Lesson Two Modern Digital Design & Digital Signal Processing

Text A: Introduction to Digital Signal Processing

1. What is Digital Signal Processing?

Digital Signal Processing, or DSP, as the term suggests, is the processing of signals by digital means. A signal in this context can represent a number of different things. Historically the origins of signal processing are in electrical engineering, and a signal here means an electrical signal carried by a wire or telephone line, or perhaps by a radio wave. More generally, however, a signal is a stream of information representing anything from stock prices to data from a remote-sensing satellite. The term "digital" comes from "digit", meaning a number (you count with your fingers - your digits), so "digital" literally means numerical; the French word for digital is numerique. A digital signal consists of a stream of numbers, usually (but not necessarily) in binary form. The processing of a digital signal is done by performing numerical calculations.

Digital Signal Processing is one of the most powerful technologies that will shape science and engineering in the twenty-first century. Revolutionary changes have already been made in a broad range of fields: communications, medical imaging, radar & sonar, high fidelity music reproduction, and oil prospecting, to name just a few. Each of these areas has developed a deep DSP technology, with its own algorithms, mathematics, and specialized techniques. This combination of breath and depth makes it impossible for any individual to master all of the DSP technology that has been developed. DSP education involves two tasks: learning general concepts that apply to the field as a whole, and learning specialized techniques for your particulararea of interest.

2. Analog and digital signals

In many cases, the signal of interest is initially in the form of an analog electrical voltage or current, produced for example by a microphone or some other type of transducer. In some situations, such as the output from the readout system of a CD (compact disc) player, the data is already in digital form. An analog signal must be converted into digital form before DSP techniques can be applied. An analog electrical voltage signal, for example, can be digitized using an electronic circuit called an analog-to-digital converter or ADC. This device generates a digital output as a stream of binary numbers whose values represent the input electrical voltages at each sampling instant.

3. Signal processing

Signals commonly need to be processed in a variety of ways. For example, the output signal from a transducer may well be contaminated with unwanted electrical "noise". The electrodes attached to a patient's chest when an ECG is taken measure tiny electrical voltage changes due to the activity of the heart and other muscles. The signal is often strongly affected by "mains pickup" due to electrical interference from the mains supply. Processing the signal using a filter circuit can remove or at least reduce the unwanted part of the signal. Increasingly nowadays, the filtering of signals to improve signal quality or to extract important information is done by DSP techniques rather than by analog electronics.

4. Development and Applications of DSP

The development of digital signal processing dates from the 1960's with the use of mainframe digital computers for number-crunching applications such as the Fast Fourier Transform (FFT), which allows the frequency spectrum of a signal to be computed rapidly. These techniques were not widely used at that time, because suitable computing equipment was generally available only in universities and other scientific research institutions.

Because computers were expensive during this time, DSP was limited to only a few critical applications. Pioneering efforts were made in four key areas: radar & sonar, where national security was at risk; oil exploration, where large amounts of money could be made; space exploration, where data are irreplaceable; and medical imaging, where lives could be saved.

The personal computer revolution of the 1980s and 1990s caused DSP to explode with new applications. Rather than being motivated by military and government needs, DSP was suddenly driven by the commercial marketplace. Anyone who thought they could make money in the rapidly expanding field was suddenly a DSP vendor. DSP reached the public in such products as:mobile telephones, compact disc players, and electronic voice mail.

This technological revolution occurred from the top-down. In the early 1980s, DSP was taught as a graduate level course in electrical engineering. A decade later, DSP had become a standard part of the undergraduate curriculum. Today, DSP is a basic skill needed by scientists and engineers in many fields. As an analogy, DSP can be compared to a previous technological revolution: electronics. While still the realm of electrical engineering, nearly every scientist and engineer has some background in basic circuit design. Without it, they would be lost in the technological world. DSP has the same future. DSP has revolutionized many areas in science and engineering. A few of these diverse applications are shown in Figure 1.

