{{Unreferenced, date=July 2008 In

broadcast engineering Broadcast engineering is the field of electrical engineering, and now to some extent computer engineering and information technology, which deals with radio and television broadcasting. Audio engineering and RF engineering are also essential ...

, a frequency extender is an

electronic Electronic may refer to: *Electronics, the science of how to control electric energy in semiconductor * ''Electronics'' (magazine), a defunct American trade journal *Electronic storage, the storage of data using an electronic device *Electronic co ...

device that allows high-fidelity

analog audio Analog recording is a technique used for the recording of analog signals which, among many possibilities, allows analog audio for later playback. Analog audio recording began with mechanical systems such as the phonautograph and phonograph. L ...

to be sent over regular POTS

telephone line A telephone line or telephone circuit (or just line or circuit industrywide) is a single-user circuit on a telephone communication system. It is designed to reproduce speech of a quality that is understandable. It is the physical wire or ot ...

s, without the loss of higher

audio frequencies An audio frequency or audible frequency (AF) is a periodic function, periodic vibration whose frequency is human hearing range, audible to the average human. The International System of Units, SI unit of frequency is the hertz (Hz). It is the pr ...

( treble). 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 ...

of a

telephone hybrid A telephone hybrid is the component at the ends of a subscriber line of the public switched telephone network (PSTN) that converts between two-wire and four-wire forms of bidirectional audio paths. When used in broadcast facilities to enable the a ...

. The

uses frequency shifting to overcome the

narrow Narrow may refer to: * The Narrow, rock band from South Africa * Narrow banking, proposed banking system that would eliminate bank runs and the need for a deposit insurance * narrow gauge railway, a railway that has a track gauge narrower than th ...

bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...

of regular telephone systems. The input

signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...

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 A band-pass filter or bandpass filter (BPF) is a device that passes frequencies within a certain range and rejects (attenuates) frequencies outside that range. Description In electronics and signal processing, a filter is usually a two-po ...

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 of range, and pushing the total bandpass to 6 kHz. The sound is then acceptable for

voice The human voice consists of sound made by a human being using the vocal tract, including talking, singing, laughing, crying, screaming, shouting, humming or yelling. The human voice frequency is specifically a part of human sound producti ...

, if not for

music Music is generally defined as the art of arranging sound to create some combination of form, harmony, melody, rhythm or otherwise expressive content. Exact definitions of music vary considerably around the world, though it is an aspe ...

. It is also possible to add other lines, each increasing the bandpass by another 3 kHz. However, the

law of diminishing returns In economics, diminishing returns are the decrease in Marginalism, marginal (incremental) output of a production (economics), production process as the amount of a single Factors of production, factor of production is incrementally increased, ho ...

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 AM broadcasting is radio broadcasting using amplitude modulation (AM) transmissions. It was the first method developed for making audio radio transmissions, and is still used worldwide, primarily for medium wave (also known as "AM band") transmis ...

. A fourth line would push it up 33% to 12 kHz. FM radio 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 codec A POTS codec is a type of audio coder-decoder (codec) that uses digital signal processing to transmit audio digitally over standard telephone lines ("Plain Old Telephone Service") at a higher level of audio quality than the telephone line would n ...

See also