Amplitude modulation:
When the amplitude of high
frequency carrier wave is changed in
accordance with the intensity of the signal ,it is called amplitude modulation.
Advantages of AM:
The amplitude modulating system is relatively cheap to build which is
the reason that AM radio broadcasting has been popular for so long and is quite
likely to remain so well in future.
Major Limitations oF AM:
(It is actually for the general AM modulation or double-
sideband amplitude modulation)
1. Reception is generally noisy.
2. The
efficiency is very low.
3. Due to
low efficiency message can’t transmitted long distances.
4. Am is
wasteful of power – The carrier wave is completely independent of message wave.
So the transmission of the carrier wave therefore represents a waste of power.
5.Am is
wasteful of bandwidth. The upper and lower sidebands of an AM wave are uniquely
related to each other by the carrier frequency. It requires a transmission bandwidth
equal to twice the message bandwidth.
Some of
these limitations can be overcome with following methods as like below:
Double-sideband suppressed carrier(DSB-SC) modulation:
It is the process in which the transmitted wave consist only the
upper and lower sidebands. Transmitted power is saved here through the
suppression of the carrier wave but the channel bandwidth requirements is the
same as before(DSB).
Single sideband (SSB) modulation:
It is the process in which the modulated wave
consists of only the upper-sideband or the lower sideband. The essential
function of SSB modulation is therefore to translate the spectrum of the
modulating signal to a new location in the frequency domain. SSB modulation is
particularly suited for the transmission of voice signals by virtue of the
energy gap that exists in form of modulation in that it requires the minimum
transmitted power and minimum channel bandwidth. Its principal disadvantages is
increased cost and complexity. Demodulation of SSB is identical of DSB. SSB-SC will be called when the SSB signals without
an additional carrier.
Vestigial-sideband modulation:
It is the process in which one side-band is passed almost
completely and just a trace or vestige of the other sideband is retained. The required
channel bandwidth is therefore in excess of the message bandwidth by an amount
equal to the width of the vestigial side-band. This form of modulation is well
suited for the transmission of wide-band signals such as television signals. It
is possible to demodulate by an envelope detector ( a simple demodulator ) and
thereby simplify the receiver design. It inherits the advantages of DSB and SSB
but avoid their disadvantages at a small cost. VSB signals are relatively easy
to generate and at the same time their bandwidth is only (typically 25%)
greater than that of SSB signals.
