====== DTMF also known as Touch Tone ======

DTMF, or Dual Tone Multi Frequency, uses two tones to represent each key on the touch pad. When any key is pressed the tone of the column and the tone of the row are generated. As an example, pressing the '5' button generates the tones 770Hz and 1336Hz.

The frequencies were chosen to avoid harmonics (no frequency is a multiple of another, the difference between any two frequencies does not equal any of the frequencies, and the sum of any two frequencies does not equal any of the frequencies).

The frequencies generated must be with +/- 1.5%, the highest frequency must be as loud as the lowest frequency and as much as 4db louder. This level difference is refered to as 'twist'.

The A,B,C and D keys were used for the U S military's Autovon phone system. They were used for Flash Override - A button, Flash - B button, Immediate - C button and Priority - D button. Pressing one of these keys gave your call priority, Flash Override being the highest. The Autovon system is no longer used, but many non telephone applications use these keys today.

1 2 3 A 697Hz
4 5 6 B 770Hz
7 8 9 C 852Hz
* 0 # D 941Hz
1209Hz 1336Hz 1477Hz 1633Hz