英语电子信息工程9.pptx

Unit 9 Digital Signal Processing Unit 9 Digital Signal Processing Passage A Basic Concepts of DSPPassage B Digital Signal Processors Passage C Comparison of DSP and ASP 1/59Unit 9 Digital Signal Processing Passage A Basic Concepts of DSPWe dont speak in a digital signal.A digital signal is a language of 1s and 0s that can be processed by mathematics.We speak in real-world,analog signals.Analog signals are real world signals that we experience everyday-sound,light,temperature,and pressure.A digital signal is a numerical representation of the analog signal.It may be easier and more cost effective to process these signals in the digital world.In the real world,we can convert these signals into digital signals through the analog-to-digital converter,process the signals,and if needed,bring the signals back out to the analog world through the digital-to-analog converter.2/59Unit 9 Digital Signal Processing 1.The essentials of analog-to-digital and digital-to-analog conversionThe first essential step in analog-to-digital(A/D)conversion(as shown in Figure 9.1)is to sample an analog signal.This step is performed by a sample and hold circuit,which samples at regular intervals called sampling intervals.The length of the sampling interval is the same as the sampling period,and the reciprocal of the sampling period is the sampling frequency fs.According to the Nyquist theorem,a signal with a maximum frequency of W Hz(called a band-limited signal)must be sampled at least 2W samples per second to ensure accurate recording.When this minimum is not respected,distortion called aliasing occurs.Aliasing causes high frequency signals to appear as lower frequency signals.To be sure aliasing will not occur,sampling is always preceded by low pass filtering.The low pass filter,called the anti-aliasing filter,removes all frequencies above half the selected sampling rate.3/59Unit 9 Digital Signal Processing Figure 9.1 Analog-to-Digital Conversions4/59Unit 9 Digital Signal Processing After a brief acquisition time,during which a sample is acquired,the sample and hold circuit holds the sample steady for the remainder of the sampling interval.This hold time is needed to allow time for an A/D converter to generate a digital code that best corresponds to the analog sample.5/59Unit 9 Digital Signal Processing The A/D converter chooses a quantization level for each analog sample.An N-bit converter chooses among 2N possible quantization levels.The larger the number of levels,the smaller the quantization errors,calculated as the difference between the quantized level and the true sample level.Most quantization errors are limited in size to half a quantization step Q.The quantization step size is calculated as Q=R/2N,where R is the full scale range of the analog signal and N is the number of bits used by the converter.The strength of the signal compared to that of the quantization errors is measured by dynamic range and signal-to-noise ratio.6/59Unit 9 Digital Signal Processing A digital signal is represented by a set of vertical lines with circles at the top to mark the quantization levels selected for each sample.The bit rate for an A/D converter is the Nfs,where fs is the sampling rate.Finally,each digital sample is assigned a digital code,which completes the A/D process.The result is a digital bit stream.It is this collection of digital codes that is processed in digital signal processing.7/59Unit 9 Digital Signal Processing To summarize,A/D comprises anti-aliasing,sampling,quantization and digitization.Once digital signal processing is complete,digital-to-analog(D/A)conversion(as shown in Figure 9.2)must occur.This process begins by converting each digital code into an analog voltage that is proportional in size to the number represented by the code.This voltage is held steady through zero order hold until the next code is available,one sampling interval later.This creates a staircase-like signal that contains frequencies above W Hz.These signals are removed with a smoothing low pass filter,the last step in D/A conversion.8/59Unit 9 Digital Signal Processing Figure 9.2 Digital-to-Analog Conversions9/59Unit 9 Digital Signal Processing The images of each frequency f present in a sampled signal appear,through sampling,at the infinite number of frequencies kfsf Hz.When the sampling rate is lower than the required Nyquist rate,that is fs2W,images of high frequency signals erroneously appear in the baseband (or Nyquist range)due to aliasing.While this undersampling is normally avoided,it can be exploited.For example,signals whose frequencies are restricted to a narrow band of high frequencies can be sampled at a rate similar to twice the width of the band instead of twice the maximum frequency.All of the important signal characteristics can be deduced from the copy of the spectrum that appears in the baseband through sampling.Depending on the relationship between the signal frequencies and the sampling rate,spectral inversion may cause