Amplitude Modulation


Audio: amplitude modulation (0:04)

Amplitude modulation
Figure: amplitude modulation

amplitude modulation plays simple Amplitude Modulation (AM). A carrier with frequency 500Hz and amplitude 0.5 is amplitude modulated by a modulator with frequency 200Hz. The amplitude modulation figure shows the change in the waveform as the AM index rises from 0% to 100%.

Amplitude Modulation (AM) uses a modulator to change the amplitude of a carrier.

Amplitude Modulation and Ring Modulation are almost identical. Both techniques produce a pair of sidebands. The only difference between them, and it is a significant one, is that in AM the carrier frequency remains, whereas in RM it disappears.

The amplitude modulation index, or AM index, is the most important factor in AM. It determines the amount, or depth, of modulation.

The AM index is the ratio of the modulator amplitude to the carrier amplitude. It has a value in the range 0-100%. An index of zero means no AM, all that is heard is the carrier. A value of 1 results in 100% Amplitude Modulation. Technically it is possible to overmodulate AM by using a value greater than 100% but the result will be distortion, unwanted noise.

In amplitude modulation the carrier has a frequency of 500Hz and an amplitude of 0.5 and the modulator has a frequency of 200Hz. The modulation index starts at 0% and gradually rises to 100%. At 0%, there is no modulation, and all that is heard is the carrier. When the index reaches 100%, the output is full on AM. The output is a sound containing three frequencies: the carrier frequency, 500Hz, the first sideband, 300Hz (500-200=300), and the second sideband, 700Hz, (500+200=700). The amplitude of the output is 1 and consists of the carrier amplitude, 0.5, the first sideband, 0.25, and the second sideband, 0.25. It is the AM index that determines the amplitude of the modulator which, in turn, determines the amplitude of the sidebands.

Simple AM and simple RM are powerful synthesis techniques. In neither case, though, is the output harmonically rich. Both simple AM and RM require two frequencies as input, RM outputs a sound containing two frequencies, and AM outputs a sound with three frequencies.

The impact of RM is arguably more pronounced than that of AM because the carrier frequency remains in AM, whereas it disappears in RM. RM is a useful sound effect that produces a distinctive hollow and metallic sound. AM is a rather subtle effect and is as useful for writing melody and harmony as it is for writing sound effects. Applying AM to a melody results in instantaneous harmony because of the additional sidebands. Applying AM to a harmony results in a complex and rich timbre because each frequency in the carrier generates a pair of sidebands in the output.