Analog Telephone Circuits

As mentioned, phone technology originally was analog,  from start to finish. Analog modulation is the technique used to convert sounds (such as your voice)  into an electromagnetic form. The analog  circuitry of the POTS tele- phone transmitter converts the voice patterns coming  from the caller’s mouth into continuous electromagnetic signal patterns. These  patterns are carried on a telephone line circuit, sometimes called  a trunk line, where they are carried to the terminating end of the circuit. There,  analog  circuitry converts the signal back into audible sounds so they can be understood by humans.

A good  basic illustration of a POTS circuit can be found in an old elementary school science experiment. My fifth-grade science teacher, Ms. Davis, had  us punch a hole  into the  end  of two tin cans and  connect them using  a long string. If we held  the  string taut, Jodie  Schnickmeister could whisper into one can and  I could hear her  in the  other. (I used to love it when  Jodie  whispered in my ear.)

This simplistic experiment taught the  basics of the  POTS network: Sound was converted to an analog signal  (vibrations) that was carried over  the taut string to the  receiving can. The string, in turn, vibrated the  can and con- verted the  analog signal  back  into audible sounds.