Selcall or 5-Tone calling systems transmit a series of tones over the radio carrier. Each tone frequency has a defined value depending on the implemented system. There are four main systems in use within the UK.
These systems use the following frequencies
| Value | CCIR | EEA | ZVEI | DZVEI |
|---|---|---|---|---|
| 1 | 1124 | 1124 | 1060 | 970 |
| 2 | 1197 | 1197 | 1160 | 1060 |
| 3 | 1275 | 1275 | 1270 | 1160 |
| 4 | 1358 | 1358 | 1400 | 1270 |
| 5 | 1446 | 1446 | 1530 | 1400 |
| 6 | 1540 | 1540 | 1670 | 1530 |
| 7 | 1640 | 1640 | 1830 | 1670 |
| 8 | 1747 | 1747 | 2000 | 1830 |
| 9 | 1860 | 1860 | 2200 | 2000 |
| 0 | 1981 | 1981 | 2400 | 2200 |
| Group | 2400 | 1055 | 2800 | 2400 |
| Repeat | 2110 | 2110 | 2600 | 810 |
DZVEI (Depressed ZVEI) was defined because some radio systems use voting tones in the range 2600 to 2900Hz to report signal strength which would interfere with the signalling.
The group tone is used as a wild card and selects all radios in that digit position. The repeat tone is used to prevent the same tone being transmitted in two sucessive positions, for example the radio 12223 would use the tones 1, 2, Repeat, 2, 3.
There is a start interval "Link establishment time" (LET) before signalling commences which is intended to ensure that the transmitter has stablised before sending the message. These intervals are listed below.
Modulation depth for AM and FM systems is 70% (+/-20%).
The way these tones are used depends on the system. In general 5 tone sequences are used with a 6th tone being sometimes employed to perform control functions such as radio mute at the end of a call. typically a radio unit will transpond its identity and enable receiver audio upon receipt of a transmission with its identity unless group tones have been employed in which case the transpond is disabled. Note that sometimes the last couple of digits of the transpond correspond to thumbwheel settings on the radio enabling a basic status report to be transmitted.
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