Developed for telephone work, dual tone multi-frequency signalling has crept into some radio systems but is slow to operate compared with other in-band systems. Two tones are transmitted simultaneously to represent each digit. In the UK they conform to the MF4 code (Table 16.3).
Table 16.3 MF4 code of DTMF tones 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
Some push button telephones signal to the local exchange by generating a combination of two frequencies. Each row and each column of the push button keypad is connected to an oscillator of set frequency. When any push button is pressed, tones corresponding to its row and column frequencies are therefore generated. The row and column oscillator frequencies are as shown below:
1209 Hz 1336 Hz 1447 HzSo-called 5-tone signalling uses a series of tones – usually five but other quantities are used in some systems. The system is popular for selective calling and addressing purposes (Table 16.4). There are a range of tones established by various organizations but, although the tones in each series may be assigned to each digit, equipment manufacturers sometimes amend the code and the usage. The relevant Radiocommunications Agency document is MPT 1316, Code of Practice, Selective Signalling for use in the Private Mobile Radio Services.
Table 16.4 5-tone codes Standard ZVEI DZVEI EIA CCIR/ CCITT EURO EEATone duration 70 70 33 100/ 100 100
(ms) 40
Pause duration 0 0 0 0 0 0
(ms)
Code/digit Tone frequency (Hz)
0 2400 970 600 1981 400 980
1 1060 1060 741 1124 697 903
2 1160 1160 882 1197 770 833
3 1270 1270 1023 1275 852 767
4 1400 1400 1164 1358 941 707
5 1530 1530 1305 1446 1209 652
6 1670 1670 1446 1540 1335 601
7 1830 1830 1587 1640 1477 554
8 2000 2000 1728 1747 1633 511
9 2200 2200 1869 1860 1800 471
Repeat 2600 2400 459 2110 2300 1063
Alarm 2800 2600 2400
Free-tone 1153
Group tone 970 825 2151 1055
ZVEI Zuverein der Electronisches Industrie. Designed to operate on 20 kHz channel spaced systems. On 12.5 kHz channel spaced systems transmission of 2800 Hz creates difficulty and depressed ZVEI (DZVEI) was adopted and recommended in MPT 1316.
EIA Electrical Industries Association.
CCIR Committee Consultatif International Radio Communication. The longer tone duration offers robustness against corruption but is slow to operate. Originally designed for marine use, each digit was transmitted twice to ensure reliability.