數(shù)據(jù)通信畢業(yè)論文外文文獻(xiàn)翻譯

1、<p><b>　　中文4700字</b></p><p>　　本科畢業(yè)設(shè)計(jì)（論文）</p><p><b>　　——英文翻譯</b></p><p>　　題 目 </p><p>　　學(xué)生姓名

2、 </p><p>　　專業(yè)班級(jí) 通信工程05-2 </p><p>　　學(xué) 號(hào) 12 </p><p>　　院 （系） 計(jì)算機(jī)與通信工程學(xué)院 </p><p&g

3、t;　　指導(dǎo)教師 </p><p>　　完成時(shí)間 2009年4月26日 </p><p><b>　　英文原文:</b></p><p>　　Data communications</p><p>　　Gilda

4、s Avoine and Philippe Oechslin</p><p>　　EPFL, Lausanne, Switzerland</p><p>　　fgildas.avoine, philippe.oechsling@ep.ch</p><p><b>　　Abstract</b></p><p>　　Data

5、 communications are communications and computer technology resulting from the combination of a new means of communication. To transfer information between the two places must have transmission channel, according to the d

6、ifferent transmission media, there is wired data communications and wireless data communications division. But they are through the transmission channel data link terminals and computers, different locations of implement

7、ation of the data terminal software and hardware and the</p><p>　　1 The development of data communications</p><p>　　The first phase: the main language, through the human, horsepower, war and oth

8、er means of transmission of original information.</p><p>　　Phase II: Letter Post. (An increase means the dissemination of information)</p><p>　　The third stage: printing. (Expand the scope of in

9、formation dissemination)</p><p>　　Phase IV: telegraph, telephone, radio. (Electric to enter the time)</p><p>　　Fifth stage: the information age, with the exception of language information, there

10、 are data, images, text and so on.</p><p>　　1.1 The history of modern data communications</p><p>　　Communication as a Telecommunications are from the 19th century, the beginning Year 30. Faraday

11、 discovered electromagnetic induction in 1831. Morse invented telegraph in 1837. Maxwell's electromagnetic theory in 1833. Bell invented the telephone in 1876. Marconi invented radio in 1895. Telecom has opened up in

12、 the new era. Tube invented in 1906 in order to simulate the development of communications.Sampling theorem of Nyquist criteria In 1928. Shannong theorem in 1948. The invention of the 20th </p><p>　　1.2 Key

13、figures </p><p>　　Bell (1847-1922), English, job in London in 1868. In 1871 to work in Boston. In 1873, he was appointed professor at Boston University. In 1875, invented many Telegram Rd. In 1876, invented

14、the telephone. Lot of patents have been life. Yes, a deaf wife. </p><p>　　Marconi (1874-1937), Italian people, in 1894, the pilot at his father's estate. 1896, to London. In 1897, the company set up the

15、radio reported. In 1899, the first time the British and French wireless communications. 1916, implementation of short-wave radio communications. 1929, set up a global wireless communications network. Kim won the Nobel Pr

16、ize. Took part in the Fascist Party. </p><p>　　1.3 Classification of Communication Systems </p><p>　　According to type of information: Telephone communication system, Cable television system ,Da

17、ta communication systems. </p><p>　　Modulation by sub: Baseband transmission,Modulation transfer.</p><p>　　Characteristics of transmission signals in accordance with sub: Analog Communication Sy

18、stem ,Digital communication system. </p><p>　　Transmission means of communication system: Cable Communications,Twisted pair, coaxial cable and so on.</p><p>　　And long-distance telephone communi

19、cation. Modulation: SSB / FDM. Based on the PCM time division multiple coaxial digital base-band transmission technology. Will gradually replace the coaxial fiber. </p><p>　　Microwave relay communications:Co

20、mparison of coaxial and easy to set up, low investment, short-cycle. Analog phone microwave communications mainly SSB / FM / FDM modulation, communication capacity of 6,000 road / Channel. Digital microwave using BPSK, Q

21、PSK and QAM modulation techniques. The use of 64QAM, 256QAM such as multi-level modulation technique enhance the capacity of microwave communications can be transmitted at 40M Channel 1920 ~ 7680 Telephone Rd PCM figure.

