What is Overmodulation?
In the image below a amplitude modulated sine wave:


0% unmodulated, the sine envelope is not visible at all;
&lt; 100% modulation depth is normal AM use;
100% modulation depth, the sine envelope touch at y=0. Maximum modulation that can be retrieved with an envelope detector without distortion;
&gt; 100% modulation depth, "overmodulation", the original sine wave can no longer be detected with an envelope detector.




A still overview of the most important modulation depths (0, 50, 100 and 200%):



The animation was created using gnuplot using the following script:

unset xtics
set yrange [-3:3]
set samples 10000
do for [d=0:200] { plot sin(2*pi*3*x)*(1+(sin(2*pi*x/10))*d/100) title sprintf("%3i%%",d); }