and Digital Transmission • •

~-:~-=--~=--=~~------- -------~~·~
4 sA~S_E~B;A:N:D~A_N_D_B_A_N_D_PA_S~S~S ~IG=N~A~L~S~-----------
107

2,1
~ a n d Signal
1. 1311s . • h . .
The information 01 t e rnput signal to a co 1 . .
. be digital e.g. the computer d t ~~ murnca ~ion sys~om can hu analo~ i.e. ao\.ind, pict~ re
1
o~· it c/a_n known as the baseband sig al a. l,e electrical equ,iualent of lh iR original in(ormatwn
signo 1,s . na .
In other words, we can defme a baseband s· 1 . .
. data and picture signals are called as t igna as th e ?ne which 1s not modulatc<l . All the
voice, . • h • f' he baseband signals Th e frequ ency spectrum of a
1
baseba nd signa is s own 111 igure 2 ·28 · (a). It generally occupies the frequency spectr um right
fro!11 OHz.

f Hz

(a) Spectrum of a baseband signal (b) spectrum of a bandpass signal

Fig. 2.28.

2. Bandpass signal
It may be defined as a signal which has a non zero lowest freqtic cy in its spectrum. This
means that the frequency spectrum of a bandpass signal extends from f to f Hz. The modulated
signal is called as the bandpass signal. It is obtained by shifting the bas~band signal in frequency
domain. The spectrum of bandpass signal is shown in figure 2.28. (b). Here, it may be noted that
the lowest frequency in its spectrum is f 1 Hz where the highest frequency is f Hz. All the bandpass
signals are not necessarily modulated signals. They can be available naturallly as well. Examples
of bandpass signals are the ultrasound waves, visible light, radio waves etc.

2.15 CLASSIFICATION BASED ON THE TECHNIQUE OF TRANSMISSION

Based on the technique used for the signal transmission, we can categories the electronic
communication system as under:
(i) Baseband transmission system.
(ii) Communication systems using modulation.

2.15.1. Baseband Transmission

1. Basic Aspects
In some systems, called the baseband transmission systems, the baseband signals (original
information signals) are directly transmitted. Example of these type of systems are telephone
networks where the sound signal converted into the electrical signal is placed directly on the
telephone lines for transmission. Another example of baseband transmission is computer data
transmission over the coaxial cables in the computer networks. Thus, the baseband transmission
is the transmission of the original information signal as it is.
2. Limitations of Baseband Transmission
The baseband transmission cannot be used with certain mediums e.g., it cannot be used for the
radio transmission where the medium is free space. This is because the voice signal (in the electrical
for~) cannot travel long distance in air. It gets suppressed after a short dist~nce. Therefore, for the
radio communication of baseband signals, a technique called modulation 1s used.
2-15.2. Modulation
·
· signa
, modulation process, two signals are used namely the modulating
In tne l an d the carrier ·
 eo 0 \111 Com1n11r111•., 11r
tot • • m IJ,\tj (
-· ~ rn• 'Jt
, llll (jlf!nA1 t nnt hm 1 liu 1 t lu h 1!l' hno cf ~<yJ.,tnti,XJ
·nu OlOIIU A
1
j,( I Rl ' lflAI \ t hi! , (" ''I( I l •t II hrnh
{
~" :.,rj1_,, 1
1.(,,1
~ u t,,~.""
I OI rn lOt mA .,~-
•. . . .
,,in • f;!-qn ,il
c.1 ~rnft ~• •·rntl J ,.
1
.:;1n1l' :0J1J~ 1
In th • m111J11
rr, qu • nc,· •
d th, < "n◄, '~ " ' '
td111I\
I ~

1
ir ..,,.,
1Sf! ..:,llll• p81Al li ' ,1, l ( "' '
proc · 1 1 1rn~ ,,,iu v,u 11,l111
\lr l\l, 1 ,
11m 1)htuU 1 q 111
I , 1
,,,.,, ... ,,,
~fl l . ,,
·c"1·<l A ni ,, \\ 1\ l 1 1 .. n11, dulot1 1, ., ' " " "' I 11 1 "' "' ,r 1
r lij ') / ti. , "
RI , d 01 ft1 lS th('ll llt\11 1.rnll ll t d hv t lH
m,du At~ i=i,,. • V ~l r,rJ,ta•,;.r
,11 , "''<'t't \lll lh•mm l\llnt l''( I l w 1,•r
11nflm11t • r ,,1~ ,, 1•1v ,, , I ln<H l 11 I 11t1•d 11 1v.r,1tl
tr Jtl( 1 ~r.t
mfor""illuon 'HJ, 11hnl'k Thu s , dc mocluluiwn IA t •x1H I lv oppo.-ot,, tJ) rtH,,li.litt to ,
th " ra rritr \\ HVP nctun lh nc1s fl H n n 1rr lf•r wn1 l ''>t1 f r1 th ,, '
;n°'1o lat1on . l ch irirri , ~ t ►,, f!Pl' r
J l Al) fro m 1hr tnrn i.
.
m1lt r r to
. 'J
'h . 1 1nf,,
r , 'If
, m()o U ll11HjZ t- 10"' 11
fPC'P1 v c•r 1i:1 .I A R1milnr 1, 1 A i nM1,,r,,
. li:i hi 1- cAr or on I h h
hi s h1k(l from one p HCC' to t e ot Pr . 'fh,, p,,r'l <> n '11 tJ;i• 1 r r ~ti
p<>r~cm t ra' P 111 ' h .
1 1
1r A/ 1

