The condition for achieving distortion-less demodulation of an amplitude-modulated signal using an envelope detector is

Overmodulation results in sharp edges or bends in the waveform of the envelope of the transmitted signal, regardless of the audio frequency. It results in false emissions by the modulated carrier and distortion of the recovered modulating signal.

For μ > 1, the AM wave is said to be over-modulated where μ is the modulation index.

Important Points

• For μ < 1, the AM wave is said to be under-modulated and distortion-less demodulation is achieved using envelop-detector.
• For μ = 1, the AM wave is said to be critically modulated.
• 100% modulation is where the modulating signal drives the carrier to zero and is theoretically the maximum modulation that can be successfully demodulator by a regular AM envelope detector.