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Thread: Cable Impedance

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    Cable Impedance

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    I was thinking about this last night on the toilet, and thought I should run it by here before I come to any conclusions.

    It seems that in audio signals, the impedance of the cable is completely irrelevant. According to wikipedia, transmission line equations only come into effect when the length of the wire is comparable in length to the wavelength of the transmitted signal.

    "In many electric circuits, the length of the wires connecting the components can for the most part be ignored. That is, the voltage on the wire at a given time can be assumed to be the same at all points. However, when the voltage changes in a time interval comparable to the time it takes for the signal to travel down the wire, the length becomes important and the wire must be treated as a transmission line. Stated another way, the length of the wire is important when the signal includes frequency components with corresponding wavelengths comparable to or less than the length of the wire."

    Assuming that we are using a copper wire and electricity flows at about 2/3 the speed of light (200,000 m/sec), the wavelength of a 30kHz signal is 6,667 meters long. So unless our cables are 4 miles long, the impedance doesn't matter. Am I the only person who didn't know this? or am I missing something?

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    To some degree yes, but there are other culprits as the (unbalanced) line gets longer like RF interference, and of course the impedance of the source. A high impedance source (passive electric guitar for instance) will be more greatly effected by the resistance in the cable run as opposed to a low impedance source (dynamic mic for instance).

    That's why you don't plug your electric guitar into the stage snake without a direct-box to that FOH mixer 200' away. If you haven't tried it, it sounds terrible...and also why your unbalanced studio runs need to be less than around 25'...they'll start to induce hum and RFI.

    That reference from wiki...transmission lines...that is likely refering to scenarios like telephone transmission lines which have a vastly limited bandwidth expectation over hi-fidelity audio, so it is apples and oranges to some degree...a great degree, really.

    BTW,

    I was thinking about this last night on the toilet
    That was over-sharing.

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    That is correct. At audio frequencies cable is not a transmission line. As noted there are other factors....But you could take any coax and use it for a mic cable (unbalanced) or take twinax and use it as a balanced mic cable. It would not matter the cables impedence (50 or 75 ohms).

    E
    In Sunny Vancouver ..... (Washington)

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    For the record, don't EVER take anything on wikipedia as gospel. Not saying they are always wrong there, but remember wiki stands for 'What I Know Is' and sometimes web denizens don't know shit.


    AK

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    Quote Originally Posted by evm1024 View Post
    That is correct. At audio frequencies cable is not a transmission line. As noted there are other factors....But you could take any coax and use it for a mic cable (unbalanced) or take twinax and use it as a balanced mic cable. It would not matter the cables impedence (50 or 75 ohms).

    E
    Thank you. It's a little embarrassing that it took me this long to realize that.

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    Quote Originally Posted by analogkid View Post
    For the record, don't EVER take anything on wikipedia as gospel. Not saying they are always wrong there, but remember wiki stands for 'What I Know Is' and sometimes web denizens don't know shit.


    AK
    I know wikipedia is not the gospel, but this quote is true.

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    Are you guys serious? I take everything wiki says as fact. I have won many a argument by going to the wiki... Sometimes I even have won money. I guess I should give that guy his 20 bucks back...
    I own: Tascam TSR-8
    Tascam M-1600 mixer

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    Quote Originally Posted by poopchute View Post
    Are you guys serious? I take everything wiki says as fact. I have won many a argument by going to the wiki... Sometimes I even have won money. I guess I should give that guy his 20 bucks back...
    If wikipedia is winning you money, then I see no reason not to love it. Is there an analog wikipedia?

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    Is there an analog wikipedia?
    Yes there is!

    It is rudimentary at this point. Ethyrvalve got it started.

    We haven't won any money yet.

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    Quote Originally Posted by sweetbeats View Post
    That was over-sharing.
    Not so!!! I do all my best thinking on the toilet!

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