22、 </p><p>　　Optical Fiber Communication: Optical fiber communication is the use of lasers in optical fiber transmission characteristics of long-distance with a large communication capacity, communication, lon

23、g distance and strong anti-interference characteristics. Currently used for local, long distance, trunk transmission, and progressive development of fiber-optic communications network users. At present, based on the long

24、-wave lasers and single-mode optical fiber, each fiber road approach more than 10,00</p><p>　　Satellite communications: Distance communications, transmission capacity, coverage, and not subject to geographic

25、al constraints and high reliability. At present, the use of sophisticated techniques Analog modulation, frequency division multiplexing and frequency division multiple access. Digital satellite communication using digita

26、l modulation, time division multiple road in time division multiple access. </p><p>　　Mobile Communications: GSM, CDMA. Number of key technologies for mobile communications: modulation techniques, error corr

27、ection coding and digital voice encoding. Data Communication Systems. </p><p>　　1.4 Five basic types of data communication system: </p><p>　?。?）Off-line data transmission is simply the use of a

28、telephone or similar link to transmit data without involving a computer system.The equipment used at both ends of such a link is not part of a computer, or at least does not immediately make the data available for comput

29、er process, that is, the data when sent and / or received are 'off-line'. This type of data communication is relatively cheap and simple. </p><p>　?。?）Remote batch is the term used for the way in whi

30、ch data communication technology is used geographically to separate the input and / or output of data from the computer on which they are processed in batch mode. </p><p>　?。?）On-line data collection is the

31、method of using communications technology to provide input data to a computer as such input arises-the data are then stored in the computer (say on a magnetic disk) and processed either at predetermined intervals or as r

32、equired. </p><p>　　（4）Enquiry-response systems provide, as the term suggests, the facility for a user to extract information from a computer.The enquiry facility is passive, that is, does not modify the info

33、rmation stored.The interrogation may be simple, for example, 'RETRIEVE THE RECORD FOR EMPLOYEE NUMBER 1234 'or complex.Such systems may use terminals producing hard copy and / or visual displays. </p><

34、p>　?。?）Real-time systems are those in which information is made available to and processed by a computer system in a dynamic manner so that either the computer may cause action to be taken to influence events as they

35、occur (for example as in a process control application) or human operators may be influenced by the accurate and up-to-date information stored in the computer, for example as in reservation systems. </p><p>

36、　　2 Signal spectrum with bandwidth </p><p>　　Electromagnetic data signals are encoded, the signal to be included in the data transmission. Signal in time for the general argument to show the message (or data

37、) as a parameter (amplitude, frequency or phase) as the dependent variable. Signal of their value since the time variables are or not continuous, can be divided into continuous signals and discrete signals; according to

38、whether the values of the dependent variable continuous, can be divided into analog signals and digital Signal. </p><p>　　Signals with time-domain and frequency domain performance of the two most basic forms

39、 and features. Time-domain signal over time to reflect changing circumstances. Frequency domain characteristics of signals not only contain the same information domain, and the spectrum of signal analysis, can also be a

40、clear understanding of the distribution of the signal spectrum and share the bandwidth. In order to receive the signal transmission and receiving equipment on the request channel, Only know the t</p><p>　　Mo

41、re or less the voice signal spectrum at 20 Hz ~ 2000 kHz range (below 20 Hz infrasound signals for higher than 2000 KHz. For the ultrasonic signal), but with a much narrower bandwidth of the voice can produce an acceptab

42、le return, and the standard voice-frequency signal gnal 0 ~ 4 MHz, so the bandwidth of 4 MHz. </p><p>　　As a special example of the monostable pulse infinite bandwidth. As for the binary signal, the bandwidt