5. Digital Signal Processors (DSPs)

The introduction of the microprocessor in the late 1970's and early 1980's made it possible for DSP techniques to be used in a much wider range of applications. However, general-purpose microprocessors such as the Intel x86 family are not ideally suited to the numerically-intensive requirements of DSP, and during the 1980's the increasing importance of DSP led several major electronics manufacturers (such as Texas Instruments, Analog Devices and Motorola) to develop Digital Signal Processor chips - specialized microprocessors with architectures designed specifically for the types of operations required in digital signal processing. (Note that the acronym DSP can variously mean Digital Signal Processing, the term used for a wide range of techniques for processing signals digitally, or Digital Signal Processor, a specialized type of microprocessor chip). Like a general-purpose microprocessor, a DSP is a programmable device, with its own native instruction code. DSP chips are capable of carrying out millions of floating point operations per second, and like their better-known general-purpose cousins, faster and more powerful versions are continually being introduced. DSPs can also be embedded within complex"system-on-chip" devices, often containing both analog and digital circuitry.

Although some of the mathematical theory underlying DSP techniques, such as Fourier and Hilbert Transforms, digital filter design and signal compression, can be fairly complex, the numerical operations required actually to implement these techniques are very simple, consisting mainly of operations that could be done on a cheap four-function calculator. The architecture of a DSP chip is designed to carry out such operations incredibly fast, processing hundreds of millions of samples every second, to provide real-time performance: that is, the ability to process a signal "live" as it is sampled and then output the processed signal, for example to a loudspeaker or video display. All of the practical examples of DSP applications mentioned earlier, such as hard disc drives and mobile phones, demand real-time operation.

The major electronics manufacturers have invested heavily in DSP technology. Because they now find application in mass-market products, DSP chips account for a substantial proportion of the world market for electronic devices. Sales amount to billions of dollars annually, and seem likely to continue to increase rapidly.

6. The Depth of DSP

As you go through each application, note that DSP is very interdisciplinary, relying on the technical work in many adjacent fields. As Fig.2 suggests, the borders between DSP and other technical disciplines are not sharp and well defined, but rather fuzzy and overlapping. If you want to specialize in DSP, these are the allied areas you will also need to studys areas of science, engineering and mathematics.

7. Some area of DSP affected:

1) Telecommunications

DSP has revolutionized the telecommunications industry in many areas: signaling tone generation and detection, frequency band shifting, filtering to remove power line hum, etc. Three specific examples from the telephone network will be discussed here: multiplexing, compression, and echo control.

(1) Multiplexing

There are approximately one billion telephones in the world. At the press of a few buttons, switching networks allow any one of these to be connected to any other in only a few seconds. The immensity of this task is mind boggling! Until the 1960s, a connection between two telephones required passing the analog voice signals through mechanical switches and amplifiers. One connection required one pair of wires. In comparison, DSP converts audio signals into a stream of serial digital data. Since bits can be easily intertwined and later separated, many telephone conversations can be transmitted on a single channel. This technology is called multiplexing.

(2) Compression

When a voice signal is digitized at 8000 samples/sec, most of the digital information is redundant. That is, the information carried by any one sample is largely duplicated by the neighboring samples. Dozens of DSP algorithms have been developed to convert digitized voice signals into data streams that require fewer bits/sec. These are called data compression algorithms. Matching uncompression algorithms are used to restore the signal to its original form. These algorithms vary in the amount of compression achieved and the resulting sound quality. In general, reducing the data rate from 64 kilobits/sec to 32 kilobits/sec results in no loss of sound quality.

(3) Echo control

Echoes are a serious problem in long distance telephone connections. When you speak into a telephone, a signal representing your voice travels to the connecting receiver, where a portion of it returns as an echo. If the connection is within a few hundred miles, the elapsed time for receiving the echo is only a few milliseconds. The human ear is accustomed to hearing echoes with these small time delays, and the connection sounds quite normal. As the distance becomes larger, the echo becomes increasingly noticeable and irritating. The delay can be several hundred milliseconds for intercontinental communications, and is particularly objectionable. Digital Signal Processing attacks this type of problem by measuring the returned signal and generating an appropriate antisignal to cancel the offending echo. The same technique allows speakerphone users to hear and speak at the same time without fighting audio feedback (squealing).It can also be used to reduce environmental noise by canceling it with digitally generated antinoise.

2) Audio Processing

The two principal human senses are vision and hearing. Correspondingly, much of DSP is related to image and audio processing. People listen to both music and speech. DSP has made evolutionary changes in both these areas.

