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Frequency Division Multiplexing in Telephone

  1. Apr 19, 2017 #1
    "A standard FDM scheme for telephone signals is to multiplex 12 voice channels, each occupying 4kHz bandwidth, into a group signal with 48kHz. The next basic building block is the 60-channel supergroup, which is formed by frequency division multiplexing five group signals. The next level of the hierarchy is the mastergroup, which combines 10 supergroup inputs."

    My question is why do we need this grouping of voice channels at different stages and with different carriers? Why don't we just modulate our individual voice channels with the supergroup carrier frequencies?
  2. jcsd
  3. Apr 19, 2017 #2
    The idea of having a tier of multiplexing came about as technology developed. At first, FDM was used over the short distance copper pairs used for junction circuits between local exchanges. Then longer distances were wanted for trunk circuits over open wire, and eventually coaxial cable was used carrying hypergroups - hundreds of circuits. It is much easier to manage a network when traffic can be patched in large blocks such as supergroups or hypergroups. It also helps to ensure that the transmission levels are accurately controlled, because adjusting 960 individual circuits to the same precise level would be a problem.
    Another factor is the economy of generating the SSB at a low frequency, and the possibility of producing identical group modulating equipment in large quantities. If each supergroup circuit was individually generated we would require a special crystal for each one. The tier arrangement avoids having too many crystal frequencies involved. But I do agree with you that there is a historical element here.
    It is interesting that in the digital era, there was a move away from the tiered arrangement with its "multiplexer mountains" needed to drop channels from a high capacity system, and this was the move from the Plesiochronous to the Synchronous (SONET in the USA) Digital Hierarchies.
  4. Apr 19, 2017 #3
    So that means it is done to get economy and avoid complexity.
  5. Apr 19, 2017 #4
    Yes, or perhaps economy, simplification of the equipment and making a reliable network.
  6. Apr 19, 2017 #5

    jim hardy

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    Ahh how far we've come... in mid 1960's i worked on Lenkurt telephone carrier systems for the railroad. They had their own telephone system on the poles that run alongside their tracks.
    We thought 20 khz carrier with five voice signals was high tech. We had to "transpose" the wires at every pole to make them a 'twisted pair' that'd carry such a remarkably high frequency across Kansas and Missouri......

    So i think the remark about "historical" is your answer.

    Here's a transposition at a pole - the wires criss-cross, swapping places.


    Makes them effectively a twisted pair. Higher the carrier frequency the more twists per mile.

    old jim
  7. Apr 20, 2017 #6


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    Wow. Learn something new everyday. You the man.
  8. Apr 20, 2017 #7

    jim hardy

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    Thanks Don

    i'm glad you enjoy my boring anecdotes !

    old jim
  9. Apr 20, 2017 #8


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    Them were the days :wink:. I was doing that as well for telecom in New Zealand during the late '70 and through the '80's

    Even for just standard voice open wire lines we still transposed wires but the twist was done "in-span" rather than at the pole, unless a single pair circuit

    wire position.GIF
    numbers equal wire position on pole. this was mainly to stop pickup of electrical interference from external sources

    EDIT .... I should have drawn successive poles in the other order. in the order seen they are from right to left :rolleyes:

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