Analog telephony


The original application of the ring modulator was in the field of analog telephony for frequency-division multiplexing for carrying multiple voice signals over telephone cables.Frank A. Cowan invented it back in 1934 and patented in 1935 as an improvement on the invention of Clyde R. Keith at Bell Labs. It has since applied to a wider range of uses such as voice inversions, radio transceivers and as an effect in electronic music.


The History behind Ring modulator


Ring modulator is quite an old effect. However, it was not until diodes were used in the circuitry that ring modulators were found to be useful in sound synthesis. One of the first synthesizers to use ring modulator effect was the Melo-chord build in 1947 by Harald Bode. Due to the success of Bode`s ring modulator, he went on to design a designated, stand-alone ring modulation. The Bode`s ring modulator was a later built in module format for inclusion in Moog modular systems. Buchla, Oberheim, EMS and Yamaha all followed and began including ring modulators into their synthesizers. Today, you can find a ring modulator in virtually any synthesizer. Ring modulators have become a synthesis staple. For that reason, they are abundant in a numerous synthesizers, both analog and digital based.


Metalic Sound


The ring modulator effect sounds ear-opening, metallic or robotic. The overtones produced by the ring modulator tend to be inharmonic. Furthermore, the effect can be useful for sounding “out” or for playing with music that has no obvious tone centre. In a ring modulator, the multiplication of a carrier signal and an input signal (guitar, for example) occurs. Consequently, the character of the effect depends on the frequency of the carrier signal. If the frequency is very low (below the range of human hearing), instead of overtones, you will hear a tremolo effect.


Mixing Waveforms


Ring modulators frequency mix or heterodyne two waveforms, and output the sum and difference of the frequencies present in each waveform. This process of ring modulation produces a signal rich in partials. Additionally, neither the carrier nor the incoming signal are prominent in the outputs, and ideally, not at all. Two oscillators, whose frequencies were harmonically related and ring modulated against each other, produce sounds that still adhere to the harmonic partials of the notes, but contain a very different spectral makeup.


Inharmonious sounds


When the oscillators’ frequencies are not harmonically related, ring modulation creates inharmonics. This produces bell-like or otherwise metallic sounds. The ring modulator includes an input stage, a ring of four diodes excited by a carrier signal, and an output stage. The input and output stages typically include transformers with centre-taps towards the diode ring. It is important to note that while the diode ring has some similarities to a bridge rectifier the diodes in a ring modulator all point in the same clockwise or counter-clockwise direction.


Bidirectional flow


A particular elegance of the ring modulator is that it is bidirectional. The signal flow can be reversed allowing the same circuit with the same carrier to be used either as a modulator or demodulator, for example in low-cost radio transceivers.


Additional resources & source texts