the shape of the spectrum in the baseband to be inverted from the true spectrum of the signal.110/59Unit 9 Digital Signal Processing 2.Technologies for digital signal processingIf a universal microprocessor solution existed with which every design could be realized,the electronics industry wouldnt be a very competitive place.2 However,typically in most electronic designs,more than one processor technology can be used to implement the required functions.The trick is,of course,to choose the one that best delivers the performance,size,power consumption,features,software and tools to get the job done fast-without breaking the budget.After almost two decades of development,digital signal processors continue to take the place of competitive processors.Digital signal processors are,after all,at the center of signal processing.11/59Unit 9 Digital Signal Processing A digital signal processor(DSP)is a type of microprocessor-one that is incredibly fast and powerful.A DSP is unique because it processes data in real time.This real-time capability makes a DSP perfect for applications that cannot tolerate any delays.For example,did you ever talk on a cell phone where two people couldnt talk at once?You had to wait until the other person finished talking.If you both spoke simultaneously,the signal was cut-you didnt hear the other person.With todays digital cell phones,which use DSP,you can talk normally.The DSP inside cell phones process sounds so rapidly you hear them as quickly as you can speak-in real time.Here are just some of the advantages of designing with DSPs over other microprocessors:12/59Unit 9 Digital Signal Processing Single-cycle multiply-accumulate operations;Real-time performance simulation and emulation;Flexibility;Reliability;Increased system performance;Reduced system cost.However,there are some of the other alternatives available for digital signal processing.How they compare to DSPs?13/59Unit 9 Digital Signal Processing The FPGA Alternative Field-Programmable Gate Arrays(FPGA)have the capability of being reconfigurable within a system,which can be a big advantage in applications that need multiple trial versions within development,offering reasonably fast time to market.They also offer greater raw performance per specific operation because of the resulting dedicated logic circuit.However,FPGAs are significantly more expensive and typically have much higher power dissipation than DSPs with similar functionality.As such,even when FPGAs are the chosen performance technology in designs such as wireless infrastructure,DSPs are typically used in conjunction with FPGAs to provide greater flexibility,better price/performance ratios,and lower system power.14/59Unit 9 Digital Signal Processing The ASIC Alternative Application-specific ICs(ASIC)can be tailored to perform specific functions extremely well,and can be made quite power efficient.However,since ASICS are not field-programmable,their functionality cannot be iteratively changed or updated while in product development.As such,every new version of the product requires a redesign and trips through the foundry,an expensive proposition,and an impediment to rapid time-to-market.3 Programmable DSPs,on the other hand,can be updated without changing the silicon,merely change the software program,greatly reducing development costs,and availing aftermarket feature enhancements with mere code downloads.Consequently,more often than not,when you see ASICs in real time signal processing applications,they are typically employed as bus interfaces,glue logic,and/or functional accelerators for a programmable DSP-based system.15/59Unit 9 Digital Signal Processing The GPP Alternative In contrast to ASICs that are optimized for specific functions,general-purpose microprocessors(GPPs)are best suited for performing a broad array of tasks.However,for applications in which the end product must process answers in real time,or must do so while powered by consumer batteries,GPPs comparatively poor real time performance and high power consumption all but rules them out.4More and more,these processors are being seen as the dinosaurs of the industry,too encumbered with PC compatibility and desktop features to adapt to the changing real time market place.As the world embraces tiny hand-held wireless-enabled products that require power dissipation measured in milliwatts-not the watts that these processors consume-DSPs are the programmable technology of choice.That trend is bound to continue as digital Internet appliances get smaller,faster and more portable.16/59Unit 9 Digital Signal Processing Notes1 Depending on the relationship between the signal frequencies and the sampling rate,spectral inversion may cause the shape of the spectrum in the baseband to be inverted from the true spectrum of the signal.依据信号频率和采样频率之间关系不一样,可能出现“频谱反转”现象基带频谱形状和信号真实频谱形状恰好相反。be inverted from.和相反。所谓基带是指不加任何调制过程数字信号传输。基带带宽全部用来传送数字脉冲信号,并能够经过时分复用方式进行多路数字信号传输。