In telephone systems, multifrequency signaling in which standard set combinations of two specific voice band frequencies, one from a group of four low frequencies and the other from a group of four higher frequencies, are used. ( 188 ) Synonyms multifrequency pulsing , multifrequency signaling. Note 1: DTMF signals, unlike dial pulses, can pass through the entire connection to the destination user , and therefore lend themselves to various schemes for remote control after access , i.e. , after the connection is established. Note 2: Telephones using DTMF usually have 12 keys. Each key corresponds to a different pair of frequencies. Each pair of frequencies corresponds to one of the ten decimal digits, or to the symbol "#" or "*", the "*" being reserved for special purposes. Note 3: The standard signal frequency pairs transmitted by DTMF equipment used by the public exchange carriers are as follows:
Button or Digit Frequencies (Hz) 1 697/1209 2 697/1336 3 697/1477 4 770/1209 5 770/1336 6 770/1477 7 852/1209 8 852/1336 9 852/1477 0 941/1336 * 941/1209 # 941/1477
Note 4: Tactical telephones have 16 keys, the extra 4 being used for precedence . For DSN ( Defense Switched Network ) the signal frequency pairs transmitted for the ten decimal digits and the * and # are the same as those used by the public exchange carriers. The additional four keys, corresponding to four different frequency pairs and the precedence, are as follows:
Button or Key Frequencies (Hz) FO ( Flash Override ) 697/1633 F ( Flash ) 770/1633 I (Immediate) 852/1633 P ( Priority ) 941/1633