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, 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 3kHz, which is normally the upper bandpass limit in telephony. Thus, an audio frequency of 5kHz is sent at 2kHz. 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 6kHz. 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 3kHz. 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 9kHz, equivalent to AM radio. A fourth line would push it up 33% to 12kHz. FM radio quality would require five telephone lines to be installed, pushing the bandpass up 25% to 15kHz. The audio is shifted down by 6,9, and 12kHz respectively for each additional line.

See also: telephone hybrid, remote broadcast