SINGLE SIDEBAND MODULATION USING TMS32020 DIGITAL SIGNAL PROCESSOR by RABINDER SINGH MOKHA School of Electrical Engineering & Computer Science University of New South Wales Kensington, NSW, Australia. A project report (18 credit points) submitted in partial fulfillment of the requirement for the degree of Master of Engineering Science April, 1988 UNIVERSITY OF N.S.W. 2 1 MAR 1989 LIBRARY I hereby declare that this submission is my own work and that, to the best of my knowledge and belief, it contains no material previously published or written by another person nor material which to a substantial extent has been accepted for the award of any other degree or diploma of a university or other institute of higher learning, except where due acknowledgement is made in the text. Signed Date ABSTRACT In modern day communications, single sideband (SSB) modulation is the preferred means of transmission of speech signals, where its main attractions are low power and minimum bandwidth requirements. Theoretically, Weaver’s method of SSB generation is superior to the other methods available. Though Weaver’s method was first presented in 1956, the inability to produce two identical paths in analog, with components like modulators and filters, lead to the non-recognition of this method. In digital systems, where signal processing involves only arithmetic operations and the processing components comprise of numbers only, identical paths can very readily be obtained. With the availibility of high speed digital signal processors like the TMS320 family from Texas Instruments, such systems, operating on software, are possible. The work presented in the thesis involves the implementation of Weaver’s method for speech transmission, using the TMS32020 digital signal processor. For efficient implementation, the design procedures for the processing components were studied in detail. With the aid of hardware flag pins available on the processor, a technique for synchronizing the transmitter and receiver oscillator frequencies has been presented. Finite wordlength effects introduced in the digital system were also considered. Using interpolation, design for achieving higher sampling rates has been proposed. Digital implementation of other SSB generation methods was considered and the results compared with those from the Weaver’s method. With the availibility of next generation digital signal processors, the work presented in the thesis would open new possibilities for the advancements in SSB modulation. ACKNOWLEDGEMENTS I would like to express my thanks to Dr.W.J.Dewar for his supervision, guidance, and encouragement throughtout the course of the thesis. Thanks are also extended to Dr.C.J.E.Phillips and Dr.E.H.Fooks for their useful hints and advise. I deeply appreciate the assistance given and the knowledge shared by Tom Millet and Joe Yiu, especially in the early stages of the thesis. Special thanks to Marek Szuszkiewicz for the healthy discussions and for making life much easier in the lab. I am grateful to my friends Amarjit and Ashwin for their constant support, good humour, and creating a cheerful environment. Finally, I must acknowledge my aunt without whose affection and support this work would never have eventuated. CONTENTS Abstract A cknowledgements Abbreviations CHAPTER 1 : INTRODUCTION 1 CHAPTER 2 : MODULATION TECHNIQUES 4 2.1 Introduction 2.2 Double sideband suppressed-carrier modulation 2.3 Amplitude modulation 2.4 SSB modulation CHAPTER 3 : SSB MODULATION 12 3.1 Introduction 3.2 Frequency discrimination method 3.3 Phase shifting method 3.4 Weaver’s method 3.5 Difficulty with Weaver’s method CHAPTER 4 : DIGITAL SSB SYSTEM 19 4.1 Introduction 4.2 Digital filters 4.3 Design of HR filters 4.4 Realization of digital filters 4.5 Recursive digital oscillator CHAPTER 5 : FINITE WORDLENGTH EFFECTS 32 5.1 Introduction 5.2 Arithmetic in digital systems 5.3 Coefficient quantization 5.4 Signal quantization 5.5 Roundoff noise 5.6 Overflow and Scaling 5.7 Finite wordlength in digital filters 5.8 Limit cycle oscillations 5.9 Coefficient sensitivity and roundoff noise CHAPTER 6 : TMS320 DIGITAL SIGNAL PROCESSOR 46 6.1 TMS320 family of Digital Signal Processors 6.2 TMS32020 DSP 6.3 Memory organization 6.4 Multiplication and addition 6.5 System control and speed utilization 6.6 Registers 6.7 Software Development System CHAPTER 7 : IMPLEMENTATION 58 7.1 Supporting Equipment 7.2 Weaver’s method - Implementation 7.3 Synchronization 7.4 Performance 7.5 Multiplexed system 7.6 Interpolation and Decimation 7.7 Phase shifting method CHAPTER 8 : CONCLUSION AND FUTURE WORK 81 REFERENCES 84 BIBLIOGRAPHY 88 APPENDICES 90 Appendix 1 Appendix 2 Appendix 3 Appendix 4 ABBREVIATIONS A/D Analog to Digital ADC Analog to Digital Converter AM Amplitude Modulation