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Original Telephone Design Question

  1. Nov 19, 2009 #1
    I have read about the need to have a duplex coil in a telephone to block the sound of your own voice from reaching your ear....the reason given on a few websites I have found is that the original design of the telephone had the microphone and speaker wired together.

    So here is my question....

    Why did the original telephone have this design feature? Why did they wire it this way?
  2. jcsd
  3. Nov 19, 2009 #2


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    Not quite
    The duplex coil allows you to send both sides of the conversation over the same circuit at the same time, it subtracts your outgoing microphone signal from the incoming speaker signal. This means you only need one pair for a duplex channel - pretty clever.

    See http://www.epanorama.net/circuits/teleinterface.html for an explanation of telephones
  4. Nov 19, 2009 #3
    1. I have already looked at that website....

    2. There is only one website I can find that seems to agree with your use of terminology (even though that wasnt the question I asked) found here:

    From this website:
    http://en.allexperts.com/q/Electrical-Engineering-1356/Duplex-Coil.htm [Broken]

    Even if this one source is correct and all the others are wrong in what they are calling a "duplex coil"....I really dont care.

    I am not looking for a lesson on electrical enigneering (I'm an EE so I'm fine thanks)....my question is a historical one....here is the question in case you missed it...

    Why did the original telephone have this design feature? Why did they wire it this way?
    Last edited by a moderator: May 4, 2017
  5. Nov 19, 2009 #4


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    sorry i had only used that site once to work out how to power something from a phone line.
    historically they wire a phone like that so they only needed a single pair of wires, so having their infrastructure bill. I'm guessing the first phones ha a single coil as a microphone an speaker so only one person could talk at once an an isolation transformer and separate speakers/mics were a later invention.

    There are reasons for allowing some outgoing audio back into your ear - it makes it easier for you to talk if you can hear what you are saying for some psychological reason. Even digital or VOIP phones feed some of the output back into the earpiece deliberately. some even degrade the quality to make it the feedback sound like a tradiational phone!
  6. Nov 19, 2009 #5
    That sounds very reasonable to me....I figured it was likely an infrastructure/saving cost thing....

    I don't know for sure but that sounds reasonable since it would be an easy way to do it as long as you had the iso transformer...

    It also seems to tell the user that the phone is on and working...so there isn't dead silence when you are speaking.

    I know that if the phone stops working the 2 things I notice are no ring tone and a "dead air" sound when I talk into the phone....
  7. Nov 19, 2009 #6


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    I know Skype had to deliberately add some noise into the earpiece rather than having it silent when there was no incoming data because people thought it wasn't working.

    It's an interesting area of interface engineering, the little analog clues you get from the real world that you have to duplicate in digital systems.
  8. Nov 19, 2009 #7
    lol I never thought about the need to add in that stuff with the digital systems....but yeah it makes sense.
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