Amplitude Modulation (AM) is the name given the to the technique of varying the amplitude or loudness of one signal known as the carrier (typically an audio signal, swinging both above and below 0 volts) with a second signal called the modulator. In the typical amplitude modulation (AM) scenario, a low frequency oscillator with a positive voltage (say, between 0v and 5v, or maybe something smaller such as between 1v and 2v) is fed into the control input of a voltage controlled amplifier to add vibrato to an audio signal passing through it. Technically, this is known as a two-quadrant multiplier or modulator, as any negative swings in the modulation signal are ignored; when patching tremolo, you may need to make sure an offset voltage is being added to your LFO to make sure the sound doesn’t cut out on the lower excursions of the LFO’s waveform.
A special case of amplitude modulation is ring or balanced modulation, (or four-quadrant modulation for the really geeky), where both the carrier and modulator are bipolar, swinging both above and below 0v. The result is a more complex set of component tones that don’t follow typical “musical” spacing based on octaves above the fundamental that harmonics usually follow. Namely, the modulation frequency is both added to and subtracted from the carrier’s frequency; the resulting harmonics replace the original carrier and modulator. Say the carrier was a sine wave (only the fundamental harmonic present) at 600Hz, and the modulator was a sine wave at 100Hz. The result would be a tone that had frequency components at 500 and 700Hz.
Yes, you can do audio rate amplitude modulation. The result is slightly different: you still get the new sum and difference harmonics, but they are at half the strength of ring modulation. However, the carrier is also present, at full strength. Click here for the full technical explanation.
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