43、h depends on the generalThe exact shape of the signal waveform, as well as the order of 0,1. The greater the bandwidth of the signal, it more faithfully express the number of sequences. </p><p>　　3 The cut-o

44、ff frequency channel with bandwidth </p><p>　　According to Fourier series we know that if a signal for all frequency components can be completely the same through the transmission channel to the receiving en

45、d, then at the receiving frequency components of these formed by stacking up the signal and send the signal side are exactly the same, That is fully recovered from the receiving end of the send-side signals. But on the r

46、eal world, there is no channel to no wear and tear through all the Frequency components. If all the Fourier components </p><p>　　Increase the frequency characteristics, resulting in output signal distortion.

47、 Usually are frequency of 0 Hz to fc-wide channel at Chuan harmonic lost during the attenuation does not occur (or are a very small attenuation constant), whereas in the fc frequency harmonics at all above the transmissi

48、on cross Decay process a lot, we put the signal in the transmission channel of the amplitude attenuation of a component to the original 0.707(that is, the output signal Reduce by half the power) when th</p><p&

49、gt;　　Cut-off frequency transmission medium reflects the inherent physical properties. Other cases, it is because people interested in Line filter is installed to limit the bandwidth used by each user. In some cases, beca

50、use of the add channel Two-pass filter, which corresponds to two-channel cut-off frequency f1 and f2, they were called up under the cut-off frequency and the cut-off frequency. </p><p>　　This difference betw

51、een the two cut-off frequency f2-f1 is called the channel bandwidth. If the input signal bandwidth is less than the bandwidth of channel, then the entire input signal Frequency components can be adopted by the Department

52、 of channels, which the letter Road to be the output of the output waveform will be true yet. However, if the input signal bandwidth greater than the channel bandwidth, the signal of a Frequency components can not be mor

53、e on the channel, so that the signal out</p><p>　　4 Data transfer rate</p><p>　　Channel maximum data transfer rate Unit time to be able to transfer binary data transfer rate as the median. Impro

54、ve data transfer rate means that the space occupied by each Reduce the time that the sequence of binary digital pulse will reduce the cycle time, of course, will also reduce the pulse width. </p><p>　　The pr

55、evious section we already know, even if the binary digital pulse signal through a limited bandwidth channel will also be the ideal generated wave Shape distortion, and when must the input signal bandwidth, the smaller ch

56、annel bandwidth, output waveform distortion will be greater. Another angle Degree that when a certain channel bandwidth, the greater the bandwidth of the input signal, the output signal the greater the distortion, so whe

57、n the data transmission </p><p>　　Rate to a certain degree (signal bandwidth increases to a certain extent), in the on-channel output signal from the receiver could not have been Distortion of the output sig

58、nal sent to recover a number of sequences. That is to say, even for an ideal channel, the limited bandwidth limit System of channel data transfer rate. </p><p>　　At early 1924, H. Nyquist (Nyquist) to recogn

59、ize the basic limitations of this existence, and deduced that the noise-free Limited bandwidth channel maximum data transfer rate formula. In 1948, C. Shannon (Shannon) put into the work of Nyquist 1 Step-by-step expansi

60、on of the channel by the random noise interference. Here we do not add on to prove to those now seen as the result of a classic. </p><p>　　Nyquist proved that any continuous signal f (t) through a noise-free

61、 bandwidth for channel B, its output signal as a Time bandwidth of B continuous signal g (t). If you want to output digital signal, it must be the rate of g (t) for interval Sample. 2B samples per second times faster tha

62、n are meaningless, because the signal bandwidth B is higher than the high-frequency component other than a letter has been Road decay away. If g (t) by V of discrete levels, namely, the likely outcome of each sam</p&g

63、t;<p>　　Rmax = 2Blog 2 V (bit / s)</p><p>　　For example, a 3000 Hz noise bandwidth of the channel should not transmit rate of more than 6,000 bits / second binary digital signal. </p><p>

