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Selcall (selective calling) is a type of squelch protocol used in radio communications systems, in which transmissions are preceded by 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.

Motorola's name is Select 5 in sales brochures for obsolete equipment marketed in Europe such as Syntor mobiles, Syntor X mobiles, Mitrek mobiles, Mostar mobiles, and Maxar mobiles.

The similar-sounding name SelCal is a different selective-calling system used by the International Civil Aviation Organization. It uses pairs of tones mostly corrsponding to the Z-chart of Motorola's obsolete Quik Call I system. SelCal was used over HF SSB radios on ICAO trans-oceanic routes.

Contents 1 Details 1.1 As push-to-talk identifier 2 Four Main Protocols: 3 Trivia 4 External link

Most selcall systems utilise a sequence of five tones, each of which can have any of ten frequencies, plus two extra frequencies for 'group' and 'repeat'. This provides almost 100,000 different sequences, plus a broadcast mode allowing multiple receivers to be addressed. Selcall is suitable for SSB and other amplitude modulation schemes, and is a popular option for many commercial HF and VHF radio transmitters and receivers. It should not be confused with CTCSS (Continuous Tone Coded Squelch System), which relies upon a continuous superimposed low-frequency audio tone, and which is mainly used for VHF and UHF FM communications systems.

A similar proprietary Motorola format used a seven-tone sequence and was called MODAT. Radios with this option were marketed in the US during the 1970s and 1980s. MODAT encoders in Motorola radios can be configured to send five-tone sequences with code plans compatible to CCIR, ZVEI, or the proprietary Motorola seven-tone-sequential format. These systems send tone sequences to identify a unit (unit ID) rather than for selective calling. Some systems used CTCSS and MODAT. In a unit ID application, every radio has a different five- or seven-tone code. Each time the push-to-talk is pressed, the tone sequence is transmitted. This code is displayed at the dispatch console to identify which unit has called. In some cases the code is translated to a vehicle number or other identifier.

Tone plans are different for each protocol.

• CCIR: 100mS (+/- 10mS) tone duration;
• EEA: 40mS (+/- 4mS) tone duration;
• ZVEI: 70mS (+/- 15mS) tone duration;
• DZVEI: 70mS (+/- 15mS) tone duration.

In the U.S. feature film Dirty Pretty Things, a scene inside a taxi dispatch office has Selcall tones in the background audio.

More information can be found at http://www.x31.com/~andy/keywords/Selcall.html. Suitable encoder and decoder ICs are available from a variety of manufacturers.