net 04 signal encoding
DESCRIPTION
Signal encoding techniquesTRANSCRIPT
Lecture 4:
Encoding and Modulation Techniques
Encoding and Modulation Techniques
Digital Data Digital Signal
! Digital signal – Discrete discontinuous voltage ulses
– Each ulse is a signal ele.ent
/ser inut at a ,- is converted into a strea. o0 %inar&
digits (1s and 2s)3 'inar& 1 is reresented %& 5v and
%inar& 2 %& 65v3
So.e Ter.s
! /niolar – ll signal ele.ents have sa.e sign (all ositive or all negative)
! ,olar – One logic state reresented %& ositive voltage the other %&
negative voltage
! Data signaling rate (or 7ust data rate) – #ate o0 data trans.ission in %its er second (%s)
! Duration or length o0 a %it – Ti.e ta8en 0or trans.itter to e.it the %it (0or data rate # %it
duration is 19#)
– Measured in %aud signal ele.ents er second
;nterreting Signals
! To interret digital signals the receiver needs to 8now – Ti.ing o0 each %it when a %it starts and ends
– Signal level (high <2= or low <1=) 0or each %it osition
! +actors a00ecting success0ul interreting o0 signals – Signal to noise ratio (S"#)
– Data rate
– Encoding sche.e (i3e3 mapping data bits to signal elements)
! >ith other 0actors held constant the 0ollowing are true: – n increase in data rate increases %it error rate ('E#)3
– n increase in S"# decreases 'E#3
we need to consider
! Signal Sectru. – Lac8 o0 high 0requencies reduces required %andwidth
– Lac8 o0 dc co.onent allows ac couling via trans0or.er roviding electrical isolation
– -oncentrate ower in the .iddle o0 the trans.ission %andwidth
! -loc8ing – S&nchroni?ing trans.itter and receiver
– E@ternal cloc8 or sel0 cloc8ingA
! Error detection – #esonsi%ilit& o0 data lin8 control
– 'ut can %e %uilt into signal encoding to detect errors .ore quic8l&
! Signal inter0erence and noise i..unit& – So.e codes are %etter than others er0or.ance .easured %& 'E#
! -ost and co.le@it& – Bigher signaling rate lead to higher costs
– So.e codes require signaling rate greater than data rate
"onreturn to $eroLevel ("#$L)
! Two di00erent voltage levels 0or two %inar& %its
! Coltage constant during %it interval
– no transition i3e3 no return to ?ero voltage
! ;.le.entations:
– /niolar (a%sence o0 voltage 0or one constant ositive voltage 0or
?ero)
– ,olar (negative voltage 0or one value and ositive 0or the other)
2 high level
1 low level
"onreturn to $ero ;nverted ("#$;)
! -onstant voltage ulse 0or duration o0 %it
! Data encoded as resence or a%sence o0 signal transition at %eginning o0 %it ti.e
! Transition (low to high or high to low) denotes a %inar& 1
! "o transition denotes %inar& 2
! "#$; is a t&e o0 di00erential encoding – Di00erential encoding rule: i0 the current %it is a %inar& 2 then the
current %it is encoded with the sa.e signal as the receding %it i0 the current %it is a %inar& 1 then the current %it is encoded with a di00erent signal than the receding %it3
"#$ ,ros and -ons
! ,ros – Eas& to engineer
– Ma8e good use o0 %andwidth (requires hal0 the %andwidth o0 Manchester in so.e cases)
! -ons – dc co.onent
– Lac8 o0 s&nchroni?ation caa%ilit&
e3g3 with a long string o0 1s or 2s 0or "#$L or a long string o0 2s 0or "#$; the outut is a constant voltage over a long eriod o0 ti.e3 So an& dri0t %etween the cloc8s o0 trans.itter and receiver will result in loss o0 s&nchroni?ation %etween the two
! /sed 0or digital .agnetic recording
! "ot o0ten used 0or signal trans.ission
! 'iolarM; (lternate Mar8 ;nversion)
– 2’ reresented %& no line signal (2v)
– 1’ reresented %& alternating ositive and negative ulses (e3g3 65 5 65)
– 'inar& 1 ulses alternate in olarit&
– "o loss o0 s&nc i0 a long string o0 1s occurs (2s still a ro%le.)