17/59Unit 9 Digital Signal Processing 2 If a universal microprocessor solution existed with which every design could realized,the electronics industry wouldnt be a very competitive place.假如存在可用以实现任何设计通用微处理器话,电子行业就不会竞争得如此激烈了。本句为虚拟语气。18/59Unit 9 Digital Signal Processing 3 As such,every new version of the product requires a redesign and trips through the foundry,an expensive proposition,and an impediment to rapid time-to-market.而且,每次推出一个新产品都需要重新设计并经历全部制造流程。这么做不但造价昂贵,而且不利于快速上市。time-to-market意思为“上市时间”。foundry是指为第三方制造芯片半导体制造商,能够指那些出售其充裕生产能力大型芯片制造企业,也能够是指专为其它企业制造芯片企业。19/59Unit 9 Digital Signal Processing 4 However,for applications in which the end product must process answers in real time,or must do so while powered by consumer batteries,GPPs comparatively poor real time performance and high power consumption all but rules them out.然而,在最终产品必须实时响应应用中,或者必须在电池供电下实现实时响应应用中,因为GPP实时性能较差、功耗大，所以就被排除在外了。all but此处作rule them out状语,涵义为“几乎,差一点”。rule out意思是“将排除在外”。20/59Unit 9 Digital Signal Processing Exercises1.Fill in the blanks.(1)The first essential step in conversion is to sample an analog signal.(2)The length of the is the same as the ,and the reciprocal of the sampling period is the sampling frequency fs.(3)To summarize,A/D comprises ,and .(4)A is a type of microprocessor-one that is incredibly fast and powerful.21/59Unit 9 Digital Signal Processing(5)In contrast to ASICs that are optimized for specific functions,are best suited for performing a broad array of tasks.(6)have the capability of being reconfigurable within a system,which can be a big advantage in applications that need multiple trial versions within development,offering reasonably fast time to market.22/59Unit 9 Digital Signal Processing 2.Translate the following paragraph into Chinese.The first step in converting an analog signal is sampling.This step is accomplished using a sample and hold circuit,which grabs a sample from the signal and holds it steady until the next sampling time.Samples are normally collected at regular time intervals,called sampling periods.If the rate of the sampling is not adequate,distortion called aliasing can occur.The output from the sample and hold circuit is passed to an A/D converter,which chooses a quantization level that is closest to the actual amplitude of the signal.This is the second step in A/D conversion.The number of levels to choose from depends on the number of bits used by the converter:for N bits,2N levels are possible.Since analog amplitudes cannot in general be represented perfectly in a digital system,quantization errors occur.23/59Unit 9 Digital Signal Processing 3.Translate the following sentences into English.(1)数字信号处理是无数家用和商用系统中关键部分,其应用领域与日俱增。所以,DSP正在成为技术教授和工程师专业知识一个主要组成部分。(2)数字信号处理可分为定点和浮点两大类。定点和浮点指是器件内用于存放和操作数据格式。定点数字信号处理通惯用最少16位来表示每个数据,经典浮点数字信号处理器用最少32位来存放每个数据。(3)和其它科学和工程应用一样,数字信号处理器通常使用汇编语言或者C语言进行编程。然而,DSP程序和传统软件任务不一样。首先,程序通常短得多,如100行而不是1万行。其次,执行速度往往是其应用关键部分。24/59Unit 9 Digital Signal Processing Passage B Digital Signal ProcessorsDigital signal processing tasks can be performed by all processors.Specialized digital signal processors(DSPs),however,perform these tasks most efficiently and most quickly.While traditional processors follow the Von Neumann architecture model(as shown in Figure 9.3),which assumes a shared single memory to be used for both program instructions and data,DSPs use the Harvard or modified Harvard architecture(as shown in Figure 9.4),which includes multiple program and data memories,along with multiple buses to access them.1 This arrangement means that much less waiting is required when instructions or25/59Unit 9 Digital Signal Processing numbers are fetched from memory.In fact at least one of each can be fetched simultaneously.Such overlapping of tasks is called pipelining.In addition to multiple memories and buses,all DSPs have fast multipliers,accumulators,and shifters,and many have hardware support for circular buffers.Address generators can speed up accesses to memory locations referenced by registers.26/59Unit 9 Digital Signal Processing Figure 9.3 Von Neumann Architecture27/59Unit 9 Digital Signal Processing Figure 9.4 Harvard Architecture28/59Unit 9 Digital Signal Processing DSPs are available in two major classes:fixed point and floating point.The fixed point class represents real numbers in a fixed number of bits.The position of the binary point(similar to the decimal point)can be controlled by the programmer,and determines the range of numbers that can be represented.As the range increases,though,the available precision goes down,since fewer bits lie to the right of the binary point.In 16 bits,the formats 16.0,15.1,14.2,13.3,12.4,11.5,10.6,9.7,8.8,7.9,6.10,5.11,4.12,3.13,2.14,and 1.15 are possible.The dynamic range,calculated as 20log(Full Scale Range/Smallest Resolvable Difference),remains the same for all 16-bit formats,20 log216=96.3 dB.29/59Unit 9 Digital Signal Processing Floating point DSPs represent real numbers using a mantissa and an exponent,similar to scientific notation:Many combine mantissa and exponent into a 32-bit n