of damage?
     
  4. John Garcia

    John Garcia Executive Producer

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    Coaxial digital cable and video cable use the same specifications, so it should not be a problem. They are interchangeable.
     
  5. scott>sau

    scott>sau Stunt Coordinator

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    Regular RG6 (coaxial cable) your cable provider uses lets pass audio and video signal, so you could use it for audio interconnect. Many homes have their subwoofer input cable prewired with RG6Q. Try to avoid RG59. Try to use shileded coax. It is best to use cable how it was designed.
     
  6. Myo K

    Myo K Stunt Coordinator

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    what would happen if i were to use a standard audio component cable as a video cable? i tried it once and didnt seem to notice a difference. and can a coaxial cable be used for a sub cable? im wondering if i can use a generic digital coaxial cable to conenct my subs other then those ar/monster brands that cost 7 times more. id rather get a generic coaxial cable instead if it does the same thing with no potential hardware damage.

    all of them look alike to me :/
     
  7. scott>sau

    scott>sau Stunt Coordinator

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    Some video and audio interconnects are not interchangable. If you get some 75 ohm coax that your cable provider uses, strip the ends and after compression crimping F-fittings on it get some F-to-RCA adapters. You can then use the patch cable for what you choose to. Everytime I have prewired for subs I use RG6Q (quad-shieleded) coaxial cable. It is much cheaper than Monster, AR, or Kimber Kable and still is OK for signal to a sub.
     
  8. John Garcia

    John Garcia Executive Producer

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    A sub cable does not need to be a digital coax, ANY audio interconnect will work for your sub, including RG6 as mentioned. All you have to choose is your budget.
     
  9. Bob McElfresh

    Bob McElfresh Producer

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  10. Thomas Smailus

    Thomas Smailus Stunt Coordinator

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    Can I assume that if one has standard RG6, one can use the Radio Shack crimping tool and stripping tools in conjunction with the Radio Shack Type F connectors and F-to-RCA converter ends to produce an equal quality 75ohm video cable?

    I ask, as I already have the respective crimpers and strippers.
     
  11. Ernest Yee

    Ernest Yee Supporting Actor

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    Thomas - Equal quality as compared to which cables? If you crimp it yourself, it should work fine.

    Although most mid-of-the-road / cable worshippers would probably agree that the Canare cables would be superior.

    But if you're just looking for a quick and easy budget cable, your suggestion would be fine.
     
  12. Bob McElfresh

    Bob McElfresh Producer

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  13. Thomas Smailus

    Thomas Smailus Stunt Coordinator

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    Agreed.

    However, and I'm no RF engineer, as the CATV signal comming down the RG6 contains several dozen video signals modulated on top of a carrier signal, which is higher frequency I believe, then we should be able to say the RG-6 is sufficient for the signals carried on that carrier as well.

    Cableing ususally has an upper frequency limit, but not a lower one. So I don't see how a CATV grade RG-6 thats nice and sheilded could possibly have any problem carrying simple composite video.
     
  14. Bob McElfresh

    Bob McElfresh Producer

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    Thomas: A little background -

    - Since the 1940's television cameras have all been progressive. (No, it's not new just because we now have DVD players). But because we did not have the cables & antennas to handle those frequencies, they decided to use a "Interlace" format to transmit them. That is why televisions today all support interlace.

    Strange fact: the identical video signals have the following max frequencies:

    480i Video: 4 Mhz
    480p Video: 13 Mhz

    Yet they both contain the same amount of information!

    A color video signal is really 3 separate signals: a Black and white video signal, and two "Difference" signals. These are mixed together to create a "Composite" of video, then encoded (I believe) by frequency modulation on a particular carrier. Because of the encoding, the actual frequencies involved are LESS than the un-encoded video signal.

    This trick makes the max frequency needed somewhere less than 1 Mhz.

    Your CATV coax is designed to handle these sub Mhz frequencies. Not Video, Progressive Video or HD Video.

    Thomas: why do you seem to argue against using the proper coax for the different signals? Do you think that just because it looks like a cable and has RCA plugs on the ends that all coax works the same? That's kind of like thinking you can drive 60 MPH on any road that has asphalt on it like the freeway. It looks the same, right?

    Let me guess: you see CATV coax selling for $0.40/ft and component cable sets selling for $40 and you dont want to pay the difference?

    Well, I'm with you. I dont want to spend that much either.

    You can buy High Def compatible RG6 coax in a 3-conductor bundle for about $1.49/ft. Just order it from HAV Inc (New York) or from MarkerTech (Las Vegas).

    Is $1.49/ft cheap enough for you?
     
  15. scott>sau

    scott>sau Stunt Coordinator

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    " From Bob: That's
    kind of like thinking you can drive 60 MPH on any road that has asphalt
    on it like the freeway. It looks the same, right?:

    Not the roads in my neck of the woods!
    :b
     
  16. Thomas Smailus

    Thomas Smailus Stunt Coordinator

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    Bob:

    I'm arguing the case this way to try and get some conclusive information. Open debate which draws out the underlying scientific and engineering support is the only way I'm going to be convinced myself. Its not the cost so much as it is the need to buy 'special cable' that is not to be had easily locally and not of the type that is readily available and distributed.

    I'm not sure about your video and carrier frequency argument- I'll look it up at my end, but my instinct and whatever electrical engineering education from 16 years ago that stuck tells me this: You have a signal, a composite video signal, which is composed of a collection of different frequency signals. You modulate that signal on a carrier signal of some type. Even if the modulating signal is of a lower frequency (which I'm not sure about and I'll take your word on this, that it is a lower frequency modulation ), the original signal with its inherent signal frequencies is still part of the finished, modulated signal. So the combined modulated signal will still contain frequencies that are as high as the highest frequency in the modulation process, be they the modulating frequency or the signal to be transmitted. I don't see how you can transmit a signal using a broadcast signal where the highest frequency in it is less than much of the frequencies in the signal you are trying to transmit, without losing part of the original signal. I suppose I need to dig out a book on RF transmission systems and video transmission.


    Can you elaborate on the 480i=4MHz vs 480p=13MHz. If it is like HD signals, the frame rate on the progressive signal is double that of the interlaced signal as well, correct? That is, the 480i is sent at about 30fps, while the 480p is then sent at 60fps, in order to get that 13MHz figure?
     

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