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Digital Communications GATE Online Coaching Classes GATE Online Coaching Classes Digital Communications Online Class-4 By Dr.B.Leela Kumari Assistant Professor, Department of Electronics and Communications Engineering University college of Engineering Kakinada Jawaharlal Nehru Technological University Kakinada 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 1 Session -4 Baseband Transmission • Delta Modulation • advantages and Draw Backs • SNR of DM • Adaptive Delta Modulation • Comparisons • Objective Type questions and Illustrative Problems 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 2 Delta Modulation • By the DM technique an analog signal can be encoded in to bits .hence in one sense a DM is also PCM • IN DM difference signal is encoded into just a single bit ,hence in one sense a DM is also DPCM • A single bit produces just two possibilities that is used to increase or decrease the estimate 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 3 Block diagram of DM 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 4 The DM consists of Comparator Sample and Hold circuit Up-Down Counter D/A Converter 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 5 Comparator makes a comparison between the input base band signal m(t) and its quantized approximation Δ(t) =V(H) =V(L) Up-Down counter increments or decrements its count by one at each active edge of the clock waveform The count direction(incrementing or decrementing ) is determined by the voltage levels t the “count direction command “ input to the counter When this binary input which is also transmitted output S0(t) ,is at level V(H),the counter counts up, When it is at level V(L),the counter counts down The counter serves as accumulator D/ Converter: The digital output of the converter is converted to the analog quantized approximation by the D/ Converter 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 6 Response of a Delta Modulator 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 7 Operation Assume active clock edge is the falling edge For t< t1 , so that S0(t)=V(H) At t=t1,when the active clock edge appears , the counter is incremented and immediately the signal Jumps up by an mount equal to the step size S At t=t2 still so S0(t)=V(H) And there is another upward jump in At t=t3 so S0(t)=V(L) The counter decrements and there is consequent downward jump in by n mount S and so on. 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 8 Delta Modulator Transmitter and Receiver 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 9 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 10 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 11 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 12 • The input given to the decoder is processed and that output is summed up with the output of the predictor, to obtain a better output. mq(n)=eq (n)+mˆ(n-1) 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 13 Advantages of Delta Modulation 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 14 Draw Backs of Delta Modulation Hunting Effect Slope overload Error Granular Error 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 15 Hunting Effect At start up there my be a brief interval when my be a poor approximation To the base band signal m(t) Hence an initial large discrepancy Further even when has caught up to m(t),and even m(t) remains unvarying Hunts swinging up or down ,above and below m(t) 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 16 Slope overload Error The recovered waveform will be distorted when the slope of m(t) is greeter than the slope of and the DM system is said to be have Slope overload distorted 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 17 Slope overload can be avoided if S/Ts > |dm(t)/dt|max If the base band signal is a sinusoidal signal of amplitude A and frequency f, m(t)= A Cos 2πft S/Ts > |dm(t)/dt|max S/Ts > 2πfA To decrease the slope overload error ,increase the step size 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 18 Granular Error If the variations in m(t) are such that they are with in the step size ,then the is like square wave. This will be recovered as DC, but original signal m(t) is not DC. The noise due to this reason is known as Granular noise Granular noise comes because chosen value of step size is greeter than optimum step size. To reduce the slope overload error , step size needs to be decreased 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 19 Noise in Delta Modulation system 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 20 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 22 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 23 3 • (SNR)DM = (3/80)(fb/fM) • The actual value of SNR is greater than the value given above • It is found that the value of SNR is 3 (SNR)DM = (3/64)(fb/fM) 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 24 Comparison between PCM and DM • DM needs Simple circuit • Maximum possible error is ‘S’ in DM as S/2 in PCM. • Hence low SNR • For voice transmission the bit rate is 56kbps in PCM where as in DM for the same quality the bit rate is much higher • For good quality transmission , the band width needed by DM is more than PCM 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 25 DM can be recommended for following situations When band width conservation is desirable When the simple circuitry is of utmost importance 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 26 Adaptive Delta Modulation • DM are applicable for signals which are having constant slope. • If the slope of the signal is varying continuously, the step size needs to be changed , which is the basic principle of operation of ADM. • ADM is a modification of DM, in which the step size is not kept fixed 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 27 Adaptive Delta Modulation 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 28 The ADM consists of Comparator Sample and Hold circuit Digital Processor D/A Converter 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 29 • Digital Processor: it has an accumulator and at each active edge of the clock wave form , generates step S which augments or diminishes the accumulator. • The step S is not of fixed size but it is always a multiple of basic step S0 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 30 Response of a Adaptive Delta Modulator 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 31 Generation of S: The K th active clock edge generates step equal in magnitude to the step generated in response to the (k-1)st clock edge. This step is added to or subtracted from the accumulator, as required to move toward m(t) If the direction of the step k is same as at edge (k-1) , then the processor increases the magnitude of the step by an amount S0 If the directions re opposite then the processor decreases the magnitude of the step by an amount S0 S the algorithm is carried out there are clock edges when the total step S=0. In this case at the next clock edge the step is S0 in the direction gin to move toward m(t) The out put S0(t) is called e(k) which represents the error e(k) = +1 if immediately before the kth edge -1 if immediately before the kth edge at sampling time k, the step size S(k) is to be given by S(k) = |S(k-1)| e(k)+ S0 e(k-1) 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 32 • as long as the condition persists, the jump in becomes progressively larger, so that the estimate catches up with m(t) sooner than would be the case LDM • For large slope in m(t), develops large jumps. • The ADM system reduces Slope overload error but the quantization error is increased. • The ADM can operate at bit rates of 32kbps with performance comparable to that obtained when using PCM t 64kbps 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 33 Receiver of Adaptive Delta Modulator 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 34 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 35 Comparisons 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 36 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 37 Objective Type questions 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 38 1. In Delta modulation, a. One bit per sample is transmitted b. All the coded bits used for sampling are transmitted c. The step size is fixed d. Both a and c are correct 2. In digital transmission, the modulation technique that requires minimum bandwidth is a. Delta modulation b. PCM c. DPCM d. PAM 6/24/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 39 3. In Delta Modulation, the bit rate is a. N times the sampling frequency b. N times the modulating frequency c.
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