chapter-3-Pulse Carrier Modulation (1).pptx

chapter-3-PCM (1).pptx

1. 2023 Communication system II ( ECTe 3032 ) By: AD 1 Chapter Three Pulse Modulation Techniques Modulator Channel Demodulator Serial data symbols ‘analogue’ channel pulses Recovered data symbols

2. Overview 2 Introduction Analogue Pulse Modulation PAM PWM PPM Digital Pulse Modulation PCM Sampling Quantization Coding DM 2

3. Introduction Systems in which a series of recurring pulses are made to vary in amplitude, duration, shape & time, as a function of modulating signal . Analog pulse modulation: A pulse train is used as the carrier wave. Some characteristic features of each pulse (e.g., amplitude, duration, or position) is used to represent message samples . There are various Pulse Modulation schemes Pulse Amplitude Modulation Pulse Width Modulation Pulse Position Modulation Pulse Code Modulation Delta Modulation 3 Analog Pulse Modulation Digital Pulse Modulation

4. Advantages of PM Noise immunity Inexpensive digital circuitry Can be time division multiplexed with other pulse modulated signal Transmission distance is increased through the use of regenerative repeaters Digital pulse streams can be stored Error detection and correction is easily implemented 4

5. Disadvantages of PM Require greater BW to transmit and receive as compare to its analog counterpart For high transmission rates, time need specialized encoding techniques Pulse coded stream is difficult to recover For information recovery, it need synchronization between transmitter & receiver. 5

6. Analogue Pulse Modulation Pulse Amplitude Modulation (PAM) Pulse Position Modulation ( PPM ) Pulse Width Modulation ( PWM ) 6

7. Pulse Amplitude Modulation (PAM) PAM is an engineering term that is used to describe the conversion of the analog signal to a pulse-type signal in which the amplitude of the pulse denotes the analog information . In the PAM, the amplitude of periodic pulse train is varied with a amplitude of the corresponding sample value of a continuous message signal. In PAM: width and position are fixed but amplitude varies. 7

8. Sampling The sampling theorem states that a band-limited signal x(t) with a frequency, f can be reconstructed from its sample values if the sampling rate (frequency) fs =1/ T s is greater than or equal to twice the frequency of x(t) The minimum sampling rate of fs for an analog band-limited signal is called the Nyquist rate . 8 There are 3 sampling methods: Ideal - an impulse at each sampling instant. Natural - a pulse of short width with varying amplitude. Flattop - sample and hold, like natural but with single amplitude value

9. Pulse Width Modulation (PWM) In PWM, the amplitude is maintained constant but the width of each pulse is varied in accordance with instantaneous value of the analog signal . 9

10. In this type, the sampled waveform has fixed amplitude and width whereas the position of each pulse is varied as per instantaneous value of the analog signal. Pulse Position Modulation (PPM) 10

11. Digital Pulse Modulation Pulse Code Modulation Pulse Code Modulation is a process through which an analogue signal can be represented (approximated) by a digital signal PCM is a three step process which includes Sampling/Pulse amplitude modulation Quantization Coding Analog to Digital conversion is really a three step process involving Sampling Conversion from continuous-time, continuous valued signal to discrete-time, continuous-valued signal Quantization Conversion from discrete-time, continuous valued signal to discrete-time, discrete-valued signal Coding 11

12. Basic Elements of PCM 12

13. Analog to Digital Conversion 13 The Analog-to-digital Converter (ADC) performs three functions: Sampling Makes the signal discrete in time . If the analog input has a bandwidth of W Hz, then the minimum sample frequency such that the signal can be reconstructed without distortion. Quantization Makes the signal discrete in amplitude. Round off to one of q discrete levels. Encode Maps the quantized values to digital words that are n bits long. Sample Quantize Encode Digital Output Signal 111 111 001 010 011 111 011 111 110 101 100 011 010 001 000 ADC Analog Input Signal

14. Sampling A continuous-time signal has some value defined at every time instant . So it has infinite number of sample points 14

