Old-school telephone central office heterodyning

Discussion in 'The Projects Forum' started by emceedub, Jun 1, 2014.

  1. emceedub

    Thread Starter New Member

    May 29, 2014
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    0
    I'd like to recreate the voice channel pre-grouping typical of a phone company central office: taking separate voice bands (300-3400hz and upconverting to 12, 16 and 20khz carriers (as described here http://en.wikibooks.org/wiki/Communication_Systems/Frequency-Division_Multiplexing).

    I'm looking for circuit diagrams and/or discrete components that accomplish this.

    My goal is to do this analog with with no buffers and minimal latency)? I'm sure these used to be dime-a-dozen.

    Any thoughts or guidance?
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    That's not simple to do.

    You need the following:

    Three bandpass filters to limit the frequency range of your voice signals.

    Three single-sideband modulators with the three carrier frequencies to generate the three signals and combine them together in one channel.

    Three band-pass filters at the other end to separate the three signals

    Three single-sideband demodulators to recover the modulation.
     
    absf likes this.
  3. emceedub

    Thread Starter New Member

    May 29, 2014
    7
    0
    Yes, that's it exactly. At this point, I'm focusing on the first half: the upconversion.

    Question is, are there existing diagrams or pre-packaged units ot there that do this?. There had to be multiple millions of these implemented across the globe 40-80 years ago.



     
  4. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Good question. There may be diagrams of old units or old units for sale, but don't know how you would find them other than Google or other search engine.
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Start with post #43, and read on.

    http://forum.allaboutcircuits.com/showthread.php?t=97670&page=5

    Don't try to take over the thread, but if you have any questions ask here, PM me to let me know you've posted (see my signature), and I'll try to help.

    It is indeed not simple. Collins Radio made a lot of this equipment, which I repaired back in the day. SSB is never simple.
     
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