专业英语句子

Unit 9A.1. 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.根据奈奎斯特采样定理，为了确保准确记录信号，最高频率为W Hz的信号（称为带限信号），每秒内必须采集至少2W个样本。2. This process begins by converting each digital code into an analog voltage that is proportional in size to the number represented by the code.这个电压值在零阶保持器中保持到下一个码字出现，即需要保持一个采样间隔。3. 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. 根据信号频率和采样频率之间的关系的不同，可能出现“频谱反转”现象基带频谱的形状和信号真实频谱的形状正好相反。4. 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. 现场可编程门阵列具有在系统可重新配置的能力，在开发需要多次试用的应用时，这是一个巨大的优势它能提供快速的上市时间。5. 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的应用中，或者必须在电池供电下实现实时响应的应用中，由于GPP实时性能较差、功耗大，因此就被排除在外了。6. More and more, these processors are being seen as the dinosaurs of the industry, too enoumbered with PC compatibility and desktop features to adapt to the changing real time market place.GPP正在越来越多的被视为业界的庞然大物，为了适应不断变化的实时应用市场，不断增加的PC兼容性和台式电脑性能使GPP受到了拖累。B.1. While traditional processors follow the Von Neumann architecture model, which assumes a shared single memory to be used for both program instructions and data, DSPs use the Harvard or modified Harvard architecture, which includes multiple program and data memories, along with multiple buses to access them.传统的处理器遵循冯.诺依曼模型，该模型采用一个单一的共享存储器，同时存储程序指令和数据；而数字信号处理器使用的是哈佛结构或改进的哈佛结构，该结构包含多个程序和数据存储器以及访问这些存储器的多套总线。2. The specific hardware and software features offered by a particular DSP can make one choice better than another, as can the amount of on-chip memory available. Sometimes DSPs are chosen because well-matched supporting hardware, particularly A/D and D/A converters, is obtainable. Frequently, the quality and convenience of the software tools, for both low level and high level programming languages, are also major factors, as is the availability of third party software.当片上存储量足够时，特定DSP提供的软硬件功能使其成为不二之选。有时选择一个DSP是因为其匹配性良好的硬件支持，尤其是在模数和数模转换可实现的情况下。通常软件工具对于低级和高级编程语言的质量和便利性都是主要因素，第三方软件的可用性也是其一。3. While some of these boards are intended to be used as stand alone computers, most are configured to be plugged into a host, such as a personal computer.但有些电路板是作为独立电脑使用的，但大多数电脑是连接到主机上的，比如个人电脑。Unit 17A.1. A digital image is made up of many rows and columns of pixels. For gray scale images, each pixel is assigned a number that represents the gray shade assigned to that pixel.一幅数字图像是由多行和多列像素组成的。对灰度图像而言，每个像素被赋予一个代表该像素灰度级的数。2. When an image uses only a small portion of the available gray scale levels, histogram equalization can be used to spread out the usage of gray scale levels over the entire available range.当一幅图像只使用了可用灰度级的一小部分时，可以使用直方图均衡的方法将灰度级的使用扩展到整个可用的范围。3. The locations and colors of pixels not explicitly marked as control elements are determined by the locations and colors of the control elements that are nearest.那些没有明确标为控制元素的像素的位置，颜色由离其最近的控制元素的位置和颜色来确定。4. One simple compression scheme is run-length encoding, which codes more than three repetitions of a number as three copies of the number followed by a count of the additional copies needed.行程编码是一种简单的压缩方式，这种编码方法将一个数字超过三次的重复次数附加在这个数字后面。B.1. Compression rates may be generated by determining the size of the compressed image in terms of number of bits per image pixel for the original image.压缩比可以通过原始图像每像素所需压缩的比特数来确定。2.In fact, for some Land-sat scenes with urban areas and many small farms, the factor of 2 bits per pixel may not be able to achieve. The same technique applied to a Land-sat image of the mid-west where large fields occur and few shadows exist in images might produce a much better compression.实际上，对于一些郊区和有许多小农场的陆地场景，2比特每像素的系数可能无法达到。对于辽阔的中西部，图像中阴影极少，相同的技术在其陆地图像中生成的压缩会好得多。3. In image processing, a procedure called principal components has been designed to identify correlation between image bands and to create a new set of transformed bands that represent a new color space in which the new image bands are uncorrelated.在图像处理过程中，一个称为主分量的过程被设计用以确认图像频谱之间的相关性，并产生一组新的变换频谱，其中新图像频谱互不相关地表示一个新的彩色空间。