The transmission of information
through short distances does not require complicated techniques. The energy of
the information signal is sufficient enough to be sent directly. However if the
information, for example, audio frequency (20 to 20,000 Hz) needs to be
transmitted to long distances across the world, certain techniques are required
to transmit the information without any loss.
For long distance transmission, the low frequency baseband
signal (input signal) is superimposed onto a high frequency radio signal by a
process called modulation. In the modulation process, a
very high frequency signal called carrier signal (radio signal) is used to
carry the baseband signal.
As the frequency of the carrier
signal is very high, it can be transmitted to long distances with less
attenuation. The carrier signal is usually a sine wave signal. Also, the
carrier signal will be more compatible with the communication medium like free
space and can propagate with greater efficiency.
Carrier signal does not have
A sinusoidal carrier wave can be
represented as ec = Ec sin (2πνct + ϕ), where Ec is
the amplitude, νc is the
frequency and ϕ is the initial phase of the carrier wave at any instant of time
Three characteristics in the carrier
signal can be modified by the baseband signal during the process of modulation:
amplitude, frequency and phase of the carrier signal.
There are 3 types of modulation based on which parameter is modified.
(i) amplitude modulation,
(ii) frequency modulation and
(iii) phase modulation.