(1) Music

The path leading from the musician's microphone to the audiophile's speaker is remarkably long. Digital data representation is important to prevent the degradation commonly associated with analog storage and manipulation. This is very familiar to anyone who has compared the musical quality of cassette tapes with compact disks. In a typical scenario, a musical piece is recorded in a sound studio on multiple channels or tracks. In some cases, this even involves recording individual instruments and singers separately. This is done to give the sound engineer greater flexibility in creating the final product. The complex process of combining the individual tracks into a final product is called mix down. DSP can provide several important functions during mix down, including: filtering, signal addition and subtraction, signal editing, etc.

One of the most interesting DSP applications in music preparation is artificial reverberation. If the individual channels are simply added together, the resulting piece sounds frail and diluted, much as if the musicians were playing outdoors. This is because listeners are greatly influenced by the echo or reverberation content of the music, which is usually minimized in the sound studio. DSP allows artificial echoes and reverberation to be added during mix down to simulate various ideal listening environments. Echoes with delays of a few hundred milliseconds give the impression of cathedral like locations. Adding echoes with delays of 10-20 milliseconds provide the perception of more modest size listening rooms.

(2) Speech generation

Speech generation and recognition are used to communicate between humans and machines. Rather than using your hands and eyes, you use your mouth and ears. This is very convenient when your hands and eyes should be doing something else, such as: driving a car, performing surgery, or (unfortunately) firing your weapons at the enemy. Two approaches are used for computer generated speech: digital recording and vocal tract simulation.

(3) Speech recognition

The automated recognition of human speech is immensely more difficult than speech generation. Digital Signal Processing generally approaches the problem of voice recognition in two steps: feature extraction followed by feature matching. Each word in the incoming audio signal is isolated and then analyzed to identify the type of excitation and resonate frequencies. These parameters are then compared with previous examples of spoken words to identify the closest match. Often, these systems are limited to only a few hundred words; can only accept speech with distinct pauses between words; and must be retrained for each individual speaker.

8. Image Processing

Images are signals with special characteristics. First, they are a measure of a parameter over space (distance), while most signals are a measure of a parameter over time. Second, they contain a great deal of information. For example, more than 10 megabytes can be required to store one second of television video. This is more than a thousand times greater than for a similar length voice signal. Third, the final judge of quality is often a subjective human evaluation, rather than an objective criterion. These special characteristics have made image processing a distinct subgroup within DSP.

New Words and Expressions

Notes

1. Each of these areas has developed a deep DSP technology, with its own algorithms, mathematics, and specialized techniques. This combination of breath and depth makes it impossible for any one individual to master all of the DSP technology that has been developed.

译文：每个研究领域都在它自身特有的算法、数学和技术的基础上更深入的开发DSP技术，从而使DSP技术在广度和深度两个方面都得到拓展，因此，任何人都不可能掌握所有现存的DSP技术。

2. The development of digital signal processing dates from the 1960's with the use of mainframe digital computers for number-crunching applications such as the Fast Fourier Transform (FFT), which allows the frequency spectrum of a signal to be computed rapidly.

译文：数字信号处理技术源于20世纪60年代，彼时，大型计算机开始用于处理计算量较大运算，例如可以快速获得信号的频谱的快速傅立叶变换(FFT)等。

在本句中，The development of digital signal processing是主语，dates from是谓语，意思是起源于历史上的某一年代。后面以which引导的定语从句用于修饰FFT。

3. Without it, they would be lost in the technological world.

译文：没有基本的电路设计的背景(经验)，他们将会被技术界淘汰。

it是指前一句中的some background in basic circuit design.

lost的原意是丢失，这里意译为“淘汰”。

4. Note that the acronym DSP can variously mean Digital Signal Processing, the term used for a wide range of techniques for processing signals digitally, or Digital Signal Processor, a specialized type of microprocessor chip.

译文：需要注意的是，缩写DSP有多种含义，它既可以解释为“数字信号处理”，也可以解释为“数字信号处理器”，前者表示一种目前被广泛采用的数字信号处理技术，后者则表示一种专用的微处理器芯片。

5. In comparison, DSP converts audio signals into a stream of serial digital data. Since bits can be easily intertwined and later separated, many telephone conversations can be transmitted on a single channel. This technology is called multiplexing.

