SELCAL

Selective call systems are an efficient supplement to voice traffic. One method was the so-called single-tone-mode using five different tone frequencies, another one the two-tone-modes, where the call number is transmitted as frequency combinations.

Developments led to the five-tone-sequence systems. The five digits are often divided into two groups. The first digits work as radio net flags, the last three digits are user call numbers.

The entire call number is transmitted by consecutive tones in decade sequence. When two identical digits are to be transmitted consecutively, then an eleventh frequency is used as a repetition identifier. If there more than two identical digits are to be transmitted the repetition tone is appended to the digit tone (e.g. 22222 is transmitted as f2 fw f2 fw f2, where f2 is the tone for “2” and fw is the repetition tone).

In most systems the accuracy of the single frequencies has to be within +1/-1.5% of the nominal value.

Modifications of the tone allocation and tone duration has led to numerous systems despite many standards.

Only the EURO (EuroSignal) system uses six consecutive tones. The worldwide telephone signaling standard DTMF mode transmits two simultaneous tones.