e as t e earn er ai; s hown in

. .
mod\l ls ting ~Ir• cnrnl and th e rar or hik < Hn t ,, ., I\ 1
l fi~ur p 2 ''() , , )
1
,..h ~t.
• ~ • W

lnformatlon
Signal

. - - -- Carrier - - - ~
Fig. 2.30. Concept of modulation

2.15.3. Frequency Translation in Modulation Process

The baseband signal or modulating signal is a low frequency
· signal. For example h
sign. al is present m the frequency range fram 20 H z to 20 kHz . But, due to modul
. h fr
signal now gets translated to a h1g ation, tthe aud· M;

er equency range . , e 3ame

2.15.4. Multiplexing
Multiplexing may be defined as the process of combining sever
al message signal s together and
send them over the same communication channel. Three comm
only used methods of multip lellllg
are as under:
(i) Frequency Division Multiplexing (FDM).
(ii) Time Division Multiplexing (TDM).
\__ .
(iii) Code Division Multiplexing (CDM). 1

2.16. NEED OF MODULATION

1. Basic Aspects
· may b
A quest10n e asked as, when the baseb and signals can be transm1tte · dd'irect[ywhv rou, c>
the modulation? , · ·
The answer is . . . . . . h'
that the baseband transm1ss10 n has many lim1ta t10ns w ich rn n be O\~rt·oaic>
using modulation. It may be explained below
. ·
.In the process of· modulation, the baseband signal is translated · L'f'td from I t' •.i Ul' [\l\
ow rt'i
1.e., s,u e .
to high frequency . 'I'his frequ ency shift is propo
rtional to the frequ ency of earner.
2· Advantage s of
Mod ulati on
(i) Redu ction in the heigh t of anten na
(ii) Avoid s mixing of signal s
(iii) Increases th
e range of' comm umca
. t10n
.
Analog and Digital Tran sm1ss1on · - 109

(i ,·)i\1ult1plcxing 1:-- p os~ 1li lP

(,·) lmpro,·ps qunlity nf rt' t' t' pt 10 11
3, Red uction in HC'igh t of A n tl' llt lll
Fnr the t ran ~tnl i-~ l\)11 nf rn tlrn !< 1g 11 11 h1, 1lw n 1111 •n11u lw, ght 0111 , t h1 tn11lt1Jil1 ,,f (I ) ~J . H, rf · b
1
,,r
thr wn,·clc>ngt h . A= ctf \\" hen_, C' 1~ \ L'lucity of ltgh L (I n<l f IS Lhl' frC'q IJl' rlf' Y I hr '4 11{nfd trJ ~JI' t r,in nJ JttnL
The m111 11 nu m a ntc•nna hl'ight required lo t ra ns mil u busehancl ;;1gna l of f - J fJ k H 1 1 r ,dcu },1 t, d R ...
fC11low:a: :

r · h · h A C 3 X 10 8
i\1m1mum a nte nn a e1g t = - = - = - - - -3 = 7500 meters 1. e. 7 5 k m
4 4f 4 x 10 x 10
The a nten n a of t his h eigh t is practica lly im possible to install.
~ ow. let us con sider a modulate d signa l at f = 1 MHz. Th e minim um ante nna hf.:ight b gn~n

A c 3 x 10 8
Minimum antenna heigh t = - = - = - - - - = 75 mete r
4 4f 4 X 1 X 1Q 6
This antenna can be easily installed practicall y. Thus , modulat ion r educes t h e height of the
antenna.
4. Avoids Mixing of Signals
If t he baseband sound signals are transmitt ed without using the modulati on by mor e than one
e, all
transmitt er, then all the signals will be in the same frequency range i.e. 0 to 20 kHz . Therefor
the signals get mixed together and a receiver cannot separated them from each other.
3 Baseband
Signals
Signal 1_j ....,._ _ _ Frequency
o:'
'
''
Signal 2