Multilevel 'inar& – ,seudoternar&
– 1’ reresented %& no line signal (2v)
– 2’ reresented %& alternate ositive and negative ulses (e3g3
65 5 65)
– 'inar& 2 ulses alternate in olarit&
– "o loss o0 s&nc i0 a long string o0 2s occurs (1s still a ro%le.)
– "o advantage or disadvantage over %iolarM;
! /se .ore than two voltage levels to reresent data: – 'iolarM; (lternate Mar8 ;nversion)
– ,seudoternar&
– Less %andwidth than "#$
– "o loss o0 s&nchroni?ation i0 a long string o0 1s occurs in 'iolar M; or a long string o0 2s in ,seudoternar&
! Disadvantages: – S&nchroni?ation ro%le. with long runs o0 2s in the case o0 M;
or 1s in the case o0 seudoternar&
– "ot as e00icient as "#$ ! Each signal ele.ent onl& reresents one %it (although 0or Flevel
s&ste.s each signal ele.ent could reresent logGF 135H %its)
– #eceiver .ust distinguish %etween three levels (6 2)
'ihase – Manchester
! Transition serves as cloc8 and data
! Low to high reresents one
! Bigh to low reresents ?ero
! Seci0ied in ;EEE H2G3F (Ethernet) 0or %ase%and coa@ial
ca%le and twistedair %us L"s
'ihase – Di00erential Manchester
! lwa&s a transition in .iddle o0 interval
! Transition at start o0 a %it eriod reresents ?ero
! "o transition at start o0 a %it eriod reresents one
! t least one – ossi%l& G transitions er %it ti.e
! T&es: – Manchester
as a cloc8)
– "o dc co.onent
– 'uiltin error detection (a%sence o0 an e@ected transition can %e used to detect errors)
! Disadvantages – [email protected]. .odulation rate is twice that 0or "#$
– #equires .ore %andwidth than .ultilevel %inar& codes
%its er second) and
rate is 19T b where T b
%it duration3
signal ele.ents are
twice that o0 "#$;3
Scra.%ling
! Desite the sel0cloc8ing .echanis. %ihase codes are not widel& used in longdistance alications %ecause the& require a high signaling rate relative to data rate
! Scra.%ling – use 0illing sequences to relace sequences that would roduce constant voltage
! #equire.ents: – Must roduce enough transitions to .aintain s&nc
– Must %e recogni?ed %& receiver and relaced with original sequence
– Sa.e length as original
– "o dc co.onent
– "o reduction in data rate
– Error detection caa%ilit&
! ;.le.entations: – 'H$S (%iolar with H?eros su%stitution)
– BD'F (highdensit& %iolarF ?eros)
'H$S ('iolar with H$eros Su%stitution)
! 'ased on %iolarM;
! ;0 an octet o0 all ?eros occurs and last voltage ulse receding this octet was ositive encode as 222626
! ;0 an octet o0 all ?eros occurs and last voltage ulse receding this octet was negative encode as 222626
! -auses two violations o0 M; code an event unli8el& to occur as a result o0 noise
BD'F (BighDensit& 'iolarF $eros)
! 'ased on %iolarM;
! String o0 4 ?eros relaced with one or two ulses with 4 th ?ero relaced with a code violation
'valid %iolar signal
C'iolar violation
Digital Data nalog Signal
• Transmitting digital data using analog signals
! ,u%lic telehone s&ste. – Coice 0requenc& range F22B? to F422B?
– /se .ode. (.odulatorde.odulator) which converts digital data to analog signals, and vice versa
! Modulation involves oeration on one or .ore o0 the three characteristics o0 a carrier signal: a.litude 0requenc& and hase3
! Encoding9.odulation techniques – .litude shi0t 8e&ing (S*)
– +requenc& shi0t 8e&ing (+S*)
– ,hase shi0t 8e&ing (,S*)
.litude Shi0t *e&ing
! Two %inar& values reresented %& two di00erent a.litudes o0 the carrier 0requenc&
! /suall& one a.litude is ?ero
– i3e3 one %inar& digit is reresented %& the resence at constant a.litude o0 the carrier the other %& the a%sence o0 the carrier3
! Susceti%le to sudden gain changes
! ;ne00icient .odulation
! /sed to trans.it digital
cπ
! Most co..on 0or. is %inar& +S* ('+S*)
! Two %inar& values reresented %& two di00erent 0requencies (near carrier)
where 0 1 and 0 G are o00set 0ro. the carrier 0requenc& %& equal %ut oosite a.ounts
! Less susceti%le to error than S*
! / to 1G22%s on voicegrade lines
π
π
.ore than two
,hase Shi0t *e&ing
! ,hase o0 carrier signal is shi0ted to reresent data
! 'inar& ,S* – Two hases reresent two %inar& digits
! Di00erential ,S* – ,hase shi0t is relative to
revious trans.ission rather
sending a signal burst of the
same phase as the previous
signal burst sent. Binary 1 is
represented by sending a
to the preceding one.
