Amplitud Modulation

I found this document in AM (Amplitud Modulation) ( http://arcarc.xmission.com/PDF_Electronics/Amplitude%20Modulation.pdf ) where explains how it was created and how it really started. It help me to understand that AM was the first method used to modulate a frequency, and that the basic principle of AM is to take voice frequencies, and mix (or modulate) them with a radio frequency signal so that they are converted to radio frequencies which will radiate or propagate through free space.