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Chapter 5
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Reason of Conversion
 Digital data  Analog Signal
 Allows us of the public telephone system
 Allows use of optical fiber
 Analog Data  Analog Signal
 Easy
 Telephone system was primarily analog
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Digital-to-Analog Conversion
 Digital-to-analog conversion is the process of changing
one of the characteristics of an analog signal based on the
information in digital data.
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Digital-to-analog conversion
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Aspects of D/A Conversion
 Data Element:
 Smallest piece of information
 Signal Element:
 Smallest piece of signal
 Signal Rate (Baud Rate): It is supposed to be vehicle.
 Number of signals elements per second.
 Baud rate determines the bandwidth required to send the signal.
 Data Rate (Bit Rate): It is supposed to be passenger.
 Number of bits per second. (in bps)
 Describes a medium capacity.
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Carrier Signal Vs. Modulator Signals
 Carrier signal (carrier frequency)
 In analog signal, the sending device produces a high-
frequency signal that acts as a basis for the information
signal.
 Receiving device is tuned to the frequency of the carrier
signal that it expects from the sender.
 Digital information then modulates the carrier signal by
modifying one or more of its characteristics (amplitude,
frequency, or phase). This kind of modification is called
modulation (or shift keying, and the information signal is
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called the modulating signal).
Modulation & Demodulation
 The technique of superimposing the message signal on
the carrier is known as modulation.
 The process of conveying a message signal, (for example
a digital bit stream or an analog audio signal), inside
another signal that can be physically transmitted.
 The reverse operation (demodulation/detection) is
performed at the receiving end.
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Overview of Modulation
Phone
Line
Serial link
Modem
Computer
Digital
Analog
Baseband Vs. Broadband
 In Baseband, data is sent as digital signals through the media as a single channel
that uses the entire bandwidth of the media.
 Baseband communication is bi-directional, which means that the same channel
can be used to send and receive signals.
 In Baseband frequency-division multiplexing is not possible.
 Multiplexing is a process where multiple analog message signals or digital data
streams are combined into one signal over a shared medium.
 Broadband sends information in the form of an analog signal. Each transmission is
assigned to a portion of the bandwidth, hence multiple transmissions are possible at
the same time.
 Broadband communication is unidirectional, so in order to send and receive, two
pathways are needed.
 This can be accomplished either by assigning a frequency for sending and
assigning a frequency for receiving along the same cable or by using two cables,
one for sending and one for receiving. In broadband frequency-division
multiplexing is possible
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1. Amplitude Shift Keying
 Amplitude Shift Keying:
 Amplitude Shift Keying: Amplitude of carrier signal is varied
to create signal elements.
 Both Frequency and Phase remain constant.
 The strength of the carrier signal is varied to represent
binary 1 or 0.
 Types

BASK (Binary Amplitude Shift Keying) or OOK (on-off keying)

MASK (Multiple Amplitude Shift Keying)
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1. Amplitude Shift Keying

Peak amplitude of the signal during each bit duration is
constant, and its value depends on the bit (0 or 1).

ASK is susceptible to noise interference.

Noise: Unintentional voltages introduced by various
sources i-e heat or electromagnetic signals, etc
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2. Frequency Shift Keying
 Frequency Shift Keying (FSK)
 The frequency of the carrier signal is varied to represent
data.
 Frequency of the modulated signal is constant for the
duration of one signal element, but could change for the
next signal element.
 Peak Amplitude and Phase remain constant for all signal
elements.
 Frequency of the carrier signal is varied to represent
binary 1 or 0.
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2. Frequency Shift Keying

FSK avoids most of the problems from noise.
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3. Phase Shift Keying

Phase Shift Keying

The phase of the carrier is varied to represent binary 1 or 0.

Both peak amplitude and frequency remain constant.

If we start with a phase of 00 to represent binary 0, then we
can change the phase to 180 to send binary 1.

The phase of the signal during each bit duration is constant, and
its value depends on the bit (0 or 1).
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Phase Shift Keying

2-PSK or binary PSK: Two different phases (00 and 1800) are
used.
Constellation
or
phase-state
diagram
shows
the
relationship by illustrating only the phases.
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4. Quadrature Amplitude Modulation

Combination of ASK and PSK

We could have x variations in phase and y variations in
amplitude, giving us x times y possible variations and the
corresponding number of bits per variation.

Because amplitude changes are susceptible to noise and require
greater shift differences than do phase changes.
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4. Quadrature Amplitude Modulation
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Analog-to-Analog Transmission

Modulation
of
an
analog
signal
or
analog-to-analog
conversion is the representation of analog information by an
analog signal.
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Analog-to-Analog Transmission
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1. Amplitude Modulation

Amplitude Modulation

Carrier signal is modulated so that its amplitude varies with
the changing amplitudes of the modulating signal.


Frequency and phase of the carrier remains the same.
Bandwidth of an AM signal is equal to twice the bandwidth of
the modulating signal.

Bandwidth of audio signal (speech and music) is usually 5
KHz. Therefore, an AM radio station needs a minimum
bandwidth of 10 KHz. Federal Communications Commission
(FCC) allows 10 KHz for each AM station.
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1. Amplitude Modulation
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2. Frequency Modulation

Frequency of the carrier signal is modulated to follow the
changing voltage level (amplitude) of the modulating signal.

Peak amplitude and phase of the carrier signal remains
constant.

Bandwidth of an FM signal is equal to 10 times the bandwidth
of the modulating signal and, like AM bandwidths, covers a
range centred on the carrier frequency. BWt = 10 x BWm.

Bandwidth of an analog signal (speech and music) broadcast
in stereo is almost 15 KHz. Each FM radio station, needs a
minimum bandwidth of 150 KHz. FCC allows 200 KHz for
each station to provide some room for guard bands.
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2. Frequency Modulation
• FM stations are allowed carrier frequencies anywhere
between 88 and 108 MHz.
• Stations must be separated by at least 200 KHz to keep
their bandwidths from overlapping.
• To create even more privacy, FCC requires that in a
given area, only alternate bandwidth allocations may be
used. Others remain unused to prevent any possibility of
two stations interfering with each other.
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2. Frequency Modulation
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3. Phase Modulation
• Phase of the carrier signal is modulated to follow the
changing voltage level (amplitude) of the modulating
signal.
• Due to simpler hardware requirements, PM is used in
some systems as an alternative to frequency modulation.
• Peak amplitude and frequency of the carrier signal
remain constant.
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