ASK: ASK stands for
Amplitude shift keying. It is a type of encoding technique where amplitude of a
frequency is used to convert the digital data into analog signal.
Equation:
In ASK two binary values
are represented by two different amplitude of carrier frequency. Here one
amplitude is zero and other is one which represents absence and presence of
amplitude.
Advantages of
ASK:
(i) It offers high
bandwidth efficiency.
(ii) It has simple
receiver design.
(iii) ASK modulation can
be used to transmit digital data over optical fiber.
(iv) ASK modulation and
ASK demodulation processes are comparatively inexpensive.
(v) Its variant 00K is
used at radio frequencies to transmit Morse codes.
Disadvantage
of ASK:
(i) It offers lower power
efficiency.
(ii) ASK modulation is
very susceptible to noise interference. This is due to the fact that noise
affects the amplitude.
Application: Optical fiber.
FSK: FSK stands for frequency shift keying. It is a
type of encoding technique where frequency is used to convert the digital data
into analog signal. The following image is the diagrammatic representation of
FSK modulated waveform along with its input.
Equation:
Where 
and 
are typically offset from the
carrier frequency 
by equal but opposite amounts.
and are typically offset from the
carrier frequency by equal but opposite amounts.
Advantage:
(i) FSK is less
susceptible to errors than ASK.
(ii) It provides high SNR
(Signal to Noise Ratio)
Disadvantage:
(i) FSK spectrum is 2 x ASK spectrum.
(ii) It uses larger bandwidth compare to other
modulation techniques such as ASK and PSK.
Application:
Over voice lines in high-frequency radio transmission etc.
PSK: PSK stands for
Phase Shift Keying. It is a digital modulation technique which uses phase of
the analog carrier to represent digital binary data. There are mainly two
types, namely binary phase shift keying and quadrature phase shift keying.
Diagrammatic representation of wave
Equation:
Advantage:
(i) PSK is less
susceptible to errors than ASK.
(ii) More efficient use
of bandwidth.
Disadvantage:
(i) More complex signal
detection/recovery process. Than in ASK and FSK.
QAM: QAM stands for Quadrature Amplitude Modulation. It
is a popular analog signaling technique that is used in the asymmetric digital
subscriber line. QAM takes advantage of the fact that it is possible to send
two different signals simultaneously on the same carrier frequency.
Advantage:
(i) The major benefit of
QAM modulation variants is efficient usage of bandwidth.
Disadvantage:
(i) QAM modulation is
more susceptible to the noise.
(ii) QAM receiver is more
complex compare to receivers of other modulation types.
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