# CHAPTER : 04 Encoding & Modulation

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```					Engr Rehan Ali Shah
   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.
   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.
   Some time we want to convert analog
signal(like telephone) into digital signal, also
called digitization an analog signal.

   Digital to Analog signal        conversion   or
modulating a digital signal.

   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.
   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.
   The digital to digital encoding hardware and
resulting digital signal.
   Of the many mechanisms for digital to digital
encoding
   Unipolar is very simple and primitive. Digital
transmission system work by sending voltage
   Unipolar encoding is so named b/c it use only
one polarity this polarity is assign to one of
the two binary states.
   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.
   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.
   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.
   To assure synchronization, there must be
signal change for each bit. The receiver can
use these changes to buildup, update and
synchronize its clock.
signal change not b/w bits but during each
bit .like nrz-l,
   But unlike nrz-l ,halfway through each bit
represent by positive –to-zero and 0
negative-to-zero.
   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.
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.
   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.
   Bipolar alternate mark inversion      is   the
simplest type of bipolar encoding.

   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.
   A variation of   bipolar   AMI   is   called
pseudoternary.
   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.
   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).
   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.
   PAM uses the technique called sample and
hold at a given moment.
   The reason PAM is not useful to data comm:
   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.
   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.
   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.
   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.
   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.
   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.
   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.
   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.
   What is the bandwidth u known very well.
   We get a spectrum of many simple
frequencies.
   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
   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.
   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.
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).
   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
bandwidth limitation of FSK.
   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.

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