Korg MS-50 - ring modulator

(12) RING MODULATOR

[ringmod module]
- ring modulator

	SIGNAL A / B INPUT (A / B IN) For the two input signals that the ring modulator will multiply the voltage of to produce an A x B output. For example, when input voltage A is -2V, and input voltage B is +3V, the output voltage will be (-2) x (+3) x 1/3 = -2V.
	RM OUTPUT (RM OUT) This jack provides the output voltage corresponding to A x B. When input signals A and B are audio frequencies (20 Hz ~ 15 kHz), the output signal will consist of all the sum and difference frequencies that can be generated from the harmonic overtones present in the inputs. In this case, the fundamental frequencies are cancelled to produce a tone color that has no clear pitch.

The ring modulator is most commonly used to change the harmonic composition of an audio signal. Specifically, it generates a signal made up entirely of the integral (whole number) harmonics. With ring modulators having a built-in oscillator, the output signal has a metallic timbre similar to bells or chimes.

The basic principle is the same as amplitude modulation (AM) by which the VCA produces a tremolo effect. For your reference, the figure below shows the difference between tremolo and ring modulator envelopes.

[rm envelopes]
- tremolo and ring modulator envelopes

While the figure above only shows the envelopes, an oscilloscope will also reveal the modulated signals which would be located between the upper and lower lines of the envelopes drawn here. For the ring modulator, you would be able to see that the modulated signal's phase or polarity is inverted during the time when the dotted line is above the solid line in the figure.

You can also analyse the ring modulator's input and output relationships from a mathematical point of view. If input signal A is a sin at, and input signal B is b sin bt, then A x B = a sin at x b sin bt = ab/2 [cos (a + b) t = cos (a - b) t], so that we obtain an output signal having a new harmonic frequency composition consisting of the sum and difference frequencies of the harmonics in our A and B input signals.

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