• Aspects of Digital to Analog Conversion
o Before
we discuss specific
methods of digital
to analog modulation,two
basic
issues must be defined:
Bit/Baud rate
Carrier signal
Bit Rate & Baud Rate
o Two terms used frequently in data
communication
-Bit rate
-Baud rate
ŠBit rate: no of bits
transmitted during one second
ŠBaud rate: no of signal
units per second that are required
to
represent that bit
ƒ Bit Rate & Baud Rate
o In
discussion of computer
efficiency, bit rate
is more important
-we want to
know
how long it takes to process each piece of info
o In data transmission, however ,we are
more concerned with how efficiently we
can
more that data from place to place, whether in pieces or blocks
o The
fewer signal units
required, the most
efficient the system
and less
bandwidth
required to transmit more bits ,so we are more concerned with baud
rate
o The baud rate determines the B.W required
to send the signal
ƒ Relationship b/w bit rate &band rate
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o Bit
rate equals the
baud rate times
the no. of
bits represented by
each signal
units
o The baud rate equals the bit rate divided
by the no. of bits represented by each
signal
shift
o Bit rate is always greater than or equal
to Baud rate
™ Analogy for Bit rate &Baud rate
o In
transportation a band
is analogous to
a car,a bit
is analogous to a
passenger
o A car can carry one or more passengers
o If1000
cars can go
from one point
to another carrying
only one
passenger(only
driver),than 1000 passengers are transported
o However, if each car carries four
passengers, then 4000 passengers are
transported
o Note
that the number
of cars, not
the numbers of
passengers
determines
the traffic and therefore the need for wider highway
o Similarly, the baud determines the
required bandwidth, not the bit rate
Example
5.6
An
analog signal carries 4 bits in each signal element.If 1000 signal elements
are sent per
second,
find the Baud Rate and Bit Rate?
Solution:
-Baud Rate=
Number of Signal Elements
-Baud Rate
=1000 bauds/second
-Bit Rate=Baud
Rate * Number of bits per signal element
-Bit Rate= 1000
* 4 = 4000 bps
™ Carrier Signals
o In analog TX. The sending device produces
a high frequency signal, that acts as a
basis
for the information signal
o This base signal is called the Carrier
Signal or Carrier Frequency
o The receiving device is tuned to the
frequency of the carrier signal that it expects
from
the sender
o I=Digital info is then modulated on the
carrier signal by modifying one or more of
its
characteristics (Amplitude, Frequency, Phase)
o This
kind of modification is
called Modulation and
info signal is
called a
Modulating
Signal
™ Amplitude Shift Keying (ASK)
o In ASK, the strength of carrier signal is
varied to represent binary 1 or 0
o Both frequency and phase remain constant,
while the amplitude changes
o Which
voltage represents 1
and which represents
0 can be
chosen by
System
Designer
o A bit duration is the period of time that
defines one bit
o The peak amplitude of the signal during
each bit duration is constant and
its
value depends on the bit (1 or 0)
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