Ring modulation is a form of amplitude modulation in which a special circuit suppresses the carrier signal and recreates only the sidebands. Two additional frequencies are created in place of the original carrier signal. One is equal to the sum of the two input frequencies and the other is equal to the difference between them. If the input signal has many harmonics, such as a guitar or a human voice, the output signal is complex and rich, is a kind of "ghost" of the original sound. The analog ring modulator, still produced by Robert Moog, has a second input signal in the form of an oscillator. It can be adjusted to narrow or widen the distance between the two frequencies generated by the effect.