A signal flow diagram describing amplitude modulation (AM) with sinusoidal oscillators is shown in Fig. 1.
Figure 1:
A signal flow diagram for amplitude modulation.
The carrier oscillator has frequency fc and the modulating oscillator frequency is specified by fm.
A modulation index, m, is used to control the modulating oscillator amplitude in proportion to the unmodulated carrier oscillator amplitude A.
That is, when m = 0, no modulation takes place. When m = 1, the amplitude of the modulating oscillator is equal to the unmodulated carrier amplitude, which represents 100% modulation.
Using sinusoidal waveforms for both the carrier and modulator, the resulting AM spectrum (with m > 0) contains three frequencies as shown in Fig. 2: the carrier frequency (fc) and two sidebands at .
Figure 2:
The resulting AM spectrum using sinusoidal carrier and modulating waveforms.
The modulation index controls the amplitude of each of the sideband frequencies by a factor m/2, but does not affect the carrier frequency amplitude.
The perception of AM is dependent on the modulation frequency (fm):
For fm less than about 10 Hz, the amplitude envelope variations are clearly heard.
For fm > 10 Hz but small enough that the sidebands fall within the same critical band, the loudness of the resulting sound is proportional to the average amplitude of the modulating waveform.
For an fm that exceeds one-half the critical bandwidth, the sidebands are perceived individually.
.