D/A Digital to Analog DAC Digital to Analog Converter DSB-SC Double Sideband Suppressed Carrier DSP Digital Signal Processing EVM Evaluation Module FFT Fast Fourier Transform FIR Finite Impulse Response HR Infinite Impulse Response LPF Lowpass Filter LSB Lower Sideband MIPS Million Instructions Per Second PC Personal Computer PCM Pulse Coded Modulation RAM Random Access Memory ROM Read Only Memory S/H Sample and Hold SNR Signal to Noise Ratio SSB Single Sideband SWDS Software Development System TDL Tapped Delay Line USB Upper Sideband -1 - CHAPTER 1 : INTRODUCTION Digital processing of communication signals is gaining prominence and has become a practical alternative to analog processing. As long as input and output signals are band limited, conventional analog communication circuits can be replaced by their equivalent digital signal processors with the addition of analog-to-digital (A/D) and digital-to-analog (D/A) converters. Digital signal processing (DSP) involves the representation, transmission, and manipulation of signals using numerical techniques and digital processors. Over the recent years, DSP has made a tremendous progress. With the availability of large-scale integrated (LSI) circuitry and low cost A/D and D/A circuits, DSP have become economically feasible. But a bigger breakthrough has been the coming of single-chip (VLSI devices) digital signal processors which are essentially high-speed microprocessors/microcomputers designed specifically to perform DSP algorithms. The main advantages which have been the basic motivation for the use of DSP instead of traditional analog processing are : i. Low power, high speed digital signal processors with large storage capacities are now available. They are capable of performing arithmetic, storage and timing operations. ii. Since digital realizations are independent of variations in external conditions like power supply, component tolerances and temperature, the digital processing characteristics (e.g. filtering and modulation) can be reproduced to the same accuracy. iii. DSP is free from hardware problems and parasitic effects that restrict the analog realizations. iv. Digital signal processors offer a flexibility in their ability to be programmable and thus vary the characteristics as required. For example, filter characteristics can be varied simply by changing the filter tap coefficients. Because of the main advantages of stability, testability, reproducibility, accuracy, and flexibility, digital signal processors are becoming more prevalent in areas of general-purpose DSP, telecommunications, voice/speech, imaging and control. The work in this thesis investigates the feasibility of digital implementation of -2- various single sideband (SSB) generation methods on the TMS32020 digital signal processor from Texas Instruments. Until now, Weaver’s method of SSB generation has suffered severely from the limited accuracy of analog implementation of two identical paths. The digital implementation overcomes this shortcoming because two identical processing paths can be produced by having the same software and filter coefficients for the two. Besides the work presented in the thesis, a fair amount of time and effort was taken up by another two factors which are not apparent from the succeeding chapters. In the initial stages of the thesis, work involved in getting familiarized with a completely new and different set of assembler instructions & directives, and with the internal processor hardware architecture was demanding. This was necessary to make the best use of the TMS32020 instruction set and exploit the internal architecture efficiently. As explained later in Chapter 7, the processing time (or the sampling rate) plays a vital role in the feasibility of a digital system. Thus a fairly good knowledge of the processor software and hardware was important. Secondly, a TMS32020 Evaluation Module was assembled and tested to be used as a tool in software debugging. Different modulation techniques (including SSB) and a comparison between them is presented in Chapter 2. In Chapter 3, various SSB modulation methods have been presented. While Weaver’s method has been explained in detail, the other two modulation methods have been briefed. Chapter 4 reviews the digital realization of SSB signal generation using Weaver’s method. The basic components in a Weaver’s modulator are digital filters, frequency modulators, and oscillators. Design and implementation of each of these components has been presented. When an infinite number of bits are available for representing signals, the design characteristics can be implemented without any error. But in all practical digital systems, we have a finite number of bits to represent signals. This results in certain undesirable effects,