cc
c
Modulation o0 nalog Signals 0or Digital Data
nalog Data Digital Signal
! Trans0or.ing analog data into digital signals
! nalog data such as voice and video is o0ten digiti?ed to %e a%le to use digital trans.ission 0acilities3 Strictl& sea8ing the rocess o0 converting analog data into digital data is 8nown as digitization3
! Once analog data have %een converted into digital data the 0ollowing can haen: – The digital data can %e trans.itted using "#$L3 ;n this case we
have in 0act gone directl& 0ro. analog data to a digital signal3
– The digital data can %e encoded as a digital signal using a code other than "#$L3 Thus an e@tra ste is required3
– The digital data can %e converted into an analog signal via S* +S* or ,S*3
–
,ulse -ode Modulation
! ,-M is %ased on sampling theorem: ;0 a signal is
sa.led at regular intervals at a rate higher than twice the
highest signal 0requenc& the sa.les contain all the
in0or.ation o0 the original signal3
! Coice data are li.ited to %elow 4222B?3 #equire H222
sa.les er second3
! Bowever these are analog sa.les called ulse a.litude
.odulation (,M) sa.les3
! To convert to digital each o0 these analog sa.les .ust %e
assigned a %inar& code3
3030 Electronic EngineeringElectronic EngineeringELE4NET: Signal EncodingELE4NET: Signal Encoding
,ulse -ode Modulation (cont’d)
! ;0 we use an H%it sa.le which allows G5I quanti?ing levels the qualit& o0 the recovered voice signal is co.ara%le with that achieved via analog trans.ission3
! This i.lies that a data rate o0 H222 sa.les er second × H %its er sa.le I4 8%s is needed 0or a single voice signal3
,-M %loc8 diagra.
Delta Modulation
! variet& o0 techniques have %een used to i.rove the
er0or.ance o0 ,-M or to reduce its co.le@it&3 One o0
the .ost oular alternatives to ,-M is delta .odulation
(DM)3
! >ith DM analog inut is [email protected] %& a staircase
0unction that .oves u or down one quanti?ation level (δ)
at each sa.ling interval3
! The staircase 0unction has %inar& %ehavior
– Bence encode each sa.le as a single %inar& digit
– 1 0or u 2 0or down
Delta Modulation [email protected]
DM J ,-M ,er0or.ance
! Two i.ortant ara.eters in DM: ste si?e δ and sa.ling rate3
! >hen the analog wave0or. changes ver& slowl& there will %e quanti?ing noise3 This noise increases as δ increases3 >hen the analog wave0or. changes .ore raidl& than the staircase can 0ollow there is sloe overload noise3 This noise increases as δ decreases3 So δ .ust %e chosen to %alance two t&es o0 noise3
! The accurac& o0 DM can %e i.roved %& increasing the sa.ling rate3 Bowever this increases the data rate o0 the outut signal3
! DM is si.ler to i.le.ent than ,-M3
! ,-M e@hi%its %etter S"# characteristics at the sa.e data rate3
! 'andwidth issue: 0or good voice reroduction with ,-M – 1GH levels (K %its)
– >ith voice %andwidth 4 8B? we need H222 @ K 5I8%s this digital signal could require on the order o0 GH 8B? o0 %andwidth3
nalog Data nalog Signals
! nalog data can %e .odulated %& a carrier 0requenc& to roduce an analog signal in a di00erent 0requenc& %and which can %e utili?ed on an analog trans.ission s&ste.3
! >h& .odulate analog signalsA – higher 0requenc& is needed 0or e00ective trans.ission e3g3 voice
signals are transmitted over telephone lines at their original spectrum (baseband transmission), for unguided transmission, it is virtually impossible to transmit baseband signals.