Frequency, kHz
80 120 180 220 280 320

..
i 100
· Channel'
ri i 200
:-channel
i
1r:
i 300
:t,;hannel I":
i

Fig. 2.31. Modulation avoids mixing of signals

Hence, if each baseband sound signal is used to modulate a different carrier the n they will
figurt>
occu py different slots in the frequenc y domain (different channels) . This h as been s how n in
2.31. Thus, modulation avoids mixing of signal s .
5. Increase s the Range of Commu nication
The frequen cy of baseba nd sign a ls is low , a nd t h e low fr eque ncy s igna ls rn n not tnwd tl long
atton
dista nce wh en t h ey a re tra ns mitted. Th ey get heavil y atten uatl•d (sup p1·cssl' d). T he ntte nu
longt:' r di::;ta nce. The
reduces wit h incr ease in freq u en cy of t h e tra ns mitted s ign a l~. un d t hoy tr aw l
modulatio n process incr eases t h e fr eq uen cy of t he s igna l to be trnns m ittod . T he refore. it increases
on
the range of commun ication. In a ddition to t h o advu n tageo d ioc ussed t ill now. the modulati
Process has some more a dvantage s .
 Dat a Com mu ntc abo n aod Con
--pute.r t,._
110 • • ·· e~~
M ltip lex ing is pos sib le
6. u . . . oce ~c; in wh ich t wo or morC' ~1g
Mu lt.1pl~xingh1s a Pl~c.1m:ltnn eou n0Js can be tra n5m ltte d , er
om mu mc a t10n c ann e ~ sl v Th ii:. ,s pn, -.1ble onh• ,,•1th 0 ·h
c . m". ldu le.h on The mu\ '• e. ::,;an-.e
h
allo ws t e sam e c hnn n el to be use d b,· ma ny s1gno ls . H cnc e . m a.ny h n·
nnn d ~ ca n u_, p ·rl P,t>X
freq uen c). ran ge, " , ·tho u t . get t.m g mu· :ed wit . f
h enc h ot h er or d 1ff €' rPn t f r• - l,- ..-
que ncy £-lz y 1 .'.'am e A ..

tra nsm itte d nt the sa me tim e. • ... t n •~

7. Im pro ves qu ali ty of rec ept
ion . .
Wi th freq uen cy mo dul atio n (FM .
). and th_e ~g ita l com mu _rucat10
effe ct of noise is red uce d to a gre n t ech niq ue~ hke PC~L t,j"
at ext ent . Th is im proves qua lity
of rec ept ion
2.17. DIFFERENT TY PE S OF
MO DU LA TlO N SY ST EM S
Var iou s typ es of pra ctic ally use d
mo dul atio n sys tem s are a s und
er:
Modulation Systems

Analog Modulation
Systems D;gita!Modula~
Systems
+
Amplitude
Modulation (AM)
*
Angle
Modulation
•
Pulse Analog
+
Pulse Code
I
t
Modulation Modulation (PCM) Deft.a
I Moduta!ion (D\ fl
+ + +
Frequency
Modulation (FM)
Phase
Modulation (PM)
Pulse Amplitude
Modulation (PAM)
*
Pulse Width
Modulation (PWM)
+
Pulse Posffion
Modulation (PP M)
Fig. 2.32. Classification- of mo dul
ation sys tem s
2.17 .1. Am pli tud e Mo du lat ion
(AM)
In am pli tud e mo dul atio n me tho
d, the carrier is a sin ew ave wit
fre que ncy is mu ch hig her tha n h fre que ncy f_. The car rier
the fre que ncy of the mo dul atin
car rie r var ies in acc ord anc e wit g sig nal (f,,,). The a~p litu de of the
h the am pli tud e of the mo dul atin
Th e am pli tud e mo dul ate d (A.M.) g sig nal as sho wn in figure 2.33 .
sig nal is tra nsm itte d by a tra nsm
in its am pli tud e var iati on. Th e fre itte r. Th e information is con tain ed
que ncy of the car rie r rem ain s con
TV bro adc ast ing app lica tio ns . sta nt . Ai\ i is used in rad io and

..... ..... ..... ....Baseband Signal (Mo

dulating Signal)

Carrier Signal

""-; ::t+ tttt- H--H -H-l ---'- 1-1. ...U

...U U-J LJ.J LLL LU. .U.. UU- H--H
-+T- + -- nme. t

Amplitude modulated (AM) Sign

al

.. ··:.......... (Ee - E,J

i'if1~ t-H f+l~ ~~ ~W -IU -1W -lU -lW
-1W -1W -1~ ~~ h-- - - -~ nme. t
Unmodulated
Carrier
Envelope
Fig. 2.33. Am plit ude mo dul ate d
signal
 Analog and Digital Transmission
... . . 111

