Editing Ring modulation (section)

=== Analogue telephone systems ===
An early application of the ring modulator was for combining multiple analog telephone voice channels into a single wideband signal to be carried on a single cable using [[frequency-division multiplexing]]. A ring modulator in combination with [[carrier wave]] and filter was used to assign channels to different frequencies.

Early attempts at [[Privacy|securing]] analog telephone channels used ring modulators to modify the spectrum of the audio speech signals. One application is spectral inversion, typically of speech; a carrier frequency is chosen to be above the highest speech frequencies (which are low-pass filtered at, say, 3&nbsp;kHz, for a carrier of perhaps 3.3&nbsp;kHz), and the sum frequencies from the modulator are removed by more low-pass filtering. The remaining difference frequencies have an inverted spectrum: high frequencies become low, and vice versa.