Modulation of Signals
Modulation is defined as the embedding of an information-bearing signal into a second signal. The process of recovering the information is known as demodulation. Both processes are used in many of the devices commonly used today, including radios, phones, and televisions to name just a few.
A modulated signal y(t) can be seen as the product of two other signals, the information-bearing (aka Modulating) signal x(t) and the carrier signal c(t). In general, c(t) is a complex exponential or sinusoidal function that has its amplitude multiplied by x(t).
$ y(t) = x(t) c(t) \! $
c(t) is usually equal to:
$ e ^{j(\omega_c t + \theta_c)} \! $ or $ cos(\omega_c t + \theta_c)\! $
The value $ \omega_c $ is known as the carrier frequency. In order for the information to be successfully demodulated this value must always be less than the maximum frequency of the modulating signal $ \omega_m $
