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Voive over telephone cable

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  • #40942 Reply

    I have always been intrigued about how you can transmit and receive data over a cable at the same time.
    Let’s take a simple example; if I’am talking over the phone(A)and someone is hearing(B) my conversation reaches B and he hears it. Then B responds to me (A)
    and vice versa. What I say goes over the telephone line and what the other person responds goes back over the same telephone line. We don’t always speak at the same time, but, and this is my question, if we talk at the same time to each other
    how the voice signals transmited to and from as waves at the same time over the telephone line do not collide with each other if they are being transmited “up and down” at the same time over the same cable. I don’t know if I made myself understood but since I’m not a telecommunication professional I try to ask using my own words.
    Thank you for your answer.

    #40943 Reply

    Hi Manny, try to think of elecrical impulses as light, because that is exactly what it is. Our phone lines are just a medium to tranmit light. If I point my flashlight at you at the exact same time that you shine your flashlight at me, we both see each other in the dark. When we talk face to face we transmit and receive at the same time. This is because we have built in drivers(Voice) and built in receivers(ears).


    #40944 Reply

    Thank you for your answer but as far as I’m concerned what is transmited over the phone lines are electromagnetic waves. The sounds of the voice which are sound waves are converted into an AC current once you speak into the mouthpiece or microphone of the phone. This is done by the electronic circuit inside the phone.This goes over the phone line as an AC current modulated by our voice and it is analog not digital. The phone by itself does not convert from analog to digital since it is not a modem.This signal is not light because you can’t transmit light over a copper wire. You transmist light if you are using a fiber optic cable which is not our case.
    If I think of this electric wave going up the line an the other going down at the same time, they could collide with each other someplace in the line.
    I know it is a little difficult to explain and I’m pretty sure there must be some physics involved in trying to explain what really happens inside the wire
    Thank you anyway.

    #40945 Reply
    vijay Telecom Er

    your querry is ok but its purley an imagination. First the system work like this since it was invented by Edison. As discussed above, the voice change to analog electrical signal as sine wave and these from opposite side are not always in same phase. Even if little cancellation is there,human mind can reconstruct the voice.

    #40946 Reply

    The TWO wire you are refering is actually RX and TX Path.


    #40947 Reply
    vijay er

    Two wires are trans and return not Tx and Rx
    vijay Engineer

    #40948 Reply

    Well in a two wire cable I undestand your explanation about one wire to Tx and the other one to Rx, but what about a coaxial cable with one conductor only.

    #40949 Reply

    Vijay er you say “Two wires are trans and return not Tx and Rx”.
    What the purpose of the return wire?.
    Is it used to Rx?.
    Thank you

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