modulation - What is Overmodulation?

The Wikipedia article is very short and doesn't explain the concept very well, and there aren't any other sites that I can find that give a simple explanation. What does it mean by 100% modulation? I understand the basic concepts behind amplitude modulation, frequency modulation, and pulse width modulation, but I have never really understood what is meant by the "amount" of modulation or modulation depth.

Can somebody please shed some light on the subject?

Thanks!

Answer

In the image below a amplitude modulated sine wave:

• 0% unmodulated, the sine envelope is not visible at all;


• < 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;


• > 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); }