Frequency extender
Encyclopedia
In broadcast engineering
, a frequency extender is an electronic
device
that allows high-fidelity analogue audio to be sent over regular POTS
telephone line
s, without the loss of higher audio frequencies (treble
). It is an extended concept
of a telephone hybrid
.
The concept
uses frequency shifting, in order to overcome the narrow
bandwidth of regular telephone systems. The input signal is sent to one telephone line as-is, or in some cases upshifted to provide extra low-frequency response, and sent to 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
of range, and pushing the total bandpass to 6 kHz. The sound is then acceptable for voice
, if not for music
.
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 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
s.
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...
, a frequency extender is an electronic
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
device
Information appliance
In general terms, an information appliance or information device is any machine or device that is usable for the purposes of computing, telecommunicating, reproducing, and presenting encoded information in myriad forms and applications....
that allows high-fidelity analogue audio to be sent over regular POTS
Plain old telephone service
Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....
telephone line
Telephone line
A telephone line or telephone circuit is a single-user circuit on a telephone communication system...
s, without the loss of higher audio frequencies (treble
Treble (sound)
Treble refers to tones of high frequency or range. In music this corresponds to high notes, and for this reason the treble clef is often used for instruments with higher pitch. Examples of treble sounds are guitar tones, female voice , young boy voice, etc. They have frequencies above 9 KHz. Treble...
). It is an extended concept
Concept
The word concept is used in ordinary language as well as in almost all academic disciplines. Particularly in philosophy, psychology and cognitive sciences the term is much used and much discussed. WordNet defines concept: "conception, construct ". However, the meaning of the term concept is much...
of a telephone hybrid
Telephone hybrid
Telephone hybrids are an essential functional component of the Public Switched Telephone Network . The term also describes the piece of equipment used in broadcast facilities to enable the airing of telephone callers....
.
The concept
Concept
The word concept is used in ordinary language as well as in almost all academic disciplines. Particularly in philosophy, psychology and cognitive sciences the term is much used and much discussed. WordNet defines concept: "conception, construct ". However, the meaning of the term concept is much...
uses frequency shifting, in order to overcome the narrow
Narrowband
In radio, narrowband describes a channel in which the bandwidth of the message does not significantly exceed the channel's coherence bandwidth. It is a common misconception that narrowband refers to a channel which occupies only a "small" amount of space on the radio spectrum.The opposite of...
bandwidth of regular telephone systems. The input signal is sent to one telephone line as-is, or in some cases upshifted to provide extra low-frequency response, and sent to 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
Audio mixing (recorded music)
In audio recording, audio mixing is the process by which multiple recorded sounds are combined into one or more channels, most commonly two-channel stereo. In the process, the source signals' level, frequency content, dynamics, and panoramic position are manipulated and effects such as reverb may...
it with the first. This results in greatly improved audio, adding a full octave
Octave
In music, an octave is the interval between one musical pitch and another with half or double its frequency. The octave relationship is a natural phenomenon that has been referred to as the "basic miracle of music", the use of which is "common in most musical systems"...
of range, and pushing the total bandpass to 6 kHz. The sound is then acceptable for voice
Human voice
The human voice consists of sound made by a human being using the vocal folds for talking, singing, laughing, crying, screaming, etc. Its frequency ranges from about 60 to 7000 Hz. The human voice is specifically that part of human sound production in which the vocal folds are the primary...
, if not for music
Music
Music is an art form whose medium is sound and silence. Its common elements are pitch , rhythm , dynamics, and the sonic qualities of timbre and texture...
.
It is also possible to add other lines, each increasing the bandpass by another 3 kHz. However, the law of diminishing returns
Diminishing returns
In economics, diminishing returns is the decrease in the marginal output of a production process as the amount of a single factor of production is increased, while the amounts of all other factors of production stay constant.The law of diminishing returns In economics, diminishing returns (also...
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 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
POTS codec
A POTS codec is a device used in broadcast engineering to send high-fidelity digital audio over regular telephone lines. A hardware codec, implemented with digital signal processing, is used to compress the audio data enough to travel at a bitrate equivalent to a 33.6k modem...
s.