The Ring Modulator was invented by Frank A. Cowan in 1934. Its initial purpose was to experiment with carrying multiple signals on telephone cables. In the realm of music, it dates back to 1947 as a sound processing module. The device was originally developed by Harald Bode, who is also known for many other important circuits. While Bode did not invent ring modulator technology, he was among the first to apply it in musical systems. Early versions featured inputs with two audio transformers and were constructed using a ring configuration of four diodes for the ring modulator. Bob Moog was inspired by Bode's ideas and later collaborated with him, even employing Bode at Moog Music for a period. Together, they developed transformer-based ring modulator designs for the Moog Modular systems. Moog revisited this design in later years, producing various versions in different formats. Today, ring modulators are found in many synthesizers and guitar pedals, and they can be easily programmed using MAX/MSP or PD. There are also numerous well-functioning versions available in VST plug-in format. They are commonly used within synthesizer systems and are preferred for processing acoustic sounds as well.
The circuit of a ring modulator actually responds somewhat like a VCA (Voltage Controlled Amplifier). Models with transformers can read frequencies depending on the transformer's bandwidth and do not pass DC signals, which is inherent to their natural structure. Additionally, in general ring modulator applications, low frequencies are filtered out. Later transistor-based models behave more like VCAs. The main difference from VCAs is that their CV inputs act like rectifiers. This means that when the signal moves towards the positive direction, the volume increases normally. When the signal starts moving towards negative, the VCA will still open towards the positive direction due to the incoming CV signal, effectively behaving like an inverter.
In fact, when considering pedals in pedal format, ring modulators are somewhat like tremolo pedals. In a tremolo pedal, there is a signal fed into the VCA's input, and an LFO modulates the amplitude of this signal. If this LFO reaches audio rate frequencies, it results in standard AM modulation in a regular VCA. If the CV input acts like a rectifier and opens the VCA towards the positive direction when the CV goes negative, then it becomes a ring modulator. Therefore, ring modulator pedals can also be used similarly to tremolo pedals.
Fundamentally, the signal entering the Input of the ring modulator is referred to as the 'Carrier', while the signal entering as CV is termed the 'Modulator'. Starting with two sinusoidal waves or two triangle waves is generally more preferred. However, synthesis can be done with all waveforms. For example, if we input a simple 1500Hz sine wave and a 400Hz sine wave into this module, we will hear outputs at 1900Hz and 100Hz. In more complex waveforms or rich harmonic waveforms like sawtooth waves, we also need to consider their fundamental, second, and third-degree harmonics. This complexity enhances the ring modulator's capability. Synthesizing with sine waves and triangle waves certainly offers a much broader spectrum.
During the integrated circuit era, significant advancements were made in these module designs. The SID chip within the Commodore 64 enables ring modulation of triangle waves. In the Arp Odyssey, you will find a different method applied where the outputs of two oscillators' Square Wave outputs are utilized and processed through an XOR function. This method is functionally equivalent to a real ring modulator.
Ring modulators can create major chords and assist in achieving polyphony in some of your patches. Additionally, this module holds a significant place in the world of sound design. One prominent example is the sound of the Dalek character in the British television series 'Doctor Who', which was entirely created using this circuit.
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