Figure 1.

Amplitude and Frequency Modulation Illustration. Three sinusoidal waveforms are shown. The upper, slow, sine wave (black) is utilized to modulate or impress itself upon the lower two, faster “carrier” sine waves. When the slow modulating signal is impressed by means of alteration of carrier amplitude, the result is as shown in the middle red waveform – Amplitude Modulation or AM. If the AM modulated carrier were to be an audible signal such AM would cause it to wax and wane in amplitude while the carrier frequency would remain unchanged. In contrast if the modulating signal were to be impressed upon the carrier by means of alteration of carrier frequency, the result would be as shown in the lower blue waveform – Frequency Modulation or FM. If the carrier were to be an audible signal, such FM would cause it to rhythmically change frequency (or warble) but the carrier loudness or amplitude would remain unchanged. Human language appears to involve rapid changes in FM content of auditory signals especially important in the production and detection of phonemes.

Duffy et al. BMC Neurology 2013 13:12   doi:10.1186/1471-2377-13-12
Download authors' original image