Tutorials and Topics - Peabody Computer Music

Tutorial 9: Synthesis—Amplitude modulationRing modulation and amplitude modulationAmplitude modulation (AM) involves changing the amplitude of a “carrier” signal usingthe output of another “modulator” signal. In the specific AM case of ring modulation(discussed in Tutorial 8) the two signals are simply multiplied. In the more general case,the modulator is used to alter the carrier’s amplitude, but is not the sole determinant of it.To put it another way, the modulator can cause fluctuation of amplitude around somevalue other than 0. The example below illustrates the difference between ring modulationand more common amplitude modulation.1100-1-1Ring modulationAmplitude modulationThe example on the left is 1 /4 second of a 100 Hz cosine multiplied by a 4 Hz cosine; theamplitude of both cosines is 1. In the example on the right, the 4 Hz cosine has anamplitude of 0.25, which is used to vary the amplitude of the 100 Hz tone ±0.25 around0.75 (going as low as 0.5 and as high as 1.0). The two main differences are a) the AMexample never goes all the way to 0, whereas the ring modulation example does, and b)the ring modulation is perceived as two amplitude dips per modulation period (thuscreating a tremolo effect at twice the rate of the modulation) whereas the AM is perceivedas a single cosine fluctuation per modulation period.108