Ring modulation

Lore & Background

The ring modulator was invented by Frank A. Cowan in 1934 and patented in 1935 as an improvement on the invention of Clyde R. Keith at Bell Labs. Its original application was in analog telephony for frequency-division multiplexing to carry multiple voice signals over telephone cables. The name comes from the original implementation where the analog circuit of diodes takes the shape of a ring, similar to a bridge rectifier but with all four diodes polarized in the same direction.

Ring modulation approximates multiplication of the carrier and modulator signals. For two sine waves, the output is the sum of two sine waves at the sum and difference frequencies, a consequence of the trigonometric identity sin(u) sin(v) = 1/2[cos(u−v) − cos(u+v)]. If the carrier is a square wave with odd harmonics, each harmonic generates its own pair of sidebands, producing a signal rich in partials. When the oscillator frequencies are harmonically related, the output adheres to harmonic partials but with a different spectral makeup; when not harmonically related, ring modulation creates inharmonics, often producing bell-like or metallic sounds.

The ring modulator is a double-balanced mixer, suppressing both input signals in the output. The circuit includes an input stage, a ring of four diodes excited by a carrier signal, and an output stage, typically with center-tapped transformers. The carrier alternates current to make one pair of diodes conduct while reverse-biasing the other, effectively acting like a reversing switch. The ring modulator is bidirectional, allowing signal flow to be reversed with the same carrier.

Reader's Guide

Ring modulation has been applied to a wider range of uses beyond its original telephony application, including voice inversion, radio transceivers, and electronic music. Its significance lies in its ability to produce an output composed entirely of the sum and difference of the frequency components of the two inputs, suppressing the original signals. This creates a sound rich in partials, with harmonic or inharmonic content depending on the relationship between the input frequencies. In electronic music, ring modulation is notable for producing bell-like or metallic sounds when the frequencies are not harmonically related. The circuit's design, using a ring of four diodes and center-tapped transformers, allows it to function as a double-balanced mixer, and later implementations used FETs as switching elements. Its bidirectional nature adds versatility, enabling the same circuit to be used with reversed signal flow.