64、　　In front of us considered only the ideal noise-free channel. There is noise in the channel, the situation will rapidly deteriorate. Channel Thermal noise with signal power and noise power ratio to measure the signal po

65、wer and noise power as the signal-to-noise ratio (S ignal - to -- Noise Ratio). If we express the signal power S, and N express the noise power, while signal to noise ratio should be expressed as S / N. However, people U

66、sually do not use the absolute value of signal to noise ratio,</p><p>　　Rmax = Blog 2 (1 + S / N) (bits / second)</p><p>　　For example, for a bandwidth of 3 kHz, signal to noise ratio of 30 dB f

67、or the channel, regardless of their use to quantify the number of levels, nor Fast sampling rate control, the data transfer rate can not be greater than 30,000 bits / second. S h a n n o n the conclusions are derived bas

68、ed on information theory Out for a very wide scope, in order to go beyond this conclusion, like you want to invent perpetual motion machine, as it is almost impossible. </p><p>　　It is worth noting that, S h

69、annon conclusions give only a theoretical limit, and in fact, we should be pretty near the limit Difficult. </p><p><b>　　SUMMARY</b></p><p>　　Message signals are (or data) of a magne

70、tic encoder, the signal contains the message to be transmitted. Signal according to the dependent variable Whether or not a row of values, can be classified into analog signals and digital signals, the corresponding comm

71、unication can be divided into analog communication and digital communication. </p><p>　　Fourier has proven: any signal (either analog or digital signal) are different types of harmonic frequencies Composed o

72、f any signal has a corresponding bandwidth. And any transmission channel signal attenuation signals will, therefore, Channel transmission of any signal at all, there is a data transfer rate limitations, and this is Cheng

73、kui N yquist (Nyquist) theorem and S hannon (Shannon) theorem tells us to conclusions. </p><p>　　Transmission medium of computer networks and communication are the most basic part of it at the cost of the en

74、tire computer network in a very Large proportion. In order to improve the utilization of transmission medium, we can use multiplexing. Frequency division multiplexing technology has many Road multiplexing, wave division

75、multiplexing and TDM three that they use on different occasions. </p><p>　　Data exchange technologies such as circuit switching, packet switching and packet switching three have their respective advantages a

76、nd disadvantages. M odem are at Analog phone line for the computer's binary data transmission equipment. Modem AM modulation methods have, FM, phase modulation and quadrature amplitude modulation, and M odem also sup

77、ports data compression and error control. The concept of data communications Data communication is based on "data" for business communications systems, </p><p><b>　　參考文獻(xiàn)</b></p>

78、<p>　　[1]C.Y.Huang and A.Polydoros,“Two small SNR classification rules for CPM,”in Proc.IEEE Milcom,vol.3,San Diego,CA,USA,Oct.1992,pp.1236–1240.</p><p>　　[2]“Envelope-based classification schemes for c

79、ontinuous-phase binary Frequency-shift</p><p>　　-keyed modulations,”in Proc.IEEE Milcom,vol.3,Fort Monmouth,NJ,USA,Oct.1994,pp.</p><p><b>　　796–800.</b></p><p>　　[3]A.E

80、.El-Mahdy and N.M.Namazi,“Classification of multiple M-ary frequency-shift keying over a rayleigh fading channel,”IEEE Trans.Comm.,vol.50,no.6,pp.967–974,June 2002.</p><p>　　[4]Consulative Committee for Spac

81、e Data Systems(CCSDS),Radio Frequency and Modulation Systems.CCSDS,2001,no.401.</p><p>　　[5]E.E.Azzouz and A.K.Nandi,“Procedure for automatic recognition of analogue and digital modulations,”IEE Proc.Commun,

82、vol.143,no.5,pp.259–266,Oct.1996.</p><p>　　[6]A.Puengnim,T.Robert,N.Thomas,and J.Vidal,“Hidden Markov models for digital modulation classification in unknown ISI channels,”in Eusipco2007,Poznan,Poland, Sept

