Moog Music MF-102 Ring Modulator - Manual (Page 6)

Now we're ready to talk about what a ring modulator is. In electronics, modulation is
any process in which one waveform is changed in response to the contour of another
waveform. For instance, amplitude modulation is the shaping of the amplitude
(strength) of one waveform by the contour of another waveform. The "AM" in "AM Radio"
stands for amplitude modulation. An AM Radio signal coming from a transmitting tower
consists of a very high frequency, called the carrier, which has been amplitude-
modulated by an audio signal, called the program.
Any analog signal can amplitude-modulate any other analog signal. A common
example of amplitude modulation in electronic music is tremelo. Tremelo is the
amplitude modulation of an instrument signal by a sine wave of a few Hz or so.
Ring modulation is a special type of amplitude modulation. A ring modulator circuit
has two inputs and one output. If a sine wave of one frequency is applied to the first
input and a sine wave of another frequency is applied to the second input, neither of
these input frequencies appears at the output. Instead, two new frequencies appear.
One is equal to the sum of the two input frequencies, and the other is equal to the
difference between the two frequencies. For instance, if the sine wave at the first input
is 500 Hz and the sine wave at the second input is 100 Hz, then the output contains a
600 Hz sine wave and a 400 Hz sine wave. The resultant output waveform is complex. It
looks like a 500 Hz sine wave that is amplitude modulated around its center axis by a
100 Hz sine wave (See Figure 6). When you listen to it, you hear two pitches: 400 Hz and
600 Hz. You do not hear either the 500 Hz or the 100 Hz input signals.
Figure 6 - Waveform and spectrum of the result of ring-modulating a 500 Hz sine wave by a 100 Hz sine wave.