復(fù)試通信工程專業(yè)英語-theorem

1、專 業(yè) 英 語 Specialized English 被動語態(tài)及其譯法科技英語中大量使用被動語態(tài)科技英語所表達的是客觀規(guī)律，要盡量避免使用主觀色彩較強的第一、第二人稱；被動語態(tài)主要的信息置于句首，更能突出主要論證、說明對象，引人注目；在很多情況下被動結(jié)構(gòu)比主動結(jié)構(gòu)更簡短。第一部分 專業(yè)英語基礎(chǔ)知識 1.譯成漢語被動句：常用“被”、“受到”、“得到”、“讓”、“給”、“把”、“使”、“由”等措辭。A digital image is composed of a finite number of elements, each of which has a particular location

2、and value.數(shù)字圖像由有限個元素（像素）構(gòu)成，每個像素都具有特定的位置與數(shù)值。被動語態(tài)的譯法2.譯成漢語的主動句：直接將原文的被動語態(tài)按照主動語態(tài)譯成漢語的主動句。The experiment will be finished in a month.這項實驗將在一個月后完成。Using time multiplexing, a digital system can be used to process a number of digital signals.利用時分復(fù)用技術(shù)，一個數(shù)字系統(tǒng)能處理多路數(shù)字信號。被動語態(tài)的譯法3.譯成漢語的加強語句：當英語被動句不是強調(diào)被動的動作，而只是強調(diào)

3、或肯定某一事實或行為的存在時，通?？捎谩笆堑摹被颉盀樗钡染涫絹矸g此類句子。These computers were made in Xinhua Company.這些計算機是新華公司制造的。The project was completed last year.這個項目是去年完成的。被動語態(tài)的譯法4.譯成漢語的無主句：在講述什么事情時往往是強調(diào)如何去做，而不介意誰去做。對于這一類被動句的翻譯，可以譯為漢語的無主句。The instrument must be handled with great care.必須仔細操作這臺儀器?？傊?，在翻譯英語的被動句時，不能固守原句，要靈活地采用多種翻譯

4、技巧，使譯文既在內(nèi)容上忠實原文，又符合漢語的表達方式。被動語態(tài)的譯法Unit 5：第二部分 通信技術(shù) The Sampling Theorem抽樣定理Specialized English for Communication Engineering NEW WORDS AND PHRASES theoremn.定理，定律，法則exploitvt.開拓，開發(fā)，利用sequencen.次序，順序，序列instantaneous adj. 瞬間的，即刻的，即時的reproducible adj. 可再生的，可復(fù)制的cut offn.截止，中止，切掉replication n.復(fù)制，重現(xiàn)perceiv

5、ev.感知，感到，認識到aliasingn.混淆現(xiàn)象，混疊Nyquist rate n.奈奎斯特（抽樣）速率Specialized English for Communication Engineering Under certain conditions, a continuous signal can be completely represented by and recoverable from a sequence of its values, or samples, at points equally spaced in time.recoverable :可重獲的sequence

6、 :次序, 順序, 序列Space：留間隔， 隔開在一定的條件下，一個連續(xù)時間信號完全可以由該信號在時間等間隔點上的瞬時值或樣本值來表示，并且能用這些樣本值恢復(fù)出原信號來。TEXTSpecialized English for Communication Engineering This somewhat surprising property follows from a basic result that is referred to as the sampling theorem. This theorem is extremely important and useful. somew

7、hat :稍微, 有點, 有些follow from：是從根據(jù).得出的saturate:飽和這有點不可思議的性質(zhì)是根據(jù)抽樣定理這個基本結(jié)論得出的。這個理論及其重要且非常有用Specialized English for Communication Engineering It is exploited, for example, in moving pictures, which consist of a sequence of individual frames, each of which represents an instantaneous view (i.e., a sample i

8、n time) of a continuously changing scene.exploit: 使用,開拓, 開發(fā), 開采 moving pictures : 活動圖象, 電影individual :個別的, 單獨的, 個人的 frame: 幀,畫面,框架instantaneous: 瞬間的, 即刻的, 即時的例如，抽樣定理在電影里得到了利用。電影由一組按時序排列的單個畫面所組成，其中每一個畫面都代表著連續(xù)變化景象中的一個瞬時畫面（即時間樣本）。Specialized English for Communication Engineering When these samples are

9、viewed in sequence at a sufficiently fast rate, we perceive an accurate representation of the original continuously moving scene. in sequence：順次, 挨次perceive：察覺，感知，感到當我們以極快的速度來順次觀察這些瞬時畫面時，感覺到的就是對原來連續(xù)活動場景的一個準確再現(xiàn)。Specialized English for Communication Engineering Much of the importance of the sampling t

10、heorem also lies in its role as a bridge between continuous-time signals and discrete-time signals. continuous-time signals : 連續(xù)時間信號discrete-time signals: 離散時間信號抽樣定理的重要性很大程度上體現(xiàn)在：它起著連續(xù)時間信號和離散時間信號之間的橋梁的作用。Specialized English for Communication Engineering The fact that under certain conditions a contin

