Transcript CHAPTER : 04 Encoding & Modulation

Engr Rehan Ali Shah
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How information is transferred depends on
its original format and on format used by
communication hardware.
If you want to send a Love letter by smoke
signal you must know which smoke
patterns match which words in your
message before you actually build your
fire.
Words are information and puffs of smoke
are representation of that information.
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A simple signal by itself doesn’t carry
information any more than a straight line
conveys words.
The signal must be manipulated so that it
contains identifiable changes, recognizable to
sender and receiver as representing the
information.
Data stored in a computer in the form of
0,1’s to be carried from one place to another,
this is called Digital-to-digital conversion.
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Some time we want to convert analog
signal(like telephone) into digital signal, also
called digitization an analog signal.
Digital to Analog signal
modulating a digital signal.
conversion
or
Often an analog signal is sent over long
distance using analog media. This is known
as analog to analog conversion or modulating
an analog signal.
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Digital to dig: or conversion is the
representation of digital information by
digital signal.
For example you can transfer your data to the
printer, both the original data and
transmitted data are digital.
In this type of encoding, the binary 1s and 0s
generated by a computer are translated into
sequence of voltage pluses that can be
propagate over a wire.
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The digital to digital encoding hardware and
resulting digital signal.
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Of the many mechanisms for digital to digital
encoding
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Unipolar is very simple and primitive. Digital
transmission system work by sending voltage
pulses along the medium link.
Unipolar encoding is so named b/c it use only
one polarity this polarity is assign to one of
the two binary states.
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Polar encoding use the two level of voltage,
one is +ve and –ve. By using both level.
In most polar encoding methods the average
voltage level on the line is reduced that’s why
the DC component problem is alleviated.
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In nrz encoding, the level of signal is always
either positive or negative. This nrz also
divided into two categories or two method.
1-NRZ-L.
2-NRZ_I.
NRZ_L
 In nrz-l encoding the level of signal depends
on type of bit it represent. A +ve voltage
mean o and –ve 1,the level of signal depends
upon state of bit.
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In nrz-I, an inversion of voltage level
represent a 1 bit. It is the transmission b/w
positive and a negative voltage that represent
a 1 bit. A 0 bit is represented by no change.
Nrz-I is superior to nrz-l due to the
synchronization provided by the signal
change each a time a 1 bit is encountered.
Figure
shows
the
nrz-l
and
nrz-I
representation of the series of bit.
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To assure synchronization, there must be
signal change for each bit. The receiver can
use these changes to buildup, update and
synchronize its clock.
One solution is return to zero, in RZ the
signal change not b/w bits but during each
bit .like nrz-l,
But unlike nrz-l ,halfway through each bit
interval, the signal return to zero. A 1 bit
represent by positive –to-zero and 0
negative-to-zero.
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The main disadvantage of RZ encoding is that
it requires two signal changes to encode one
bit.
Probably the best existing solution to the
problem of synchronization is biphase
encoding. There are two types of biphase
encoding in use on networks today.
1- Manchester.
2- differential Manchester.
Manchester: Manchester encoding uses the inversion at
the middle of each bit interval for both
synchronization and bit representation.
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A negative to positive transition represent
binary 1and a +ve to –ve transition
represents binary 0. by using a single
transition for a dual purpose.
differential Manchester: in differential Manchester, the inversion at
the middle of the bit interval is used for
synchronization.
 the presence or absence of an additional
transition at the beginning of the internal is
used to identify the bit. A transition means
binary o and no tran: binary 1.
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Bipolar encoding, like RZ uses three voltage
level : positive, negative and zero.hower ,the
zero level in bipolar encoding is used to
represent binary 0.
The 1s are represented by alternating positive
and negative voltage. If the first 1 bit is +ve
amplitude the second is represented by –ve.
Three types of bipolar encoding are in
popular by the data communications.
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Bipolar alternate mark inversion
simplest type of bipolar encoding.
is
the
In the name AMI, the word mark comes from
telegraphy and mean 1. so AMI means
alternate 1 inversion. Zero voltage represent
binary o.
Binary 1s are represented by alternating
positive and negative voltages.
