Selective calling tones
A rapid sequence of musical tones that calls one specific radio or group.
What does it sound like?
A quick run of musical notes, like 'da-da-da-da-da'.
Who uses it?
Businesses, taxi and transport fleets, emergency services (common in Europe) and some repeaters.
Where and when can I hear it?
VHF/UHF FM channels, and some HF networks.
The science
A selcall sends an address as a quick string of single audio tones, one tone per digit. In the German ZVEI formats each tone lasts about 70 milliseconds; in the original CCIR format each lasts 100 milliseconds, with digit 1 at 1124 Hz and digit 0 at 1981 Hz. Because a decoder cannot tell two identical tones in a row apart, a special repeat tone replaces the second of two repeated digits.
Why use it?
It lets a dispatcher wake up one radio or a group without everyone else on the channel having to listen, using nothing more than audio tones that any FM voice radio can pass. It was a low-cost way to add addressing to analog fleets long before digital radio.
Lore
The ZVEI family of formats is named after the Zentralverband Elektrotechnik- und Elektronikindustrie, Germany's electrical industry association based in Frankfurt.
Easily confused with
- Touch-tone (DTMF): DTMF plays two tones at once
- MDC-1200 radio ID burst: MDC-1200 is a buzzy data burst