译文：比较而言，DSP可以将音频信号转变为数据流。由于数字比特易于组合与分离，因此，多路电话信号可以通过一条信道实现传输，这种技术称为复用。

6. When a voice signal is digitized at 8000 samples/sec, most of the digital information is redundant. That is, the information carried by any one sample is largely duplicated by the neighboring samples.

译文：当对话音信号在8000次/秒的采样率基础上进行编码时，获得的数字信号存在冗余，也就是说，某一采样点的信息在很大程度上与其它采样点的信息重复。

Exercises

I. Comprehension Questions

1. What is DSP? Please explain in a sentence.

2. Please explain the application area of DSP.

3. What areas you need to study if you want to specialize in DSP?

4. How has DSP revolutionized the telecommunications industry?

5. How does DSP solve the problem of voice recognition?

6. Why image processing is a distinct subgroup within DSP?

Ⅱ . Translate the following paragraph into Chinese

Digital Signal Processing is one of the most powerful technologies that will shape science and engineering in the twenty-first century. Revolutionary changes have already been made in a broad range of fields: communications, medical imaging, radar & sonar, high fidelity music reproduction, and oil prospecting, to name just a few. Each of these areas has developed a deep DSP technology, with its own algorithms, mathematics, and specialized techniques. This combination of breath and depth makes it impossible for any one individual to master all of the DSP technology that has been developed. DSP education involves two tasks: learning general concepts that apply to the field as a whole, and learning specialized techniques for your particular area of interest.

Text B: Modern Digital Design

1. OVERVIEW

The speed of light is just too slow. Commonplace, modern, volume-manufactured digital designs require control of timings down to the picosecond range. The amount of time it takes light from your nose to reach your eye is about 100 picoseconds (in 100 ps, light travels about 1.2 in.). This level of timing must not only be maintained at the silicon level, but also at the physically much larger level of the system board, such as a computer motherboard. These systems operate at high frequencies at which conductors no longer behave as simple wires, but instead exhibit high-frequency effects and behave as transmission lines that are used to transmit or receive electrical signals to or from neighboring components. If these transmission lines are not handled properly, they can unintentionally ruin system timing. Digital design has acquired the complexity of the analog world and more. However, it has not always been this way. Digital technology is a remarkable story of technological evolution. It is a continuing story of paradigm shifts, industrial revolution, and rapid change that is unparalleled. Indeed, it is a common creed in marketing departments of technology companies that "by the time a market survey tells you the public wants something, it is already too late."

This rapid progress has created a roadblock to technological progress that this book will help solve. The problem is that modern digital designs require knowledge that has formerly not been needed. Because of this, many currently employed digital system designers do not have the knowledge required for modern high-speed designs. This fact leads to a surprisingly large amount of misinformation to propagate through engineering circles. Often, the concepts of high-speed design are perceived with a sort of mysticism. However, this problem has not come about because the required knowledge is unapproachable. In fact, many of the same concepts have been used for several decades in other disciplines of electrical engineering, such as radio-frequency design and microwave design. The problem is that most references on the necessary subjects are either too abstract to be immediately applicable to the digital designer, or they are too practical in nature to contain enough theory to fully understand the subject. This book will focus directly on the area of digital design and will explain the necessary concepts to understand and solve contemporary and future problems in a manner directly applicable by practicing engineers and/or students. It is worth noting that everything in this book has been applied to a successful modern design.

2. THE BASICS

As the reader undoubtedly knows, the basic idea in digital design is to communicate information with signals representing 1s or 0s. Typically this involves sending and receiving a series of trapezoidal shaped voltage signals such as shown in Figure 1 in which a high voltage is a 1 and a low voltage is a 0. The conductive paths carrying the digital signals are known as interconnects. The interconnect includes the entire electrical pathway from the chip sending a signal to the chip receiving the signal. This includes the chip packages, connectors, sockets, as well as a myriad of additional structures. A group of interconnects is referred to as a bus. The region of voltage where a digital receiver distinguishes between a high and a low voltage is known as the threshold region. Within this region, the receiver will either switch high or switch low. On the silicon, the actual switching voltages vary with temperature, supply voltage, silicon process, and other variables. From the system designers point of view, there are usually high-and low-voltage thresholds, known as Vih and Vil, associated with the receiving silicon, above which and below which a high or low value can be guaranteed to be received under all conditions. Thus the designer must guarantee that the system can, under all conditions, deliver high voltages that do not, even briefly, fall below Vih, and low voltages that remain below Vil, in order to ensure the integrity of the data.