– ,er.its 0requenc& division .ultile@ing (chater H)
! T&es o0 .odulation – .litude modulation (AM)
– +requenc& modulation (FM)
– ,hase modulation (PM)
Encoding and Modulation Techniques
Encoding and Modulation Techniques
Digital Data Digital Signal
! Digital signal – Discrete discontinuous voltage ulses
– Each ulse is a signal ele.ent
/ser inut at a ,- is converted into a strea. o0 %inar&
digits (1s and 2s)3 'inar& 1 is reresented %& 5v and
%inar& 2 %& 65v3
So.e Ter.s
! /niolar – ll signal ele.ents have sa.e sign (all ositive or all negative)
! ,olar – One logic state reresented %& ositive voltage the other %&
negative voltage
! Data signaling rate (or 7ust data rate) – #ate o0 data trans.ission in %its er second (%s)
! Duration or length o0 a %it – Ti.e ta8en 0or trans.itter to e.it the %it (0or data rate # %it
duration is 19#)
– Measured in %aud signal ele.ents er second
;nterreting Signals
! To interret digital signals the receiver needs to 8now – Ti.ing o0 each %it when a %it starts and ends
– Signal level (high <2= or low <1=) 0or each %it osition
! +actors a00ecting success0ul interreting o0 signals – Signal to noise ratio (S"#)
– Data rate
– Encoding sche.e (i3e3 mapping data bits to signal elements)
! >ith other 0actors held constant the 0ollowing are true: – n increase in data rate increases %it error rate ('E#)3
– n increase in S"# decreases 'E#3
we need to consider
! Signal Sectru. – Lac8 o0 high 0requencies reduces required %andwidth
– Lac8 o0 dc co.onent allows ac couling via trans0or.er roviding electrical isolation
– -oncentrate ower in the .iddle o0 the trans.ission %andwidth
! -loc8ing – S&nchroni?ing trans.itter and receiver
– E@ternal cloc8 or sel0 cloc8ingA
! Error detection – #esonsi%ilit& o0 data lin8 control
– 'ut can %e %uilt into signal encoding to detect errors .ore quic8l&
! Signal inter0erence and noise i..unit& – So.e codes are %etter than others er0or.ance .easured %& 'E#
! -ost and co.le@it& – Bigher signaling rate lead to higher costs
– So.e codes require signaling rate greater than data rate
"onreturn to $eroLevel ("#$L)
! Two di00erent voltage levels 0or two %inar& %its
! Coltage constant during %it interval
– no transition i3e3 no return to ?ero voltage
! ;.le.entations:
– /niolar (a%sence o0 voltage 0or one constant ositive voltage 0or
?ero)
– ,olar (negative voltage 0or one value and ositive 0or the other)
2 high level
1 low level
"onreturn to $ero ;nverted ("#$;)
! -onstant voltage ulse 0or duration o0 %it
! Data encoded as resence or a%sence o0 signal transition at %eginning o0 %it ti.e
! Transition (low to high or high to low) denotes a %inar& 1
! "o transition denotes %inar& 2
! "#$; is a t&e o0 di00erential encoding – Di00erential encoding rule: i0 the current %it is a %inar& 2 then the
current %it is encoded with the sa.e signal as the receding %it i0 the current %it is a %inar& 1 then the current %it is encoded with a di00erent signal than the receding %it3
"#$ ,ros and -ons
! ,ros – Eas& to engineer
– Ma8e good use o0 %andwidth (requires hal0 the %andwidth o0 Manchester in so.e cases)
! -ons – dc co.onent
– Lac8 o0 s&nchroni?ation caa%ilit&
e3g3 with a long string o0 1s or 2s 0or "#$L or a long string o0 2s 0or "#$; the outut is a constant voltage over a long eriod o0 ti.e3 So an& dri0t %etween the cloc8s o0 trans.itter and receiver will result in loss o0 s&nchroni?ation %etween the two
! /sed 0or digital .agnetic recording
! "ot o0ten used 0or signal trans.ission
! 'iolarM; (lternate Mar8 ;nversion)
– 2’ reresented %& no line signal (2v)
– 1’ reresented %& alternating ositive and negative ulses (e3g3 65 5 65)
– 'inar& 1 ulses alternate in olarit&
– "o loss o0 s&nc i0 a long string o0 1s occurs (2s still a ro%le.)