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A variation of
pseudoternary.
bipolar
AMI
is
called
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We some times need to digitize an analog
signal for example, to send human voice over
a long distance, we need to digitize, the
digital signal are less prone to noise.
This require a reduction of the potentially
infinite number of values in an analog
message so that they can be represented as a
digital steam with a minimum loss of
information.
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In analog to digital conversion, we r
representing the information contained in a
continuous wave form as a series of digital
pulses(1s or 0s).
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The first step in analog to digital conversion
is called PAM.
This technique takes an analog signal,
samples it and generates a series of pulses
based on the result of the sampling.
The method of sampling used in PAM is more
useful to other area of engineering then it is
to Data communication.
PAM is the foundation of an important
analog-to-digital conversion method called
PCM.
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PAM uses the technique called sample and
hold at a given moment.
The reason PAM is not useful to data comm:
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PCM modifies the pulses created by PAM to
create a completely digital signal. To do so,
the PCM first quantizes the PAM pulses.
Quantization is a method of
assigning
integral values in a specific range to sampled
instances.
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The binary digits are then transformed into a
digital signal using one of the digital to
digital encoding techniques.
PCM is actually made up of four separate
processes .
PCM is the sampling method used to digitize
voice in T-line transmission in the North
American Telecommunication system.
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So the question is that, how many samples
are sufficient , actually, it require remarkably
little information for the receiving device to
reconstruct an analog signal.
Nyquist Theorem, to ensure the accurate
reproduction of an original signal using PAM,
the sampling rate must be at least twice of
the original signal.
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Digital to Analog conversion or D-to-A
Modulation is the process of changing one of
the characteristics of an analog signal based
on the information in digital signal.
When you transmit data from one computer
to another across a public access phone line.
The digital data must be modulated on an
analog signal that has been manipulated to
look like two distinct values corresponding to
binary 1 and o.
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Before we discuss specific methods of D-to-A
two basic issues must be define:
1- Bit rate.
2- Baud rate.
The fewer signal unit required, the more
efficient the system and the less bandwidth
required to transmit more bits, so we r more
concerned to the baud rate.
The baud rate determines the bandwidth
required to send the signal.
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In analog transmission, the sending device
produce a high frequency signal that acts as a
basis for information signal.
The base signal is called the carrier signal.
Digital information is then modulated on
carrier signal by modifying one or more of its
characteristics .
Modulation or shifting keys and modulation
signal.
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In Amplitude shift keying , the strength of
carrier signal varied to represent binary 1 or
0. both frequency and phase remain constant
while the Amplitude changes.
The peak amplitude of the signal during each
bit duration is constant and its value depends
on the bit 0 or 1.
Unfortunately, ASK transmission is highly
susceptible to noise interference.
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A popular ASK technique is called OOK. In
OOK one of the bit value represented by no
voltage .
This advantage is a reduction in the amount
of energy required to transmit information.
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What is the bandwidth u known very well.
We get a spectrum of many simple
frequencies.
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In FSK , the frequency of the carrier signal is
varied to represent binary 1 or 0.
The frequency of the signal during each bit
duration is constant and its value depends on
the bit (0,1) both peak amplitude and phase
remain constant .see the fig:
FSK avoids most of the noise problem of ASK.
It can ignore voltage spikes
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Although FSK shifts b/w two carrier
frequencies. It is easier to analyze as two
coexisting frequencies.
the bandwidth required for FSK transmission
is equal to the baud rate of the signal plus
the frequency shift. BW=(fc1-fc0)+N .
The process of modulation produces a
composite signal.
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In phase shift keying , the phase of the carrier
is varied to represent binary 1 or 0. both peak
amplitude and frequency remain constant as
the phase change.
If we start with a phase of 0 degree to
represent binary 0, then we can change the
phase to 180 degree to send binary 1.
The phase of the signal during each bit
duration is constant and its value depends on
bit (0,1).
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The method is often called 2-PSK or BPSK,
b/c two different phases (0 and 180) are
used.
PSK is not susceptible to the noise
degradation that effects ASK. Nor the
bandwidth limitation of FSK.
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This means that smaller variation in the
signal can be detected reliably by the receiver
,instead of utilizing only two variation of a
signal , each represent 1 bit ,we can use four
variation and let each phase shift represent
two bits.