Mike,
On a standard 10 or 12-button telephone there are actually only seven different frequencies used.
They are arranged on a row/column basis:
.....1209..1336..1477..1633 Hz
697....1.......2.......3.......A
770....4.......5.......6.......B
852....7.......8.......9.......C
941....*.......0.......#.......D
So when you press 5 for example, you send two tones of 770 and 1336 Hz.
It won't work on new phones, but for anyone with an original Western Electric 1500 or 2500-set, you can hear each tone individually by holding down two buttons in the same row or column, e.g. if you hold down 7 and 9 you'll hear a single 852Hz tone.
The 1633Hz column was in the original DTMF specification, and can be used for special signaling functions, although only special phones are fitted with these extra buttons (they were used extensively in the old AUTOVON military system in America).
By the way, if you listened to the Jane Barbe clip above, the burst of tone signaling at the beginning is not the same DTMF tones as heard when dialing a phone.
The tones you hear there are the ones which were commonly used for inter-office signaling in the past, and they use a different scheme:
1 = 900 + 700 Hz
2 = 1100 + 700
3 = 1100 + 900
4 = 1300 + 700
5 = 1300 + 900
6 = 1300 + 1100
7 = 1500 + 700
8 = 1500 + 900
9 = 1500 + 1100
0 = 1500 + 1300
KP = 1700 + 1100
ST = 1700 + 1500
[This message has been edited by pauluk (edited 09-21-2003).]
