The modulation index should be a number between 0 and 1. If the amplitude of the modulating voltage is higher than the carrier voltage, m will be greater than 1, causing distortion of the modulated waveform.
If the distortion is great enough, the intelligence signal becomes unintelligible. Distortion of voice transmissions produces garbled, harsh, or unnatural sounds in the speaker. Distortion of video signals produces a scrambled and inaccurate picture on a TV screen.
Simple distortion is illustrated in Fig. 3-4. Here a sine wave information signal is modulating a sine wave carrier, but the modulating voltage is much greater than the carrier voltage, resulting in a condition called overmodulation. As you can see, the waveform is flattened at the zero lines. The received signal will produce an output waveform in the shape of the envelope, which in this case is a sine wave whose negative peaks have been clipped off. If the amplitude of the modulating signal is less than the carrier amplitude, no distortion will occur. The ideal condition for AM is when Vm 5 Vc, or m 5 1, which gives 100 percent modulation. This results in the greatest output power at the transmitter and the greatest output voltage at the receiver, with no distortion.
Preventing overmodulation is tricky. For example, at different times during voice transmission voices will go from low amplitude to high amplitude. Normally, the amplitude of the modulating signal is adjusted so that only the voice peaks produce 100 percent modulation. This prevents overmodulation and distortion. Automatic circuits called compression circuits to solve this problem by amplifying the lower-level signals and suppressing or compressing the higher-level signals. The result is a higher average power output level without overmodulation
Distortion caused by overmodulation also produces adjacent channel interference. Distortion produces a nonsinusoidal information signal. According to Fourier theory, any nonsinusoidal signal can be treated as a fundamental sine wave at the frequency of the information signal plus harmonics. Obviously, these harmonics also modulate the carrier and can cause interference with other signals on channels adjacent to the carrier.
Amplitude Modulation Concepts (Overmodulation)
The Relationship Between Rise Time and Bandwidth (Overmodulation)