科技通信课程教案10月16日样稿样稿.doc

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 mind is the unavoidable presence of noise in a communication system. Noise refers to 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. For example, we 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 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 sources of information: speech, music, pictures, and computer data. A source of information may be characterized in terms of the signal that carries the information. A signal is
defined as a single-valued function of time that plays the role of the independent variable; at every instant of time, the fu