Selcall

Selcall (selective calling) is a type of squelch protocol used in radio communications systems, in which transmissions include a brief burst of sequential audio tones. Receivers that are set to respond to the transmitted tone sequence will open their squelch, while others will remain muted.

Selcall is a radio signalling protocol mainly in use in Europe, Asia, Australia and New Zealand, and continues to be incorporated in radio equipment marketed in those areas.

The transmission of a selcall code involves the generation and sequencing of a series of predefined, audible tones. Both the tone frequencies, and sometimes the tone periods, must be known in advance by both the transmitter and the receiver. Each predefined tone represents a single digit. A series of tones therefore represents a series of digits that represents a number. The number encoded in a selcall burst is used to address one or more receivers. If the receiver is programmed to recognise a certain number, then it will un-mute its speaker so that the transmission can be heard; an unrecognised number is ignored and therefore the receiver remains muted.

A selcall tone set contains 16 tones that represent 16 digits. The digits correspond to the 16 hexadecimal digits, i.e. 0-9 and A-F. Digits A-F are typically reserved for control purposes. For example, digit "E" is typically used as the repeat digit.

There are eight, well known, selcall tone sets.

The physical characteristics of the transmitted sequence of tones is tightly controlled. Each tone is generated for a predefined period, in the order of tens of milliseconds. Each subsequent tone is transmitted immediately after the preceding one for the same period, until the sequence is complete.

Typical tone periods include; 20ms, 30ms (sometimes 33ms), 40ms, 50ms, 60ms, 70ms, 80ms, 90ms and 100ms.

The longer the tone period, the more reliable the decoding of the tone sequence. Naturally, the longer the tone period, the greater the duration of the selcall tone burst; longer bursts may be enough to force the user pause before speaking, especially if using the leading-edge ANI scheme.

A typical tone period selection is 40ms, so for a 5-tone sequence this represents a total selcall duration of 5 x 40ms = 200ms.

Each tone in a selcall sequence must be unique. Typically, the receiving device cannot discriminate between two consecutive tones, where the frequency of those two tones is the same; that is, two consecutive tones with the same frequency will be decoded as a single digit. Therefore, where there are two consecutive digits to be transmitted that are the same, the second digit will be replaced by the repeat digit. The repeat digit is nearly always assigned as "E". On reception, if the receiving device decodes a sequence that contains a repeat digit, then it will substitute it with the preceding digit, thereby reconstituting the original sequence.

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