In order to maximize the speed of operation of a digital system, the timing uncertainty of a transition through the threshold region must be minimized. This means that the rise or fall time of the digital signal must be as fast as possible. Ideally, an infinitely fast edge rate would be used, although there are many practical problems that prevent this. Realistically, edge rates of a few hundred picoseconds can be encountered. The reader can verify with Fourier analysis that the quicker the edge rate, the higher the frequencies that will be found in the spectrum of the signal.Herein lies a clue to the difficulty. Every conductor has a capacitance, inductance, and frequency-dependent resistance. At a high enough frequency, none of these things is negligible. Thus a wire is no longer a wire but a distributed parasitic element that will have delay and a transient impedance profile that can cause distortions and glitches to manifest themselves on the waveform propagating from the driving chip to the receiving chip. The wire is now an element that is coupled to everything around it, including power and ground structures and other traces. The signal is not contained entirely in the conductor itself but is a combination of all the local electric and magnetic fields around the conductor. The signals on one interconnect will affect and be affected by the signals on another. Furthermore, at high frequencies, complex interactions occur between the different parts of the same interconnect, such as the packages, connectors, and bends. All these high-speed effects tend to produce strange, distorted waveforms that will indeed give the designer a completely different view of high-speed logic signals. The physical and electrical attributes of every structure in the vicinity of the interconnect has a vital role in the simple task of guaranteeing proper signaling transitions through Vih and Vil with the appropriate timings. These things also determine how much energy the system will radiate into space, which will lead to determining whether the system complies with governmental emission requirements. We will see in later chapters how to account for all these things. When a conductor must be considered as a distributed series of inductors and capacitors, it is known as a transmission line. In general, this must be done when the physical size of the circuit under consideration approaches the wavelength of the highest frequency of interest in the signal. In the digital realm, since edge rate pretty much determines the maximum frequency content, one can compare rise and fall times to the size of the circuit instead, as shown in Figure 2 On a typical circuit board, a signal travels about half the speed of light (exact formulas will be in later chapters). Thus a 500 ps edge rate occupies about 3 in. in length on a circuit trace. Generally, any circuit length at least 1/10th of the edge rate must be considered as a transmission line.

One of the most difficult aspects of high-speed design is the fact that there are a large number of codependent variables that affect the outcome of a digital design. Some of the variables are controllable and some force the designer to live with the random variation. One of the difficulties in high-speed design is how to handle the many variables, whether they are controllable or uncontrollable. Often simplifications can be made by neglecting or assuming values for variables, but this can lead to unknown failures down the road that will be impossible to "root cause" after the fact. As timing becomes more constrained, the simplifications of the past are rapidly dwindling in utility to the modern designer. This book will also show how to incorporate a large number of variables that would otherwise make the problem intractable. Without a methodology for handling the large amount of variables, a design ultimately resorts to guesswork no matter how much the designer physically understands the system. The final step of handling all the variables is often the most difficult part and the one most readily ignored by a designer. A designer crippled by an inability to handle large amounts of variables will ultimately resort to proving a few "point solutions" instead and hope that they plausibly represent all known conditions. While sometimes such methods are unavoidable, this can be a dangerous guessing game. Of course, a certain amount of guesswork is always present in a design, but the goal of the system designer should be to minimize uncertainty.