2.11.2. Frequency Modulation (FM)

In frequency modulation m ethod tt f
mplitude of the modulat · . ' .I ~~ t oqu e ncy of Lh c cnrri.-r is chnngNI in ncco rdanre with
tl1e8
c m g stgnn . J hu s tho fr • · ·1 . . ·
Jacr as shown in figure 2. ;14 _ Th o FM . r ', cqu ? ncy co mprtJF1A 1on n~ r Px pans wn will ta ke
p- "ormation in the frequen"v V' ,· t · sT1hgnal l fl L_runam1Lled hy th 0 trnn Hm1 UN und it carries the
Hu '-. cllH\ ton c nmphtud 0 0 f th · · ·
uQcd for TV, sound nn d radio hr d · . . _ e ca rri e r remarn e constunt. Also, .F'M is
~ on casting and police wireless tran smi ss ion .

Baseband Signal (Modulating Signal)

+ ······--·

i
Frequency Modulated (~Ml Signal
'
•
t·---... .....
I

+········ .. . ·.-! ··· .... ... .. .. .......

1 --- 1· ----- --- ----i ------
No -+i
Frequency ! Maximum Minimum
Change fc Frequency Frequency

Fig. 2.34. Frequency modulated signal

2.17.3. Phase Modulation (PM)

In PM the phase shift of the carrier signal is varied in accordance with the amplitude of the
modulating signal.
The amplitude of carrier remains constant. Thus, the information is carried in the phase
variations of the modulated signal. The phase modulated signal is shown in figure 2.35. Phase
modulation is very similar to the frequency modulation. The only difference is that the phase of the
carrier is varied instead of varying the frequency. The amplitude of the carrier remains constant.
Also, PM is used for generation of FM.
Baseband Signal (Modulating Signal)
Modulating voltage
+

01--------~--- -..______,~-----------:r .--+

..'

Un~,~~~t•d~ Phase Modulated (PM) Signal

- - -: . . ........ l . . .·- - . . . ---·-· · ·-·

i
:
:
:

I
.. .... \. . --.··· .... ··•··· --~...........

No Phase
Change
__J
i
\ I Carrier Stretching
j
Carrl~r Compression ·
!

(Phase Lag) (Phase Lead)

Fig. 2.35. Phase modulated signal
r
112 ee
2.17.4. Pulse Analog Modula tion (PAM , PWM and PPM)
F or tht> .\~1. F~t nncl PM r1, ... c ui,. i-P cl 1t1 tlw p1 P\ 1ou ,. 11 rl 1, 11 • 1 h, < rum • a r :1w.
H ow< ,·<•r 1n t hr pul'-(' mndulnt,0 11 . 1lw cnn re r ,._ rn r h, forn , ,,r ,,,d ' in u ~,,d o 4 t).f.
Jo PA.\f (pub•(' ,l mplitudt• modulat wn), tJH' umpl1Lu<l1 · of th f J)UI 'd 1 o r n, r 1 v r r
with thP amplnud1 1 nf nwdulatr n g b l i:{ ll UI. [n JJ\VM (puJ , L w1tltn mc.,·duJ,,tu rJ) . ~.ad 1
and m PP>f tP ub<.. pn::-Hi~rn mochrlnt wn) , th e po&1twn n f p uJq o JS va r!' d w acc<.Hds:r nlf'
modulatr n~ ~icnal Tlw l'.\....\1 and P\\.M s jgna ls are shown in figur'1 2 ~{,;

Baseband
Signal
(Modulating
Signal)
0 ir---- -----~ .------ - - - - - - +- - ---. TM"'8

Carrier

PAM Signal
Tr= sTpling Perii
....,
i Tune

0
:
,·
:
1
:
1 , , :---♦
l"-~-+L-~-.1.-1,;;..,...,....._""9""9'_~-~_:
,, --
, Tune

---
]
PWM Signal

Time

Fig. 2.36. PAM , PWM signals (pulse analog modulation )

2.17.5. Pulse Code Modulation (PCM) and Delta Modulation (DM)

These are_the digital pulse modulat ion systems. Here, a train of digital pulses is trao~r,-,T: -c-=
by the transmi tter. All the pulses ar e of constan t amplitu de, width and position . The informa:oo:::
;..E
contain ed in combina tion of t he pulses. The digital commun ication has higher noisR 1.Ill.::::"'
"~
than the a nalog commun ica tion system.
·
2.18. DEMO DULATION OR DETECTION
The modula ted signals are tra nsmitte d by t he transmi tter via air medium or wire medium .
~
signals then reach the receiver s by tra velling over the commun ication meruum . At the ~ l \ ~r. l:l~
origina l informa tion signal is sepa rated ~rom the carrier. This ~rocess is called as demodu hoon
or
detectio n. Detecti on is exactly the opposite process of modulat 10n.