4.2 Digital TransmissionOutlinesPulse Modulation (Part 2.1)Pulse Code Modulation (Part 2.2)Delta Modulation (Part 2.3)Line Codes (Part 2.4) Basic scheme of PCM systemQuantizationQuantization ErrorCompandingBlock diagram& function of TDMPCM communication system Basic scheme of PCM systemThe most common techni!ue for using digital signals to encode analog data is PCM"Example: To transfer analog voice signals of a local loop to digital end ofce within the phone system, one uses a codec. Cont#d"""Because $oice data limited to fre!uencies %elo& '((( )z* a codec makes +((( samples,sec" -i"e"* ./0 microsecond,sample1"2f a signal is sampled at regular inter$als at a rate higher than t&ice the highest signal fre!uency* the samples contain all the information of the original signal" PCM Block Diagram Most common form of analog to digital modulationFour step process1. Signal is sampled using PAM (Sample)2. Integer values assigned to signal (PAM)3. Values converted to inar! ("uanti#ed)$. Signal is digitall! encoded for transmission(%ncoded) 4 Steps Process Cont#d3 4nalog signal is sampled" Con$erted to discretetime continuousamplitude signal (Pulse mplitude !odulation" Pulses are quantized and assigned a digital $alue. 4 5%it sample allo&s ./+ !uantizing le$els" PCM uses nonlinear encoding* i"e"* amplitude spacing of le$els is nonlinear There is a greater num#er of $uanti%ing steps for low amplitude This reduces overall signal distortion. This introduces $uanti%ing error (or noise". PCM pulses are then encoded into a digital %it stream" +((( samples,sec 6 5 %its,sample 7 08 9%ps for a single $oice channel" PCM E6ample Quantization 4 process of con$erting an in:nite num%er of possi%ilities to a :nite num%er of conditions -rounding o; the amplitudes of acent steps"The resolution 7 the magnitude of a $uantum 7 the $oltage of the minimum step size"The $uanti%ation error 7 the !uantization noise7 ? !uantum 7 -orig" sample $oltage @ !uantize le$el1PCM code 7-sample $oltage,resolution1&ont'd( 4 di;erence %et&een the e6act $alue of the analog signal & the nearest !uantization le$el"QUANTIZATION ERROR Types of QuantizationMidtread Midrise Types of Quantizer1. Uniform type : The levels of the quantized amplitude are uniformly spaced. 2. Non-uniform type : The levels are not uniform. Dynamic Aange -DA1 Bargest possi%le magnitude,smallest possi%le magnitude" Chere DA 7 a%solute $alue of dynamic rangeDma6 7 the ma6imum $oltage magnitudeDmin 7 the !uantum $alue -resolution1n 7 num%er of %its in the PCM coderesolutionVVVDRmaxminmax 1 2 nDR) log( 20 ) ( DR dB DR E6ample .." Calculate the dynamic range for a linear PCM system using .8%it !uantizing"/" Calculate the num%er of %its in PCM code if the DA 7 .E/"8 dB Coding EFciency4 numerical indication of ho& eFciently a PCM code is utilized"The ratio of the minimum num%er of %its re!uired to achie$e a certain dynamic range to the actual num%er of PCM %its used"Coding EFciency 7 Minimum num%er of %its 6 .((4ctual num%er of %its Signal to Quantization Noise Ratio (SQR) The worst-ase !oltage "#$ "#$ %or a ma&imum in'ut signal The signal 'ower-to-(uanti)ing noise 'ower ratioeQresolutionSQR (min)eQVSQRmax(max) 1]1

12212) (2 22log 10) (log 10power noise onquantizati averagepower signal averagelog 10q qRvdBvRSQR) *resistance (o+m)v * rms signal voltage, * ,uanti#ation interval E6ample /." Calculate the =QA -dB1 if the input signal 7 / Drms and the !uantization noise magnitudes 7 ("(/ D"/" Determine the $oltage of the input signals if the =QA 7 G8"+/ dBand ! 7("/ D" Efect of Non-Linear Coding Nonlinear EncodingQuantization levels not evenly spacedReduces overall signal distortionCan also be done by companding Companding -+e process of compressing and t+en e.panding.-+e +ig+er amplitude analog signals are compressed prior to transmission and t+en e.panded in receiver. Improving t+e /) of a communication s!stem. Companding Functions Method of Companding *or the om'ression+ two laws are ado'ted, the -law in -" and .a'an and the A-law in /uro'e. -law 0-law The t1'ial !alues used in 'ratie are, 2233 and A245.6. 0%ter (uanti)ation the di7erent (uanti)ed le!els ha!e to 8e re'resented in a %orm suita8le %or transmission. This is done !ia an enoding 'roess.) 1 ln() 1 ln(maxmax++VVoutinVV' ++ +11ln 1) ln( 110ln 1maxmaxmaxoutinVVoutinVVoutVVA AAA VVAAVVininVma! "a uncompressed analo# input volta#eVin! amplitude of the input si#nal at a particular of instant timeVout! compressed output amplitude$% ! parameter define the amount of compression E6ample G 4 companding system &ith H 7 /00 used to compand from (D to .0 D sinusoid signal" Dra& the characteristic of the typical system"Dra& an + le$el nonuniform !uantizer characteristic that corresponds to the mentioned H" Cont#d"""01la2A1la2 PCM Line Speed The data rate at which serial PCM bits are clocked out of the PCM encoder onto the transmission line. WhereLine speed = the transmission rate in bits per secondSample/second = sample rate, fsBits/sample = no of bits in the compressed PCM codesamplebitsXseconsamplesspee line E6ample 'Ior a single PCM system &ith a sample rate fs 7 8((( samples per second and a 5 %its compressed PCM code* calculate the line speed" Virtues & Limitation of PCMThe most important advantages of PCM are:Robustness to channel noise and interference.Efcient regeneration of the coded signal along the channel path.Efcient exchange between BT and SNR.Uniform format for diferent kind of base-band signals.Flexible TDM. Cont#d3Secure communication through the use of special modulation schemes of encryption.These advantages are obtained at the cost of more complexity and increased BT.With cost-efective implementations, the cost issue no longer a problem of concern.With the availability of wide-band communication channels and the use of sophisticated data compression techniques, the large bandwidth is not a serious problem. TimeDi$ision Multiple6ingThis technique combines time-domain samples from diferent message signals (sampled at the same rate) and transmits them together across the same channel.The multiplexing is performed using a commutator (switch). At the receiver a decommutator (switch) is used in synchronism with the commutator to demultiplex the data" Cont#d3TDM system is $ery sensiti$e to sym%ol dispersion* that is* to $ariation of amplitude &ith fre!uency or lack of proportionality of phase &ith fre!uency" This pro%lem may %e sol$ed through e$uali%ation of %oth magnitude and phase" Jne of the methods used to synchronize the operations of multiple6ing and demultiple6ing is to organize the multiple6ed stream of data as frames &ith a special pattern" The pattern is kno&n to the recei$er and can %e detected $ery easily" Block diagramof TDM-PCM communication system EKD JI P4AT /"/