{{Unreferenced, date=July 2008 In broadcast engineering, a frequency extender is an electronic

device A device is usually a constructed tool. Device may also refer to: Technology Computing * Device, a colloquial term encompassing desktops, laptops, tablets, smartphones, etc. * Device file, an interface of a device driver * Peripheral, any devi ...

that allows high-fidelity analog audio to be sent over regular

POTS Pot may refer to: Containers * Flowerpot, a container in which plants are cultivated * Pottery, ceramic ware made by potters * A type of cookware Places * Ken Jones Aerodrome, IATA airport code POT * Palestinian Occupied Territories, the We ...

telephone lines, without the loss of higher audio frequencies (

treble Treble may refer to: In music: *Treble (sound), tones of high frequency or range, the counterpart of bass *Treble voice, a choirboy or choirgirl singing in the soprano range *Treble (musical group), a three-piece girl group from the Netherlands *T ...

). It is an extended

concept Concepts are defined as abstract ideas. They are understood to be the fundamental building blocks of the concept behind principles, thoughts and beliefs. They play an important role in all aspects of cognition. As such, concepts are studied by s ...

of a telephone hybrid. The

uses frequency shifting to overcome the narrow bandwidth of regular telephone systems. The input signal is sent on one telephone line as-is, or in some cases upshifted to provide extra low-frequency response, and sent on a second line shifted down by 3 kHz, which is normally the upper bandpass limit in telephony. Thus, an audio frequency of 5 kHz is sent at 2 kHz. A receiver on the other end then shifts the second line back up and mixes it with the first. This results in greatly improved audio, adding a full

octave In music, an octave ( la, octavus: eighth) or perfect octave (sometimes called the diapason) is the interval between one musical pitch and another with double its frequency. The octave relationship is a natural phenomenon that has been refer ...

of range, and pushing the total bandpass to 6 kHz. The sound is then acceptable for voice, if not for

music Music is generally defined as the The arts, art of arranging sound to create some combination of Musical form, form, harmony, melody, rhythm or otherwise Musical expression, expressive content. Exact definition of music, definitions of mu ...

. It is also possible to add other lines, each increasing the bandpass by another 3 kHz. However, the law of diminishing returns takes over, because each successive octave is double the size of the last. A third line pushes the bandpass up 50 % to 9 kHz, equivalent to AM radio. A fourth line would push it up 33% to 12 kHz.

FM radio FM broadcasting is a method of radio broadcasting using frequency modulation (FM). Invented in 1933 by American engineer Edwin Armstrong, wide-band FM is used worldwide to provide high fidelity sound over broadcast radio. FM broadcasting is c ...

quality would require five telephone lines to be installed, pushing the bandpass up 25% to 15 kHz. The audio is shifted down by 6,9, and 12 kHz respectively for each additional line. Frequency extenders have been nearly eliminated by POTS codecs.

See also