83、ember 2007,pp.1882–1885.</p><p>　　[7]E.Vassalo and M.Visintin,“Carrier phase synchronization for GMSK signals,”Int.J.Satell. Commun.,vol.20,no.6,pp.391–415,Nov.2002.</p><p>　　[8]J.G.Proakis,Digi

84、tal Communications.Mc Graw Hill,2001.</p><p>　　[9]L.Rabiner,“A tutorial on hidden Markov models and selected applications in speech recognition,”Proc.IEEE,vol.77,no.2,pp.257–286,1989.</p><p><

85、;b>　　英文譯文：</b></p><p><b>　　數(shù)據(jù)通信</b></p><p>　　Gildas Avoine and Philippe Oechslin</p><p>　　EPFL, Lausanne, Switzerland</p><p>　　fgildas.avoine, phil

86、ippe.oechsling@ep.ch</p><p><b>　　摘要</b></p><p>　　數(shù)據(jù)通信是通信技術(shù)和計(jì)算機(jī)技術(shù)相結(jié)合而產(chǎn)生的一種新的通信方式。要在兩地間傳輸信息必須有傳輸信道，根據(jù)傳輸媒體的不同，有有線數(shù)據(jù)通信與無線數(shù)據(jù)通信之分。但它們都是通過傳輸信道將數(shù)據(jù)終端與計(jì)算機(jī)聯(lián)結(jié)起來，而使不同地點(diǎn)的數(shù)據(jù)終端實(shí)現(xiàn)軟、硬件和信息資源的共享。</p&

87、gt;<p>　　1 數(shù)據(jù)通信的發(fā)展:</p><p>　　第一階段：以語言為主，通過人力、馬力、烽火等原始手段傳遞信息。 </p><p>　　第二階段：文字、郵政。（增加了信息傳播的手段） </p><p>　　第三階段：印刷。（擴(kuò)大信息傳播范圍） </p><p>　　第四階段：電報(bào)、電話、廣播。（進(jìn)入電器時(shí)代） <

88、/p><p>　　第五階段：信息時(shí)代，除語言信息外，還有數(shù)據(jù)、圖像、文本等。</p><p>　　1.1 現(xiàn)代數(shù)據(jù)通信歷史</p><p>　　通信作為電信是從19世紀(jì)30年度開始的。1831年法拉第發(fā)現(xiàn)電磁感應(yīng)。1837年莫爾斯發(fā)明電報(bào)。1873年馬克斯韋爾的電磁場(chǎng)理論。1876年貝爾發(fā)明電話。1895年馬可尼發(fā)明無線電，開辟了電信的新紀(jì)元。1906年發(fā)明電子管，從而

89、模擬通信得到發(fā)展。1928年奈奎斯特準(zhǔn)則和取樣定理。1948年山農(nóng)定理。20世紀(jì)50年代發(fā)明半導(dǎo)體，從而數(shù)字通信得到發(fā)展。20世紀(jì)60年代發(fā)明集成電路。20世紀(jì)40年代提出靜止衛(wèi)星概念，但無法實(shí)現(xiàn)。20世紀(jì)50年代航天技術(shù)。1963年第一次實(shí)現(xiàn)同步衛(wèi)星通信。20世紀(jì)60年代發(fā)明激光，企圖用于通信，未成功。20世紀(jì)70年代發(fā)明光導(dǎo)纖維，光纖通信得到發(fā)展。</p><p><b>　　1.2 重要人物<

90、;/b></p><p>　　貝爾（1847－1922），英國人，1868年在倫敦工作。1871年，去波士頓工作。1873年，任波士頓大學(xué)教授。1875年，發(fā)明多路電報(bào)。1876年，發(fā)明電話。一生曾獲許多專利。妻子是一位聾人。 </p><p>　　馬可尼（1874－1937），意大利人，1894年，在父親的莊園試驗(yàn)。1896年，去倫敦。1897年，建立無線電報(bào)公司。1899年，首次

