02-Tw2103eu01_eg0001_Principles of PCM Technology

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Principles of PCM Technology Siemens

2 Principles of PCM Technology

Contents Pages

1 Principles of PCM Technology 12 Objectives 23 Basic Components of a PCM System 34 Conversion nalog ! "igital 3# Ban$ %imiting 4& Sampling 4' (nco$ing #) M*ltiple+ing ), M*ltiple+ing - "em*ltiple+ing ,1. S*mmary 11

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Siemens Principles of PCM Technology

2.1 Objectives

0pon completion of this chapter yo* ill *n$erstan$ the principles of convertinganalog telephone signals into $igital PCM signals as ell as the principle of signaltransmission5

6o* ill be able to

7ame an$ e+plain the in$ivi$*al steps of analog!$igital signal conversion

"escribe the m*ltiple+ing an$ $em*ltiple+ing of binary co$es

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2.2 Basic Components of a PCM System

P*lse co$e mo$*lation 8PCM9 converts an analog aveform into a $igital signal5 PCM signal can be transmitte$ either alone or interleave$ ith the binary co$es ofother PCM signals5

(very PCM system has the folloing basic components:

Fig. 1

2.3 Analog / Digital Convesion

Three steps are re;*ire$ to convert an analog telephone signal into a $igital signal

Fig. 2

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2.3.1 Ban! "imiting

/ith the ai$ of a lopass filter the a*$ible fre;*encies are limite$ to 3.. to 34.. es samples from the telephone signals at reg*lar intervals5

Fig. 4

Warning: The sampling fre;*ency m*st be higher than tice the highest fre;*encyof the analog signal5

n international sampling fre;*ency of )...


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The first stage in the conversion to a $igital signal is the ;*anti=ing of the PMsignal5 Aor this the hole range of possible amplit*$e val*es is $ivi$e$ into

;*anti=ing intervals 8in o*r e+ample 1& intervals b*t generally 2#& are *se$95

Fig. 7

The ;*anti=ing intervals are n*mbere$ 1 to ) in the positive range an$ -1 to -) inthe negative range of the telephone signal5

The appropriate ;*anti=ing interval is $etermine$ for each sample5

Fig. 8

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Decision valuesform the bo*n$aries beteen a$jacent ;*anti=ing intervals5 On thetransmitter si$e several $ifferent analog val*es fall ithin the same ;*anti=inginterval5 On the receiver si$e onesignal val*e correspon$ing to the mi$point of the;*anti=ing interval is recovere$ for each ;*anti=ing interval5 This ca*ses small$iscrepancies to occ*r beteen the original telephone signal samples on thetransmitter si$e an$ the recovere$ val*es5 The $iscrepancy for each sample can be*p to half a ;*anti=ing interval5 The ;*anti=ing $istortion hich may arise on thereceiver si$e as a res*lt of this manifests itself as noise s*perimpose$ on the *sef*lsignal5 *anti=ing $istortion $ecreases hen the n*mber of ;*anti=ing intervals isincrease$5 Df the ;*anti=ing intervals are ma$e s*fficiently small the $istortion ill be

minimal an$ the noise imperceptible5

The secon$ stage is the enco$ing of the ;*anti=ing intervals5

Fig. 9

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2.3.$ M%ltiple&ing

The ) bit binary co$es of several telephone signals are transmitte$ consec*tively inrepeate$ cycles5 Beteen to binary co$es of the same telephone signal the binaryco$es of other telephone signals are aligne$ in consec*tive or$er5 This creates aPCM time$ivision m*ltiple+ signal5

Fig.10

Dn this e+ample the transmission of an ) bit binary co$e re;*ires only one thir$ of asampling interval

Fig. 11

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Dn the time left the binary co$es of 2 f*rther signals can be transmitte$5

Fig. 12

2.3.' M%ltiple&ing ( Dem%ltiple&ing

The processes involve$ in !ul"iple#ingare carrie$ o*t f*lly electronically5

To e+plain the principle of m*ltiple+ing an$ $em*ltiple+ing 4 PCM inp*t signals 8S1to S49 are sample$ cyclically by a rotating sitch 5

Sitch moves from one inlet to the ne+t synchronouslyith the incoming binary

co$es5 The PCM time$ivision m*ltiple+ signal is then available at the o*tlet ofsitch 5 the time interval ithin hich a PCM binary co$e is transmitte$ is >non asa time slot5

On the receiver si$e the in$ivi$*al PCM signals are recovere$ from the time$ivisionm*ltiple+ signal i5e5 the ) bit PCM binary co$es are $istrib*te$ to the respectiveo*tlets5

The m*ltiple+ing an$ $em*ltiple+ing process can only f*nction if the transmitter si$e8Sitch 9 an$ the receiver si$e 8sitch B9 r*n synchrono*sly5

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Aor this reason the transmitter si$e $oes not only relay the PCM signals to thereceiver si$e b*t also information for synchroni=ation5 This can be $one in ana$$itional time slot as ith PCM 3.5

s ith m*ltiple+ing processes on the transmitter si$e the %e!ul"iple#ingprocesses are controlle$ f*lly electronically5 The e+ample shos ho sitch B hichis synchroni=e$ ith sitch $istrib*tes the PCM binary co$es to the 4 o*tlets5

Bit se;*ence containing onePCM binary co$e from everyinp*t signal is >non asa pulse fra!e5 Dn the e+ample a p*lse frame consists of 4 consec*tive PCM binaryco$es one PCM binary co$e being from each of the inp*t signals S1 to S45

Fig. 13

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2.$ S%mmay

Pinciples of t)e PCM *ec)nology

Tas>s of the transmitter si$e:

Ban$ limiting of a telephone signal

Creation of a PCM signal 8by sampling the telephone signal9

*anti=ing an$ enco$ing samples

Dnterleaving of PCM binary co$es in a telephone signal ith the PCMbinary co$es from other telephone signals to form a PCM time$ivision m*ltiple+ signal

Tas>s of the receiver si$e:

"istrib*ting of binary co$es to the in$ivi$*al lines 8$em*ltiple+ing9

Eestoring a PM signal from the co$e or$ 8$eco$ing9

Eestoring the original analog telephone signal from the PM signal

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02-Tw2103eu01_eg0001_Principles of PCM Technology

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