Professional Mobile Radio - Selective Calling

Selective Calling

Many systems operate with the remote or mobile stations being able to hear all the calls being made. This may not always be satisfactory and a system of selective calling may be required. There are several ways of achieving this, including Dual Tone Multiple Frequency (DTMF) signalling and Continuous Tone Coded Squelch System (CTCSS).

DTMF is a system that is widely used for telephone signalling and is almost universally used for touch tone dialing for landline telephones today. It uses set pairs of tones that are used to carry the information. The eight frequencies used are 697, 770, 852, 941 Hz which are termed the "low tones" and 1209, 1336, 1477, and 1633 Hz which are "high tones". One high and one low tone is used together and the various combinations are used to represent different numbers and symbols.

The relevant code consisting of one or more digits is sent and the station programmed to respond to the number, typically one or two digits responds by opening the squelch on the receiver to let the audio through. The disadvantage of this system is that if the receiver does not pick up the code at the instant the DTMF signalling takes place then it will not respond to any of the message. This can be a significant disadvantage because mobile stations often lose the signal for short periods as they are on the move.

Another widely used system is CTCSS, also referred to as subaudible tones or PL tones (a Motorola trademark). As the name suggests it uses subaudible tones (below about 250 Hz) to carry the selection information. These are transmitted in addition to the normal voice channel, but as they appear below the audio range passed by most mobile radios (roughly 300-3000Hz), they are filtered out and therefore not heard.

Only when the correct tone for the required station is transmitted will the squelch for that receiver be opened and the transmitted audio will be heard. The advantage of this system is that the subaudible tones are transmitted for the whole period of the transmission so if the signal fades at the beginning of the transmission is lost but later then increases in strength, the continuously transmitted tones will enable the squelch to open and the audio to be heard. Systems typically are able to provide up to 37 different tones, the lowest frequency of which is 67 Hz and the highest 254.1 Hz. This enables a variety of different mobiles to be called selectively..