3. THE PAST AND THE FUTURE

Gordon Moore, co-founder of Intel Corporation, predicted that the performance of computers will double every 18 months. History confirmed this insightful prediction. Remarkably, computer performance has doubled approximately every 1.5 years, along with substantial decreases in their price. One measure of relative processor performance is internal clock rates. Figure 3 shows several processors through history and their associated internal clock rates. By the time this is in print, even the fastest processors on this chart will likely be considered unimpressive. The point is that computer speeds are increasing exponentially. As core frequency increases, faster data rates will be demanded from the buses that feed information to the processor, as shown in Figure 4, leading to an interconnect timing budget that is decreasing exponentially. Decreased timing budgets mean that it is evermore important to properly account for any phenomenon that may increase the timing uncertainty of the digital waveform as it arrives at the receiver. This is the root cause of two inescapable obstacles that will continue to make digital system design difficult. The first obstacle is simply that the sheer amount of variables that must be accounted for in a digital design is increasing. As frequencies increase, new effects, which may have been negligible at slower speeds, start to become significant. Generally speaking, the complexity of a design increases exponentially with increasing variable count. The second obstacle is that the new effects, which could be ignored in designs of the past, must be modeled to a very high precision. Often these new models are required to be three-dimensional in nature, or require specialized analog techniques that fall outside the realms of the digital designer's discipline. The obstacles are perhaps more profound on the subsystems surrounding the processor since they evolve at a much slower rate, but still must support the increasing demands of the processor system increases.

All of this leads to the present situation: There are new problems to solve. Engineers who can solve these problems will define the future. This book will equip the reader with the necessary practical understanding to contend with modern high-speed digital design and with enough theory to see beyond this book and solve problems that the authors have not yet encountered.

New Words and Expressions

Notes

1. These systems operate at high frequencies at which conductors no longer behave as simple wires, but instead exhibit high-frequency effects and behave as transmission lines that are used to transmit or receive electrical signals to or from neighboring components.

译文：当系统工作于高频段时，导体不再是简单的导线，此时，导体将具表现出高频特性，即具有传输线的功能和特性，该特性使得导体能与相邻器件之间实现信号的发送和接收。

transmit to和receive from是固定搭配。

2. From the system designer's point of view, there are usually high-and low-voltage thresholds, known as Vih and Vil, associated with the receiving silicon, above which and below which a high or low value can be guaranteed to be received under all conditions.

译文：从系统设计者的角度来说，高电压门限(Vih)和低电压门限(Vil)的取值与硅的品质有关，在任何条件下，高于Vih的值或低于Vil的值都可以保证被正确接收。

from one's point of view：从某人的角度来看，从某人的观点来看。

3. Every conductor has a capacitance, inductance, and frequency-dependent resistance. At a high enough frequency, none of these things is negligible. Thus a wire is no longer a wire but a distributed parasitic element that will have delay and a transient impedance profile that can cause distortions and glitches to manifest themselves on the waveform propagating from the driving chip to the receiving chip.

译文：每个导体都有电容和电感，它们的阻抗因频谱而变，当频率足够高时，必须要考虑这些因素的影响，此时，导线不再仅仅是简单的导线，而是分布参数元件，这样的元件会带来延时和瞬变阻抗，从而导致驱动芯片和接收芯片之间传输的波形发生畸变。

4. The signal is not contained entirely in the conductor itself but is a combination of all the local electric and magnetic fields around the conductor.

译文：信号不再仅仅存在于导体内，也在导体外产生磁场。

Exercises

Ⅰ. Comprehension Questions

1. Why are the concepts of high-speed design often perceived with a sort of mysticism?

2. How to communicate information with signal 1s or 0s in digital design?

3. What does the interconnect include?

4. What does the designer must do in order to ensure the integrity of the data?

5. What will you do to maximize the speed of operation of a digital system?

6. Please tell the difference of a conductor between high frequency and low frequency.

7. What is the most difficult thing in high-speed designing?

8. What is the two inescapable obstacles that will make digital system design difficult?

9. According to the author, what will happen in the future?

Ⅱ. Translating the following paragraph into Chinese

One of the most difficult aspects of high-speed design is the fact that there are a large number codependent variables that affect the outcome of a digital design. Some of the variables are controllable and some force the designer to live with the random variation. One of the difficulties in high-speed design is how to handle the many variables, whether they are controllable or uncontrollable. Often simplifications can be made by neglecting or assuming values for variables, but this can lead to unknown failures down the road that will be impossible to "root cause" after the fact. As timing becomes more constrained, the simplifications of the past are rapidly dwindling in utility to the modern designer.