A subaudible tone is a tone that is used to

trigger Trigger may refer to: Notable animals and people ;Mononym * Trigger (horse), owned by cowboy star Roy Rogers ;Nickname * Trigger Alpert (1916–2013), American jazz bassist * "Trigger Mike" Coppola (1900–1966), American gangster ;Surname * Bru ...

automated Automation describes a wide range of technologies that reduce human intervention in processes, namely by predetermining decision criteria, subprocess relationships, and related actions, as well as embodying those predeterminations in machines ...

event at a

radio station Radio broadcasting is transmission of audio (sound), sometimes with related metadata, by radio waves to radio receivers belonging to a public audience. In terrestrial radio broadcasting the radio waves are broadcast by a land-based radi ...

. A subaudible tone is audible; however, it is usually at a low level that is not noticeable to the average listener at normal

volume Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). Th ...

s. It is a form of

in-band signaling In telecommunications, in-band signaling is the sending of control information within the same band or channel used for data such as voice or video. This is in contrast to out-of-band signaling which is sent over a different channel, or even o ...

Overview

These tones are included in the audible main portion of audio in the case of

satellite A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioi ...

; on tape, these often are filtered. Normally, subaudible tones are at one of the following frequencies: 25, 35, 50, 75

hertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that o ...

(Hz), or combinations of those frequencies. Until computerized radio automation became inexpensive and common, 25 and 35 Hz were used either in the audio stream or, in the case of tape cartridges used in radio broadcasting (better known as "

carts A cart or dray (Australia and New Zealand) is a vehicle designed for transport, using two wheels and normally pulled by one or a pair of draught animals. A handcart is pulled or pushed by one or more people. It is different from the flatbed ...

"), on a special track on the tape to indicate to a radio station's automation system that it was time to trigger another event. With the advent of computers and digital satellite, these tones are relegated to triggering commercial announcements and legal IDs on a dwindling number of

radio network There are two types of radio network currently in use around the world: the one-to-many (simplex communication) broadcast network commonly used for public information and mass media, mass-media entertainment, and the two-way radio (Duplex (telecomm ...

s, as tones in the audio have been supplanted by external data channels sent independent of audio on digital satellite feeds for radio. These trigger

relay A relay Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts An automotive-style miniature relay with the dust cover taken off A relay is an electrically operated switch ...

closure terminals on the satellite receiver itself (Starguide being a prominent system).

Use for filmstrips

Subaudible tones have also been used by later

filmstrip The filmstrip is a form of still image instructional multimedia, once commonly used by educators in primary and secondary schools (K-12), overtaken at the end of the 1980s by newer and increasingly lower-cost full-motion videocassettes and la ...

projector A projector or image projector is an optical device that projects an image (or moving images) onto a surface, commonly a projection screen. Most projectors create an image by shining a light through a small transparent lens, but some newer types ...

s to advance to the next frame in a filmstrip presentation. Previously, the

phonographic record A phonograph, in its later forms also called a gramophone (as a trademark since 1887, as a generic name in the UK since 1910) or since the 1940s called a record player, or more recently a turntable, is a device for the mechanical and analogu ...

or audio cassette accompanying a filmstrip to provide its soundtrack would have an audible tone to signal the person operating the projector to advance the film to the next frame. But automatic filmstrip projectors were introduced in the 1970s (that had an integrated cassette player) that would read a subaudible tone of 50 Hz recorded on the cassette to automatically trigger the projector to advance to the next frame. Most of the cassettes accompanying filmstrips from the 1970s and 80s would have one side of the cassette with audible tones for use with older manual projectors, and the other side with the same program audio, but with 50 Hz subaudible tones instead for automatic projectors. Some filmstrip releases would have both audible & subaudible tones combined, making the filmstrip and its companion cassette compatible with any filmstrip projector.

External links

Examples of subaudible tone units
Radio technology Broadcast engineering {{broadcast-stub