3. Signal Encoding/Modulation
◦ We are concerned with transmitting
digital data.
◦ Some transmission media will only
propagate analog signals e.g., optical
fiber and unguided media
◦ Therefore, we will discuss transmitting
digital data using analog signals.
◦ The most familiar use of this
transformation is transmitting digital data
through the public telephone network.
4. Encoding/Decoding
◦ Encoding is the process of converting
the data or a given sequence of
characters, symbols, alphabets etc.,
into a specified format, for the secured
transmission of data.
◦ Decoding is the reverse process of
encoding which is to extract the
information from the converted format.
5. Modulation
◦ The process of superimposing a low-frequency signal
on a high-frequency carrier signal.
◦ Two signals are involved in the modulation process.
Message signals are also known as baseband signals.
◦ Baseband signals are the band of frequencies
representing the original signal. This is the signal to be
transmitted to the receiver.
6. Modulation
◦ The frequency of such a signal is usually low. The
other signal involved in this is a high-frequency
sinusoidal wave.
◦ This signal is called the carrier signal. The frequency
of the carrier signals is almost always higher than that
of the baseband signal.
7. Modulation
◦ Modulation is how an information signal is added to a
carrier signal
◦ Modulation is the process of changing one or more
properties of the carrier signal in proportion with the
information signal. In modulation, some parameter of the
carrier wave (such as amplitude, frequency or phase ) is
varied in accordance with the modulating signal . This
modulated signal is then transmitted by the transmitter
8. Modulation
Information / Message / Modulating Signal
It is the base signal to be transmit. It is low frequency signal.
Carrier Signal
It is the high frequency signal use to carry information signal.
Modulator
It is the circuit which performs modulation
Modulated Wave / Signal
It is the carrier signal that has been acted on by information
signal. It is high frequency signal.
Demodulator
Demodulator is a circuit in receiver which performs
demodulation.
11. CONT. ◦ The way to think of this is like a letter
◦ The envelope is the carrier and the
letter is the information
◦ The envelope is only needed during
transmission
12. Types of
Modulation ◦ There are many common analog
modulation methods, including the
following:
Amplitude Modulation (AM)
Frequency Modulation (FM)
Phase Modulation (PM)
13. EM Spectrum
◦ Amplitude Modulation in which the height ,It is a type of
modulation in which only the amplitude of the carrier
signal is varied to represent the data being added to the
signals whereas the phase and the frequency of the
signal are kept unchanged.
14. Cont.
◦ Frequency Modulation It is a type of modulation in
which only the frequency of the carrier signal is
varied to represent the frequency of the data
whereas the phase and the amplitude of the signals
are kept unchanged.
15. CONT.
◦ Phase modulation in which the phase of the carrier waveform
is varied to reflect changes in the frequency of the data.
◦ In PM, the frequency is unchanged while the phase is changed
relative to the base carrier frequency.
◦ It is similar to FM.
16. Amplitude Shift Keying
◦ Amplitude Shift Keying (ASK) is a type of Amplitude Modulation which
represents the binary data in the form of variations in the amplitude of a
signal.
◦ The two binary values are represented by two different amplitudes of the
carrier frequency.
◦ One binary digit represented by presence of carrier, at constant amplitude
◦ Other binary digit represented by absence of carrier
17. ASK ◦ Susceptible to sudden gain
changes
◦ Inefficient modulation technique
◦ On voice-grade lines, used up to
1200 bps
◦ Used to transmit digital data over
optical fiber
18. Binary Frequency Shift-Keying (BFSK)
◦ FSK changes the frequency of the sine wave as
time goes by, without changing the height
◦ Two binary digits represented by two different
frequencies near the carrier frequency
19. BFSK ◦ Less susceptible to error than ASK
◦ On voice-grade lines, used up to
1200bps
◦ Used for high-frequency (3 to 30
MHz) radio transmission
◦ Can be used at higher frequencies
on LANs that use coaxial cable
20. Phase-Shift Keying (PSK)
◦ PSK changes the phase of successive sine
waves
◦ Two-level PSK (BPSK)
◦ Uses two phases to represent binary
digits
21. Need for
Modulation
◦ The baseband signals are
incompatible for direct
transmission. For such a signal, to
travel longer distances, its strength
has to be increased by modulating
with a high frequency carrier wave,
which doesn’t affect the
parameters of the modulating
signal
