The telephone has some pretty impressive, clever ideas behind it. First is the concept of tone dialing which older people will be familiar with, where the number to dial is expressed as a set of digits in unary expressed through quick breaking of the phone circuit. For example, to dial 911, you could quickly press and release the “hang up” button that the headset normally resides on nine times, then (after a pause) once, and finally (after another pause) once more. This is why old phones had these funny circular dial pads — each rotation of the pad from one digit to the next would break the circuit once so if you reached for the digit “9″ and then released the pad, it would break the circuit nine times.

The next-generation — used until cell phones showed up — system was just as clever. The idea was for the exchange to listen to the sound from the headset and interpret certain frequencies as digits. The frequencies that would stand for digits were actually sets of two sounds which were not harmonics of one another — i.e. if you tried to play the sound on any instrument, or whistled, you couldn’t generate these frequencies. What a brilliant way to minimize conflicts!

For posterity, and because I’m strangely attracted to it, here is the Dual-Tone Multifrequency keypad — the row/column headers correspond to the frequencies, in Hz. The cells tell you which two frequencies are used (so the digit “7″ generates a tone that’s a combination of an 852 Hz and a 1209 Hz sounds):


·····1209·1336·1477·1633
687···1····2····3····A
770···4····5····6····B
852···7····8····9····C
941···*····0····#····D


This entry was posted on Sunday, July 11th, 2010 at 21:26 and is filed under clever, technology. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.