For the use of subaudible tones in Two-way radio

Two-way radio

A two-way radio is a radio that can both transmit and receive , unlike a broadcast receiver which only receives content. The term refers to a personal radio transceiver that allows the operator to have a two-way conversation with other similar radios operating on the same radio frequency...

 see: CTCSS

CTCSS

In telecommunications, Continuous Tone-Coded Squelch System or CTCSS is a circuit that is used to reduce the annoyance of listening to other users on a shared two-way radio communications channel. It is sometimes called tone squelch...

A subaudible tone is a tone

Note

In music, the term note has two primary meanings:#A sign used in musical notation to represent the relative duration and pitch of a sound;#A pitched sound itself....

 that is used to trigger

Trigger

-Technology:* Trigger , a mechanism that actuates the firing of firearms* Image trigger, a device used in highspeed cameras* Schmitt trigger, an electronic circuit* Trigger circuit, IBM's name for a flip-flop...

 an automated

Broadcast automation

Broadcast automation incorporates the use of broadcast programming technology to automate broadcasting operations. Used either at a broadcast network, radio station or a television station, it can run a facility in the absence of a human operator...

 event at a radio station

Radio station

Radio broadcasting is a one-way wireless transmission over radio waves intended to reach a wide audience. Stations can be linked in radio networks to broadcast a common radio format, either in broadcast syndication or simulcast or both...

. A subaudible tone is not inaudible; however, it is usually at a low level

Amplitude

Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...

 that is not noticeable to the average listener at normal volumes. It is a form of in-band signaling

In-band signaling

In telecommunications, in-band signaling is the sending of metadata and control information in the same band or channel used for data.-Telephone:...

.

Overview

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

Satellite

In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

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

Hertz

The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

 (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

Fidelipac

The Fidelipac, commonly known as an NAB cartridge or simply cart, is a magnetic tape sound recording format, used for radio broadcasting for playback of material over the air such as radio commercials, jingles, station identifications, and music. Fidelipac is the official name of this industry...

"), 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

Radio network

There are two types of radio networks currently in use around the world: the one-to-many broadcast type commonly used for public information and mass media entertainment; and the two-way type used more commonly for public safety and public services such as police, fire, taxicabs, and delivery...

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

Relay

A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...

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

Use for filmstrips

Subaudible tones have also been used by later filmstrip

Filmstrip

The filmstrip was a common form of still image instructional multimedia, once commonly used by educators in primary and secondary schools , now overtaken by newer and increasingly lower-cost full-motion videocassettes and DVDs...

 projector

Film projector

Film projection or Film projector may refer to:*Movie projector for projection of moving images from film*Slide projector for projection of still images from film...

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