15. Shannon- Nyquist Sampling Theorem… Critical Sampling When the sampling frequency is chosen to be equal to twice the max. frequency component ( Fs=2Fmax ). Ideally, we should be able to recover the Analog signal from Digital samples. Under Sampling When the sampling frequency is chosen to be less than twice the max. frequency component ( Fs<2Fmax ). We would not be able to recover the analog signal from Digital samples. Over Sampling When the sampling frequency is chosen to be greater than twice the max. frequency component We would be able to recover the Analog signal from Digital samples It would also help to control the effect of Aliasing 15

16. … cont’d 16

17. Examples For the following analog signal, find the Nyquist sampling rate, also determine the digital signal frequency and the digital signal ‹#›

18. Quantization Now we have converted the continuous-time, continuous-valued signal into a discrete-time, continuous-valued signal We still need to make it discrete valued => Quantization. “The process of converting analog voltage with infinite precision to finite precision ” e.g . if a digital processor has a 4-bit word, the amplitudes of the signal can be segmented into 16 levels 18

19. … cont’d 19

20. General rules for Quantization 20

21. Example 21

22. Coding Conversion from a discrete-time, discrete-valued signal to an efficient digital data format Represent as bit. 22

23. Encoding Techniques (Line Codes) ‹#› A  line code  is the code used for data transmission of a digital signal over a transmission line Used to avoid overlap and distortion of signal such as inter-symbol interference Types of Line coding

24. Coding: Uni -Polar encoding Unipolar signaling is also called as  On-Off Keying  or simply OOK In uni -polar encoding, only two voltage levels are used. It uses only one polarity of voltage level 24 Unipolar Return to Zero RZ 1- Half of the bit duration remains high but it immediately returns to zero and shows the absence of pulse during the remaining half of the bit duration 0-  absence of pulse  Unipolar Non-Return to Zero NRZ 1- a High in data is represented by a positive pulse 0- A Low in data input has no pulse

25. Polar encoding Polar encoding technique uses two voltages One positive level and the other one is negative Four different polar encoding techniques 25

26. Non-return to Zero-Level (NRZ-L) Two different voltages (+ and –) for 0 and 1 bits Voltage constant during bit interval More often, negative voltage for one value and positive for the zero ‹#›

27. Non-return to Zero Invert (NRZ-I) Considered as a differential encoding techniques because of its level is a function of the transition at the beginning of the signal element. If signal is logic 0 , there is no change to opposite logic level, either at beginning or throughout cell. If signal is logic 1 , then transition occurs at beginning of the cell and logic level change to the opposite value throughout cell. 27

28. Return to Zero (RZ) In Bipolar RZ encoding , two non-zero voltages are used; a logic 1 and 0 alternate in polarities, each taking half the bit time before running to zero. Bipolar RZ is a self clocking code ‹#›

29. Self Clocking Codes A self-clocking signal is one that can be decoded without the need for a separate  clock signal  or other source of  synchronization Self clocking codes are encoding techniques used to ensure that each bit time associated with the binary serial bit stream contains at least one level transition (1 to 0, or 0 to 1). Self clocking codes include Bipolar RZ, Manchester and differential Manchester, Miller . Telephones trunks circuits uses a self clocking RZ encoding called bipolar with 8 zeros substitution(B8ZS ) ‹#›

30. Manchester Encoding In Manchester encoding, a logic 1 is represented with low-to-high transition in the center(or beginning) of the cell and Logic 0 is represented with a high-to-low transition in the center(or beginning) of the cell Ethernet LAN systems employ Manchester encoding 30

31. 31 In differential Manchester, a transition from low-to-high or high-to-low occurs at center of each cell, which provides for self clocking mechanism. A logic 1 or 0 are determined by the logic level of the previous cell . If the previous cell is a logic 1 , there is no change at the beginning of the current cell. If the previous cell is a logic 0 , then a transition occurs at the beginning of the current cell. Differential Manchester

32. Miller Encoding Self clocking techniques in which a transition occurs at the center of each cell for logic 1s only No transition is used for a logic 0 unless it is followed by another 0 , in which case transition is placed at the end of the cell for the first 0 Miller encoding used in digital magnetic recording . ‹#›

33. Bi-polar encoding Bi-polar, uses the three voltage levels (+, -, 0) ‹#›