= How the telephone works
* https://www.quora.com/How-is-a-voice-transmitted-from-one-phone-to-another
* https://www.quora.com/How-many-wires-does-a-telephone-use/answer/Peter-Yardley-1
> Basic analogue phones connected to the public exchange use two wires mainly arranged as a twisted pair to reduce noise. The voice signal is differential (the voltage in one wire equal and opposite to the other) biased above ground by 48V. Using a twisted pair reduces induced noise because the noise signal will induce an equal voltage in each wire and because the signal is transmitted as the difference the effect of the induced noise will be dramatically reduced.
\Video[https://www.youtube.com/watch?v=8bwLjU90puA]
{title=Phone Intercom by Make (2014)}
{description=This video illustrates will the incredible simplicity of the connection of a telephone system. Compare that to the relative complexity of <wireless> communication, which requires <modulation>.}
\Video[https://www.youtube.com/watch?v=JPau8xDE3pk]
{title=Making a Microphone Work with an Oscilloscope by Environmental Radiation LLC (2012)}
{description=Not the most detailed setup, but good.}
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