91、實(shí)現(xiàn)英法無線通信。1916年，實(shí)現(xiàn)短波無線電通信。1929年，建立世界性無線通信網(wǎng)。曾獲諾貝爾獎(jiǎng)金。曾參加法西斯黨。 </p><p>　　1.3 通信系統(tǒng)的分類: </p><p>　　按信息類型分:電話通信系統(tǒng)，數(shù)據(jù)通信系統(tǒng)，有線電視系統(tǒng)。</p><p>　　按調(diào)制方式分:基帶傳輸，調(diào)制傳輸。</p><p>　　按傳輸信號(hào)特征分:模擬

92、通信系統(tǒng)，數(shù)字通信系統(tǒng)。</p><p>　　通信系統(tǒng)傳輸手段:電纜通信，雙絞線，同軸電纜等。 </p><p>　　微波中繼通信:易架設(shè)、投資小、周期短。模擬電話微波通信主要采用SSB/FM/FDM調(diào)制，通信容量6000路/頻道。數(shù)字微波采用BPSK、QPSK及QAM調(diào)制技術(shù)。采用64QAM、256QAM等多電平調(diào)制技術(shù)提高微波通信容量，可在40M頻道內(nèi)傳送1920~7680路PCM數(shù)字

93、電話。 </p><p>　　光纖通信：光纖通信是利用激光在光纖中長距離傳輸?shù)奶匦赃M(jìn)行的，具有通信容量大、通信距離長及抗干擾性強(qiáng)的特點(diǎn)。目前用于本地、長途、干線傳輸，并逐漸發(fā)展用戶光纖通信網(wǎng)。目前基于長波激光器和單模光纖，每路光纖通話路數(shù)超過萬門，光纖本身的通信纖力非常巨大。幾十年來，光纖通信技術(shù)發(fā)展迅速，并有各種設(shè)備應(yīng)用，接入設(shè)備、光電轉(zhuǎn)換設(shè)備、傳輸設(shè)備、交換設(shè)備、網(wǎng)絡(luò)設(shè)備等。光纖通信設(shè)備有光電轉(zhuǎn)換單元和數(shù)字信

94、號(hào)處理單元兩部分組成。 </p><p>　　衛(wèi)星通信：通信距離遠(yuǎn)、傳輸容量大、覆蓋面積大、不受地域限制及高可靠性。 目前，成熟技術(shù)使用模擬調(diào)制、頻分多路及頻分多址。 數(shù)字衛(wèi)星通信采用數(shù)字調(diào)制、時(shí)分多路及時(shí)分多址。 </p><p>　　移動(dòng)通信：GSM、CDMA。 數(shù)字移動(dòng)通信關(guān)鍵技術(shù)：調(diào)制技術(shù)、糾錯(cuò)編碼和數(shù)字話音編碼。</p><p>　　1.4 五種基本的數(shù)據(jù)

95、通信系統(tǒng)：</p><p>　?。?）脫機(jī)數(shù)據(jù)傳輸是簡單地利用電話或類似的鏈路來傳輸數(shù)據(jù)，不包括計(jì)算機(jī)系統(tǒng)。這樣一條鏈路兩端所使用的設(shè)備不是計(jì)算機(jī)的部件，或至少不是立刻把數(shù)據(jù)提供給計(jì)算機(jī)處理，即數(shù)據(jù)在發(fā)送或接收時(shí)是脫機(jī)的。這種數(shù)據(jù)通信相對(duì)來說比較便宜和簡單。</p><p>　?。?）遠(yuǎn)程批處理一詞適用于這樣一種方法：采用數(shù)據(jù)通信技術(shù)來使數(shù)據(jù)的輸入和輸出在地理上遠(yuǎn)離按批處理模式處理處理它們

