Subaudible Tone (Paperback)

Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A subaudible tone is a tone that is used to trigger an automated event at a radio station. A subaudible tone is not inaudible; however, it is usually at a low level that is not noticeable to the average listener at normal volumes. It is a form of in-band signaling. These tones are included in the audible main portion of audio in the case of satellite; on tape, these often are filtered. Normally, subaudible tones are at one of the following frequencies: 25, 35, 50, 75 hertz (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"), on a special track on the tape to indicate to a radio station's automation system that it was time to trigger another event.