Digital circuits in electronics and communication eng

Digital circuit

1. UNIT-I DIGITIZATION TECHNIQUES for Analog Messages TOPICS: Introduction - Importance of Digitization Techniques Elements of Pulse Code Modulation (PCM) - Generation and Reconstruction, Quantization and Coding Quantization Error, PCM with Noise Companding in PCM Time Division Multiplexing (TDM) Delta Modulation Adaptive Delta Modulation Differential PCM Systems (DPCM) Adaptive Differential PCM System

2. INTRODUCTION – Importance of Digitization Techniques Digitization is the process of converting analog message into a digital format. Digitization is of crucial importance to - data processing , storage, and transmission, because it " allows information of all kinds in all formats to be carried with the same efficiency and also intermingled ." Though analog data is typically more stable, digital data has the potential to be more easily shared and accessed.

3. 3 ELEMENTS OF A DIGITAL COMMUNICATION SYSTEM Block diagram of a communication system: Fig.1: Basic block diagram of Digital Communication system The Overall purpose of this system is to transfer information from one point (called Source) to another point, the user destination.

4. SOURCE ENCODER / DECODER: The Source encoder ( or Source coder) converts the input i.e. symbol sequence into a binary sequence of 0‟s and 1‟s by assigning code words to the symbols in the input sequence . At the receiver, the source decoder converts the binary output of the channel decoder into a symbol sequence. Aim of the source coding is to remove the redundancy in the transmitting information, so that bandwidth required for transmission is minimized. CHANNEL ENCODER / DECODER: Error control is accomplished by the channel coding operation that consists of systematically adding extra bits to the output of the source coder . -The Channel decoder recovers the information bearing bits from the coded binary stream. Error detection and possible correction is also performed by the channel decoder.

5. MODULATOR: The Modulator converts the input bit stream into an electrical waveform suitable for transmission over the communication channel. -Modulator can be effectively used to minimize the effects of channel noise, to match the frequency spectrum of transmitted signal with channel characteristics, to provide the capability to multiplex many signals. DEMODULATOR: The extraction of the message from the information bearing waveform produced by the modulation is accomplished by the demodulator. The output of the demodulator is bit stream. The important parameter is the method of demodulation. CHANNEL: The Channel provides the electrical connection between the source and destination. The different channels are: Pair of wires, Coaxial cable, Optical fibre , Radio channel, Satellite channel or combination of any of these.

6. PCM Generation and Reconstruction Figure 12.1–1 a diagrams the functional blocks of a PCM generation system. The analog input waveform x(t) is lowpass filtered and sampled to obtain . A quantizer rounds off the sample values to the nearest discrete value in a set of q quantum levels. The resulting quantized samples are discrete in time ( by virtue of sampling) and discrete in amplitude (by virtue of quantizing). The parameters M, n, and q should be chosen to satisfy the equality, so that

7. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. (a) PCM generation system (b) quantization characteristic Figure 12.1-1

8. Thus, the number of quantum levels for binary PCM must equal some power of 2 , namely

9. QUANTIZATION ERROR Fig . 12.1–5 more convenient for the analysis of quantization noise. Here, a pulse converter in place of the sample-and-hold circuit generates the weighted impulse train.

10. PCM WITH NOISE In this section we account for the effects of random noise in PCM transmission. The resulting digital errors produce decoding noise. Random noise added to the PCM signal at the receiver causes regeneration errors that appear as erroneous digits in the codewords . The decoder then puts out a different quantum level than the one intended for a particular sample. Hence,

11. COMPANDING IN PCM The word Companding is a combination of Comp ressing and Exp anding , which means that it does both. This is a non-linear technique used in PCM which compresses the data at the transmitter and expands the same data at the receiver. The effects of noise and crosstalk are reduced by using this technique. Nonuniform quantizers are difficult to make and expensive . An alternative is to first pass the speech signal through a nonlinear amplifier before quantizing with a uniform quantizer . The nonlinearity causes the signal amplitude to be compressed . At the receiver, the signal is Expanded by an inverse to the nonlinearity . The process of compressing and expanding is called COMPANDING .

12. Eeng 360 18 Nonuniform Quantization Many signals such as speech have a nonuniform distribution. The amplitude is more likely to be close to zero than to be at higher levels. Nonuniform quantizers have unequally spaced levels The spacing can be chosen to optimize the SNR for a particular type of signal. 2 4 6 8 2 4 6 -2 -4 -6 Input sample X Output sample X Q -2 -4 -6 -8 Example: Nonuniform 3 bit quantizer

13. Eeng 360 19 A-law and m- law Companding These two are standard companding methods. u -Law is used in North America and Japan A-Law is used elsewhere to compress digital telephone signals

14. SNR of Compander The output SNR is a function of input signal level for uniform quantizing. But it is relatively insensitive for input level for a compander

15. TIME DIVISION MULTIPLEXING Definition : Time Division Multiplexing (TDM) is the time interleaving of samples from several sources so that the information from these sources can be transmitted serially over a single communication channel. At the Transmitter Simultaneous transmission of several signals on a time-sharing basis. Each signal occupies its own distinct time slot, using all frequencies, for the duration of the transmission. Slots may be permanently assigned on demand. At the Receiver Decommutator (sampler) has to be synchronized with the incoming waveform  Frame Synchronization Low pass filter ISI – poor channel filtering Feedthrough of one channel's signal into another channel -- Crosstalk Applications of TDM: Digital Telephony, Data communications, Satellite Access, Cellular radio.

16. Time Division Multiplexing Conceptual diagram of multiplexing-demultiplexing. PAM TDM System

17. DELTA MODULATION Delta modulation is a process mainly used in the transmission of voice information . It is a technique where analog-to-digital and digital-to-analog signal conversion are seen. Delta modulation (DM) is an easy way of DPCM.