5.The VQ technique is an example of asymmetric compression in that considerably more time is spent deriving the codebook than in decompressing via a codebook lookup. Since different images may have different characteristics, a robust codebook is necessary to successfully code and decode Land-sat and other satellite images. A VQ technique using between channel correlation as well as spatial correlation achieves higher compression rates with lossless than independent band VQ.矢量量化编码技术是不对称压缩的例子，其中更多时间花在推到码本而不是通过查找码本解压上。因为不同的图像可能有不同特点，要成功滴编码和解码陆地和其他卫星图像，稳定的码本必不可少。利用信道相关和空间相关的矢量技术能获得比独立的矢量技术更高的压缩率，更低的损失。6. The last type of compression considered is fractal compression. Based on Mandelbrot sets which take advantage of a self similar, scaling independent, statistical feature of nature, fractal compression and decompression involves a clustering approach to finding regions which exhibit the same characteristics as a sample region without regard to rotation and scale.最后一种可考虑的是分型压缩。它基于曼德布罗特集，利用自相关，独立规模，自然的统计特性。分型压缩和解压缩涉及一种聚类方法，这种方法是查找展现相同特点的区域作为样本区而不考虑旋转和规模。Unit 19 A1. Within a communication system the information transfer is frequently achieved by superimposing or modulating the information on to an electromagnetic wave which acts as a carrier for the information signal.在通信系统中，信息传输是通过把信息叠加在电磁波上或对电磁波进行调制来实现的，电磁波起着载送信号的作用。2. Although often simpler to implement , analog modulation with an optical fiber communication system is less efficient, requiring a far higher signal to noise ratio at the receiver than digital modulation.虽然光纤系统的模拟调制易于实现，但其效率较低，且要求在接收端有比数字调制高得多的信噪比。3. The avalanche photodiode (APD) detector is followed by a front-end amplifier and equalizer or filter to provide gain as well linear signal processing and noise bandwidth reduction.前置放大器与均衡放大器或滤波器在雪崩二极管检测器的后面以获得增益、线信号处理及噪声宽带的抑制。B.1. At present, the bandwidth available to fiber systems is not fully utilized but modulation at several gigahertz over a hundred kilometers and hundreds of megahertz over three hundred kilometers without intervening electronics (repeaters) is possible.目前，传输100km的几吉赫兹的调制信号和传输300km的几百兆赫兹的调制信号都是可能的，因此，光纤系统的可用带宽并没有得到充分利用。 2.Low transmission loss. The development of optical fibers over the last twenty years has resulted in the production of optical fiber cables which exhibit very low attenuation or transmission loss in comparison with the best copper conductors. Fibers have been fabricated with losses as low as 0.2 dB/km and this feature has become a major advantage of optical fiber communications. It facilitates the implementation of communication links with extremely wide repeater spacing (long transmission distances without intermediate electronics), thus reducing both system cost and complexity. Together with the already proven modulation bandwidth capability of fiber cable this property provides a totally compelling case for the adoption of optical fiber communication in the majority of long-haul telecommunication applications.传输损耗低。过去二十年，光纤的发展使得光缆的生产表现出相比于最好的铜导线极低的衰减或传输损失。光纤以低至0.2 dB/km的损耗生产，这一特点成为了光纤通讯的主要优点。这促使有着极宽中继距离的通讯线路得以实现（远程无中介器件传输），因此降低了系统成本和复杂度。连同已证明的光缆的调制带宽容量，这个性质使得光纤通讯在大多数长途电信中的采用令人信服。3. Potential low cost. The glass which generally provides the optical fiber transmission medium is made from sandnot a scarce resource. So, in comparison with copper conductors, optical fibers offer the potential for low cost line communication. Although over recent years this potential has largely been realized in the costs of the optical fiber transmission medium which for bulk purchases is now becoming competitive with copper wires (i.e. twisted pairs), it has not yet been achieved in all the other component areas associated with optical fiber communications. For example, the costs of high performance semiconductor lasers and detector photodiodes are still relatively high, as well as some of those concerned with the connection technology (demountable connectors, couplers, etc).潜在的低成本。通常用作光纤传输媒介的玻璃是沙子做成的，不是什么稀缺资源。所以，与铜导线相比，光纤提供了低成本有线通信的潜能。尽管近年来这种潜能在光纤传输媒介上已经广泛实现，这在大量购买的情况下相对铜导线（即双绞线）来说是极具竞争力的，但在其他与光纤通信有关的零件上还没有达到。例如，高性能半导体激光器和光电二极管检测器的成