Question

iam confused with overmodulation in AM...please explain.

Answer 1

how do phase reversals occur as a result of overmodulation?

Answer 2

In the case of amplitude modulation, the modulator receives a constant-amplitude carrier (high) frequency and an audio signal. The audio signal is adjusted to a peak-to-peak amplitude twice that of the carrier, and DC biased so the largest negative peaks just reach 0 V. Let's call these levels 0, 1 and 2. The modulator then "multiplies" the carrier by this biased audio, and thus the modulated carrier peak-to-peak output level ranges from 0 to 2, and sits at 1 when the audio is absent. This is 100% modulation.
In typical real AM transmitters, if the audio signal goes below zero (overmodulation), the carrier is cut off. Since this is an abrupt switchoff, it creates undesired harmonics, or "splatter" which may intrude into the frequency allocated to another station, and also produces noise in a properly tuned receiver, but you don't get inverted audio since there's no carrier.
However in a laboratory setting or in theoretical amplitude modulation, the audio signal below zero multiplies the carrier by a negative quantity which produces inversion of both the carrier and the modulating signal. Thus if the audio were a sinewave of amplitude +/-1.1, reaching down to -0.1, a receiver would receive a normal sinewave down to -1 and then a positive image of the remaining 0.1 part. This would look like a scoop taken out of the peak as in a volcano crater.
Note that the receiver doesn't particularly "notice" the carrier phase inversion, just that of the audio, so unless you know it's a sinewave and that overmodulation is occurring, you'd merely think you were seeing a more complex waveform that happened to have a double peak on the negative side.