Signal modulation

Modulation involves the modification of some property of a (usually sinusoidal) carrier waveform. The process of recovering the original signal from the modulated signal is called 'demodulation'. The most common modulation methods are:
• amplitude modulation
• frequency modulation
• pulse code modulation.
Modulation of a high frequency carrier by a lower frequency signal allows:

• convenient transmission by high frequency radio waves • shifting of the frequency range occupied by a signal so that multiple signals can be packed into the available bandwidth of the communication link and transmitted simultaneously.