Amplitude modulation and frequency modulation differ only by sideband phase at low indices

Evidence

Three-component complexes can be changed from representing amplitude-modulated tones to quasi-frequency modulated tones by changing one component by 1800 • When the amplitude of a tone is modulated sinusoidally, the result is the original tone (the carrier) and a sideband equally spaced in frequency to either side. The spacing between carrier and sidebands corresponds to the rate of modulation (modulation frequency). When the frequency of a tone is modulated sinusoidally, the same thing happens. For a small modulation index, i.e. the ratio between the frequency deviation and the modulation frequency, a carrier and two sidebands are again produced. Significant sidebands beyond the first pair are produced only for modulation indices larger than 0.3. The difference between AM and FM with modulation indices less than 0.3 is due to phase: relative to the phases of the components produced by AM, one of the sidebands is 1800 out of phase with FM. In other words, in first approximation, AM becomes FM if the phase of one of the sidebands is reversed. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 900

Categorization

• DEFINE: Explains the technical distinction between AM and FM in terms of sideband phase, central to the psychoacoustic phase-detection method.
• XREF: Connects to signal theory / communications-engineering knowledge of AM-FM equivalence.