How does Tone dialing work?

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Nowadays, many phones are using touch-tone dialing. A signal (tone) generator in the touch pad generates different tone combinations for each key.

	1209 Hz	1336 Hz	1477 Hz
697 Hz	1	2	3
770 Hz	4	5	6
852 Hz	7	8	9
985 Hz	*	0	#

All the keys on a particular vertical will have the same tone, as will all the keys on each horizontal line. When a particular key is pressed, both tones are generated. It is the combination of tones that is unique to each key. This is referred to as DTMF, for dual tone modulating frequencies.

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