11、uous-time signal can be completely recovered from a sequence of its samples provides a mechanism for representing a continuous-time signal by a discrete-time signal. recover：重獲，恢復(fù)，復(fù)原sequence：序列，順序, 次序mechanism ：機理， 機構(gòu), 機制在一定的條件下，可以用信號的時序樣本值完全恢復(fù)出原連續(xù)時間信號，這就提供了用一個離散時間信號來表示一個連續(xù)時間信號的機理。 Specialized Engli

12、sh for Communication Engineering In many contexts, processing discrete-time signals is more flexible and is often preferable to processing continuous-time signals.be preferable to ：比更為可取， 更好, 更優(yōu)越在許多方面，處理離散時間信號要更加靈活些，因此往往比處理連續(xù)時間更為可取。 Specialized English for Communication Engineering This is due in la

13、rge part to the dramatic development of digital technology over the past few decades, resulting in the availability of inexpensive, lightweight, programmable, and easily reproducible discrete-time signal, process the discrete-time system. dramatic：引人注目的，戲劇性的, 生動的lightweight ：重量輕的，輕質(zhì)的這在很大程度上是由于在過去幾十年

14、中數(shù)字技術(shù)的發(fā)展，導(dǎo)致了便宜的、輕質(zhì)的，處理可編程且易于再生的離散時間信號的離散時間系統(tǒng)的實用化。Specialized English for Communication Engineering We exploit sampling to convert a continuous-time signal to a discrete-time signal, process the discrete-time signal using a discrete-time system, and then convert back to continuous-time signal.通過抽樣，將連

15、續(xù)時間信號轉(zhuǎn)換為離散時間信號，再使用離散時間系統(tǒng)來處理離散時間信號，然后將其轉(zhuǎn)換回連續(xù)時間信號。Specialized English for Communication Engineering Sampling theorem can be stated as follows:Let x(t) be a band-limited signal with X(j)=0 for |m. Then x(t) is uniquely determined by its samples x(nT), n=0, 1, 2, , if s2mwhere s=2/Tband-limited signal:頻

16、帶有限的（低通）信號is uniquely determined：被唯一地確定 抽樣定理可以有下述關(guān)系表示：設(shè)x(t)是一個頻帶有限的（低通）信號，對于|m， X(j)=0 。若s2m ，其中 s=2/T，則x (t)可以被其抽樣值x(nT)唯一地確定。 m：被抽樣信號的最高頻率s ：抽樣頻率T ：抽樣間隔Specialized English for Communication Engineering Given these samples, we can reconstruct x(t) by generating a periodic impulse train in which suc

17、cessive impulses have amplitudes that are successive sample values. impulse train ：脈沖序列successive ： 連續(xù)的; 逐位的已知這些樣本值，我們可以用以下方法重構(gòu)x(t)：產(chǎn)生一個周期的脈沖序列，其中連續(xù)脈沖的幅度等于依次而來的樣本值。 Specialized English for Communication Engineering This impulse train is then processed through an ideal lowpass filter with gain T and

18、cutoff frequency greater than m and less than s-m. The resulting output signal will exactly equal x (t) . ideal lowpass filter ：理想低通濾波器gain ：增益cutoff frequency：截止頻率然后將脈沖串通過一個增益為T，截止頻率大于m，而小于s-m的理想低通濾波器，產(chǎn)生的輸出信號將精確的等于 x(t)。 Specialized English for Communication Engineering The frequency 2m, which, und

19、er the sampling theorem, must be exceeded by the sampling frequency, is commonly referred to as the Nyquist rate. exceed：超過，超越的界限successive ： 連續(xù)的; 逐位的在抽樣定理中，抽樣頻率必須超過2m ，通常將2m稱作Nyquist 速率。 Specialized English for Communication Engineering In previous discussion, it was assumed that the sampling frequ

20、ency was sufficiently high that the conditions of sampling theorem were met. sufficiently ：足夠的，十分地, 充分地在前面的討論中，假設(shè)抽樣頻率足夠高，因而滿足抽樣定理的條件。 Specialized English for Communication Engineering As illustrated in Fig.1, with s2m, the spectrum of the sampled signal consists of scaled replications of the spectru

21、m of x(t), and this forms the basis for the sampling theorem. sampled signal ：取樣（后的）信號successive ： 連續(xù)的; 逐位的如圖1所示，若s2m,取樣（后的）信號的頻譜就是對原信號 x(t)頻譜的復(fù)制, 這就構(gòu)成了抽樣定理的基礎(chǔ)。 Specialized English for Communication Engineering When s2m, X (j), the spectrum of x(t), is no longer replicated in Xp(j) and thus is no lo

22、nger recoverable by lowpass filtering. The reconstructed signal will no longer be equal to x(t). This effect is referred to as aliasing. aliasing ：頻譜折疊，混淆當s2m時， x(t)的頻譜X (j), 不再被Xp(j)所復(fù)制，也無法由低通濾波器恢復(fù)。重建的信號也不再等于x(t)。這種結(jié)果稱作頻譜折疊（混淆）。Specialized English for Communication Engineering Sapling has a number of important applications. One particularly significant set of applications relates to using sampling to process continuous-time signals with discrete-time systems, by means of puters, mic