Multilevel 'inar& – ,seudoternar&
– 1’ reresented %& no line signal (2v)
– 2’ reresented %& alternate ositive and negative ulses (e3g3
65 5 65)
– 'inar& 2 ulses alternate in olarit&
– "o loss o0 s&nc i0 a long string o0 2s occurs (1s still a ro%le.)
– "o advantage or disadvantage over %iolarM;
! /se .ore than two voltage levels to reresent data: – 'iolarM; (lternate Mar8 ;nversion)
– ,seudoternar&
– Less %andwidth than "#$
– "o loss o0 s&nchroni?ation i0 a long string o0 1s occurs in 'iolar M; or a long string o0 2s in ,seudoternar&
! Disadvantages: – S&nchroni?ation ro%le. with long runs o0 2s in the case o0 M;
or 1s in the case o0 seudoternar&
– "ot as e00icient as "#$ ! Each signal ele.ent onl& reresents one %it (although 0or Flevel
s&ste.s each signal ele.ent could reresent logGF 135H %its)
– #eceiver .ust distinguish %etween three levels (6 2)
'ihase – Manchester
! Transition serves as cloc8 and data
! Low to high reresents one
! Bigh to low reresents ?ero
! Seci0ied in ;EEE H2G3F (Ethernet) 0or %ase%and coa@ial
ca%le and twistedair %us L"s
'ihase – Di00erential Manchester
! lwa&s a transition in .iddle o0 interval
! Transition at start o0 a %it eriod reresents ?ero
! "o transition at start o0 a %it eriod reresents one
! t least one – ossi%l& G transitions er %it ti.e
! T&es: – Manchester
as a cloc8)
– "o dc co.onent
– 'uiltin error detection (a%sence o0 an e@ected transition can %e used to detect errors)
! Disadvantages – [email protected]. .odulation rate is twice that 0or "#$
– #equires .ore %andwidth than .ultilevel %inar& codes
%its er second) and
rate is 19T b where T b
%it duration3
signal ele.ents are
twice that o0 "#$;3
Scra.%ling
! Desite the sel0cloc8ing .echanis. %ihase codes are not widel& used in longdistance alications %ecause the& require a high signaling rate relative to data rate
! Scra.%ling – use 0illing sequences to relace sequences that would roduce constant voltage
! #equire.ents: – Must roduce enough transitions to .aintain s&nc
– Must %e recogni?ed %& receiver and relaced with original sequence
– Sa.e length as original
– "o dc co.onent
– "o reduction in data rate
– Error detection caa%ilit&
! ;.le.entations: – 'H$S (%iolar with H?eros su%stitution)
– BD'F (highdensit& %iolarF ?eros)
'H$S ('iolar with H$eros Su%stitution)
! 'ased on %iolarM;
! ;0 an octet o0 all ?eros occurs and last voltage ulse receding this octet was ositive encode as 222626
! ;0 an octet o0 all ?eros occurs and last voltage ulse receding this octet was negative encode as 222626
! -auses two violations o0 M; code an event unli8el& to occur as a result o0 noise
BD'F (BighDensit& 'iolarF $eros)
! 'ased on %iolarM;
! String o0 4 ?eros relaced with one or two ulses with 4 th ?ero relaced with a code violation
'valid %iolar signal
C'iolar violation
Digital Data nalog Signal
• Transmitting digital data using analog signals
! ,u%lic telehone s&ste. – Coice 0requenc& range F22B? to F422B?
– /se .ode. (.odulatorde.odulator) which converts digital data to analog signals, and vice versa
! Modulation involves oeration on one or .ore o0 the three characteristics o0 a carrier signal: a.litude 0requenc& and hase3
! Encoding9.odulation techniques – .litude shi0t 8e&ing (S*)
– +requenc& shi0t 8e&ing (+S*)
– ,hase shi0t 8e&ing (,S*)
.litude Shi0t *e&ing
! Two %inar& values reresented %& two di00erent a.litudes o0 the carrier 0requenc&
! /suall& one a.litude is ?ero
– i3e3 one %inar& digit is reresented %& the resence at constant a.litude o0 the carrier the other %& the a%sence o0 the carrier3
! Susceti%le to sudden gain changes
! ;ne00icient .odulation
! /sed to trans.it digital
cπ
! Most co..on 0or. is %inar& +S* ('+S*)
! Two %inar& values reresented %& two di00erent 0requencies (near carrier)
where 0 1 and 0 G are o00set 0ro. the carrier 0requenc& %& equal %ut oosite a.ounts
! Less susceti%le to error than S*
! / to 1G22%s on voicegrade lines
π
π
.ore than two
,hase Shi0t *e&ing
! ,hase o0 carrier signal is shi0ted to reresent data
! 'inar& ,S* – Two hases reresent two %inar& digits
! Di00erential ,S* – ,hase shi0t is relative to
revious trans.ission rather
sending a signal burst of the
same phase as the previous
signal burst sent. Binary 1 is
represented by sending a
to the preceding one.
