1.22 Analog Modulation
A message signal 
is usually an LP signal. A signal whose spectrum is in the vicinity 
is usually called a baseband signal. A baseband signal is usually voice, video, or digital data signal. Modulation converts a message signal 
from an LP spectrum to a high-pass (HP) spectrum (usually to a BP spectrum) around a carrier frequency 
. In general, a modulated signal can be described as
where the amplitude 
, the frequency 
, and the phase 
are modulated. Using the relationship 
, any narrow band signal can be presented as simultaneous AM and PM waveform
where
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