Tone burst is an obsolete method of selective calling where the radio transmits a single 0.5- to 1.5-second audio tone at the beginning of each transmission. This scheme existed before circuitry for CTCSS had been developed. This method was in wide use in the United States from the 1950s through the 1980s.
In the same way that a single CTCSS tone would be used on an entire group of radios, a single burst tone is used in a group of radios. The radio speaker turns on as soon as the tone is decoded and the speaker stays on until the carrier squelch detects that the carrier is no longer being received. At that point, the speaker mutes and the decoder resets. The receiver speaker turns off and remains muted until another valid burst tone is decoded.
In some cases, burst tones were used to select repeaters. By changing tones, the mobile radio would actuate a different repeater site. A typical tone scheme might use the tones 1,800 Hz, 2,000 Hz, 2,200 Hz, 2,400 Hz, and 2,552 Hz. This was the scheme used by most State of California agencies during the era when tone burst was in use. Some systems have been observed to use tones as low as 800 Hz. The default or standard five Motorola tones used for single tone format as of the 1980s: 1,350 Hz., 1,500 Hz., 1,650 Hz., 1,800 Hz., 1,950 Hz. These were identified in system documentation for a number of remote control equipment models as well as sales brochures for Motorola Syntor and Micor mobile radio Systems 90 accessories. A common tone burst frequency used by many amateur radio systems in Europe is 1,750Hz.
In well-designed systems, repeaters or radios usually included an audio notch filter that reduced the volume of the tone at the speaker.
A variation to the single tone scheme was seen in one-way paging receivers. In some two-tone sequential systems, sending 4-8 seconds of the second tone pages all receivers which have a code including the second tone. This is sometimes referred to as long tone B. Receivers made by Plectron and often used to page volunteer firefighters use a long single tone. The decoder in the typical Plectron receiver would not decode the tone as a valid call unless it was present for at least two to four seconds, (a very long variation of the burst tone).
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