Frequency extender
In broadcast engineering, a frequency extender is an electronic device that allows high-fidelity analogue audio to be sent over regular POTS telephone lines, without the loss of higher audio frequencies (treble). It is an extended concept of a telephone hybrid.
The concept 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 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 codecs.