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1、20XX 年 10 月 16 日Accordingly, we find that telephone circuits that respond well to this latter range of frequencies give quite satisfactory commercial telephone service. Another important point that we have to keep in mindis the unavoidable presence of noise in a communication system. Noise refers to
2、 unwanted waves that tend to disturb the transmission and processing of message signals in a communication system. The source of noise may be internal or external to the system. A quantitative way to account for the effect of noise is to introduce signal-to-noise ratio (SNR) as a system parameter. F
3、or example, we 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 1 页，共 7 页 - - - - - - - - - may define the SNR at the receiver input as the ratio of the average signal power to the average noise power, both being measured at the same point. The customary practice is to
4、 express the SNR in decibels (dBs), defined as 10 times the logarithm (to base 10) of the power ratio. For example, signal-to-noise ratios of 10, 100 and 1,000 correspond to 10, 20, and 30 dBs, respectively. 2. Source of Information The telecommunications environment is dominated by four important s
5、ources of information: speech, mus ic, pictures, and com puter data . A source of information may be characterized in terms of the signal that carries the 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 2 页，共 7 页 - - - - - - - - - information. A signal is defined as a
6、 single-valued function of time that plays the role of the independent variable; at every instant of time, the function has a unique value. The signal can be one-dimensional, as in the case of speech, music, or computer data; two-dimensional, as in the case of pictures; three-dimensional, as in the
7、case of video data; and four-dimensional, as in the case of volume data over time. In the sequel, we elaborate on different sources of information. (1) Speech is the primary method of human communication. Specifically, the speech communication processinvolv es the transfer of information from a spea
8、ker to a listener, which takes place in three success ive 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 3 页，共 7 页 - - - - - - - - - stages: (1) Production. An intended message in the speakers mind is represented by a speech signal that consists of sounds (i.e., pres
9、sure waves) generated inside the vocal tract and whose arrangement is governed by the rules of language. (2) Propagation. The sound waves propagate through the air at a speed of 300m/s, reaching the listeners ears. (3) Perception. The incoming sounds are deciphered by the listener into a received me
10、ssage, thereby completing the chain of events that culminate达到顶点 in the transfer of information from the speaker to the listener. The speech-production process may 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 4 页，共 7 页 - - - - - - - - - be viewed as a form of filte
11、r ing, in which a sound source excites a vocal tract filter. The vocal tract consists of a tube of non uniform cross-sectional area, beginning at the glottis (i.e., the opening between the vocal cords) and ending at the lip. As the sound propagates along the vocal tract, the spectrum (i.e., frequenc
12、y content) is shaped by the frequency selectivity of the vocal tract; this effect is somewhat similar to the resonance phenomenon observed in organ pipes. The important point to note here is that the power spectrum (i.e., the distribution of long-term average power versus frequency) of speech approa
13、ches zero for zero frequency and reaches a peak in the 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 5 页，共 7 页 - - - - - - - - - neighborhood of a few hundred Hertz. To put matters into proper perspective, however, we have to keep in mind that the hearing mechanism
14、is very sensitive to frequency. Moreover, the type of communication system being considered has an important bearing on the band of frequencies considered to be essential for the communication process . For example, as mentioned previously, a bandwidth of 300 to 3100Hz is considered adequate for com
15、mercial telephonic communication. (2) The second source of information, music, originates from instruments such as the pian o, violin, and flute. The note made by a musical instrument may last for a 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 6 页，共 7 页 - - - - - -
16、 - - - short time interval as in the processing of a key on a piano, or it may be sustained for a long time interval as in the example of a flute player holding a prolonged note. Typically, music has two structures: a melodic structure consisting of a set of simultaneous sounds. Like a speech signal
17、, a musical signal is bipolar. However, a musical signal differs from a speech signal in that its spectrum occupies a much wider band of frequencies that may extend up to about 15 kHz. Accordingly, musical signals demanda much wider channel bandwidth than speech signals for their transmission. 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 7 页，共 7 页 - - - - - - - - -