96、的計(jì)算機(jī)。</p><p>　?。?）聯(lián)機(jī)數(shù)據(jù)收集指的是用數(shù)據(jù)通信技術(shù)來向計(jì)算機(jī)即時(shí)提供剛產(chǎn)生的輸入數(shù)據(jù)這種方法。數(shù)據(jù)于是存儲(chǔ)在計(jì)算機(jī)里（比如磁盤上），并按預(yù)定時(shí)間間隔或者根據(jù)需要進(jìn)行處理。</p><p>　?。?）詢問——應(yīng)答系統(tǒng)，顧名思義，是為用戶提供從計(jì)算機(jī)提取信息的功能。詢問功能是被動(dòng)的。也就是說，它不修改所存儲(chǔ)的信息。提問可以很簡單，例如："檢索雇員號(hào)碼為1234的記

97、錄"也可以是復(fù)雜的。這類系統(tǒng)可能要使用能產(chǎn)生硬拷貝和（或）可視顯示的終端。</p><p>　　（5）實(shí)時(shí)系統(tǒng)是這樣一類系統(tǒng)，其中計(jì)算機(jī)系統(tǒng)是在動(dòng)態(tài)情況下取得和處理信息，以便可使計(jì)算機(jī)采取動(dòng)作來影響正在發(fā)生的事件（比如在過程控制應(yīng)用中）或者可通過存儲(chǔ)在計(jì)算機(jī)里的準(zhǔn)確且不斷更新的信息來影響人（操作員），比如在預(yù)售系統(tǒng)中。</p><p>　　基礎(chǔ)理論：這一節(jié)的主要目的是討論數(shù)據(jù)通信

98、涉及的理論基礎(chǔ)，主要內(nèi)容包括信號(hào)的頻譜與帶寬、信道的截止頻率與帶寬以及信道所能支持的最大數(shù)據(jù)傳輸率。</p><p>　　2 信號(hào)的頻譜與帶寬</p><p>　　信號(hào)是數(shù)據(jù)的電磁編碼，信號(hào)中包含了所要傳遞的數(shù)據(jù)。信號(hào)一般以時(shí)間為自變量，以表示消息（或數(shù)據(jù)）的某個(gè)參量（振幅、頻率或相位）為因變量。信號(hào)按其自變量時(shí)間的取值是否連續(xù)，可分為連續(xù)信號(hào)和離散信號(hào)；按其因變量的取值是否連續(xù)，又可分為

99、模擬信號(hào)和數(shù)字信號(hào)。</p><p>　　信號(hào)具有時(shí)域和頻域兩種最基本的表現(xiàn)形式和特性。時(shí)域特性反映信號(hào)隨時(shí)間變化的情況。頻域特性不僅含有信號(hào)時(shí)域中相同的信息量，而且通過對(duì)信號(hào)的頻譜分析，還可以清楚地了解該信號(hào)的頻譜分布情況及所占有的頻帶寬度。為了得到所傳輸?shù)男盘?hào)對(duì)接收設(shè)備及信道的要求，只了解信號(hào)的時(shí)域特性是不夠的，還必須知道信號(hào)的頻譜分布情況。信號(hào)的時(shí)域特性表示出信號(hào)隨時(shí)間變化的情況。由于信號(hào)中的大部分能量都集

100、中在一個(gè)相對(duì)較窄的頻帶范圍之內(nèi)，因此我們將信號(hào)大部分能量集中的那頻帶稱為有效帶寬，簡稱帶寬。任何信號(hào)都有帶寬。一般來說，信號(hào)的帶寬越大，利用這種信號(hào)傳送數(shù)據(jù)的速率就越高，要求傳輸介質(zhì)的帶寬也越大。下面我們將簡單介紹常見信號(hào)的頻譜和帶寬。</p><p>　　聲音信號(hào)的頻譜大致在20 Hz～2000 kHz的范圍（低于20 Hz的信號(hào)為次聲波，高于2000 KHz的信號(hào)為超聲波），但用一個(gè)窄得多的帶寬就能產(chǎn)生可接受