An interval signal, or tuning signal, is a characteristic sound or musical phrase used in international broadcasting, numbers stations, and by some domestic broadcasters, played before commencement or during breaks in transmission, but most commonly between programmes in different languages.
It serves several purposes:
It helps a listener using a radio with an analog tuner to find the correct frequency.It informs other stations that the frequency is in use.It serves as a station identifier even if the language used in the subsequent broadcast is not one the listener understands.The practise began in Europe in the 1920s and 1930s and was carried over into shortwave broadcasts. The use of interval signals has declined with the advent of digital tuning systems, but has not vanished. Interval signals were not required on commercial channels in the United States, where jingles were used as identification.
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