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http://encyclopedia2.thefreedictionary.com/Amplitude+Modulation Amplitude modulation Varying   the   voltage   of   a   carrier   or   a   direct   current   in   order   to   transmit   analog   or   digital   data.   Amplitude   modulation   (AM)   is   the   oldest   method of   transmitting   human   voice   electronically.   In   an   analog   telephone   conversation,   the   voice   waves   on   both   sides   are   modulating   the   voltage of   the   direct   current   loop   connected   to   them   by   the   telephone   company. Modulate   a   Carrier Amplitude   modulation   (AM)   is   also   widely   used   to   alter   a   carrier   wave   to   transmit   data.   For   example,   in   AM   radio,   the   voltage   (amplitude)   of a   carrier   with   a   fixed   center   frequency   (the   station's   channel)   is   varied   (modulated)   by   the   analog   audio   signal. AM   is   also   used   for   digital   data.   In   quadrature   amplitude   modulatio