cc
c
Modulation o0 nalog Signals 0or Digital Data
nalog Data Digital Signal
! Trans0or.ing analog data into digital signals
! nalog data such as voice and video is o0ten digiti?ed to %e a%le to use digital trans.ission 0acilities3 Strictl& sea8ing the rocess o0 converting analog data into digital data is 8nown as digitization3
! Once analog data have %een converted into digital data the 0ollowing can haen: – The digital data can %e trans.itted using "#$L3 ;n this case we
have in 0act gone directl& 0ro. analog data to a digital signal3
– The digital data can %e encoded as a digital signal using a code other than "#$L3 Thus an e@tra ste is required3
– The digital data can %e converted into an analog signal via S* +S* or ,S*3
–
,ulse -ode Modulation
! ,-M is %ased on sampling theorem: ;0 a signal is
sa.led at regular intervals at a rate higher than twice the
highest signal 0requenc& the sa.les contain all the
in0or.ation o0 the original signal3
! Coice data are li.ited to %elow 4222B?3 #equire H222
sa.les er second3
! Bowever these are analog sa.les called ulse a.litude
.odulation (,M) sa.les3
! To convert to digital each o0 these analog sa.les .ust %e
assigned a %inar& code3
3030 Electronic EngineeringElectronic EngineeringELE4NET: Signal EncodingELE4NET: Signal Encoding
,ulse -ode Modulation (cont’d)
! ;0 we use an H%it sa.le which allows G5I quanti?ing levels the qualit& o0 the recovered voice signal is co.ara%le with that achieved via analog trans.ission3
! This i.lies that a data rate o0 H222 sa.les er second × H %its er sa.le I4 8%s is needed 0or a single voice signal3
,-M %loc8 diagra.
Delta Modulation
! variet& o0 techniques have %een used to i.rove the
er0or.ance o0 ,-M or to reduce its co.le@it&3 One o0
the .ost oular alternatives to ,-M is delta .odulation
(DM)3
! >ith DM analog inut is [email protected] %& a staircase
0unction that .oves u or down one quanti?ation level (δ)
at each sa.ling interval3
! The staircase 0unction has %inar& %ehavior
– Bence encode each sa.le as a single %inar& digit
– 1 0or u 2 0or down
Delta Modulation [email protected]
DM J ,-M ,er0or.ance
! Two i.ortant ara.eters in DM: ste si?e δ and sa.ling rate3
! >hen the analog wave0or. changes ver& slowl& there will %e quanti?ing noise3 This noise increases as δ increases3 >hen the analog wave0or. changes .ore raidl& than the staircase can 0ollow there is sloe overload noise3 This noise increases as δ decreases3 So δ .ust %e chosen to %alance two t&es o0 noise3
! The accurac& o0 DM can %e i.roved %& increasing the sa.ling rate3 Bowever this increases the data rate o0 the outut signal3
! DM is si.ler to i.le.ent than ,-M3
! ,-M e@hi%its %etter S"# characteristics at the sa.e data rate3
! 'andwidth issue: 0or good voice reroduction with ,-M – 1GH levels (K %its)
– >ith voice %andwidth 4 8B? we need H222 @ K 5I8%s this digital signal could require on the order o0 GH 8B? o0 %andwidth3
nalog Data nalog Signals
! nalog data can %e .odulated %& a carrier 0requenc& to roduce an analog signal in a di00erent 0requenc& %and which can %e utili?ed on an analog trans.ission s&ste.3
! >h& .odulate analog signalsA – higher 0requenc& is needed 0or e00ective trans.ission e3g3 voice
signals are transmitted over telephone lines at their original spectrum (baseband transmission), for unguided transmission, it is virtually impossible to transmit baseband signals.
– ,er.its 0requenc& division .ultile@ing (chater H)
! T&es o0 .odulation – .litude modulation (AM)
– +requenc& modulation (FM)
– ,hase modulation (PM)