外文翻译--通信系统简介.doc

Introduction to Communication System It is often said that we are living in the information age. Communication technology is absolutely vital to the generation, storage, and transmission of this information. Any communication system moves information from a source to a destination through a channel. Figure 1 illustrates this very simple idea. The information from the source will generally not be in a form that can travel through the channel, so a device called a transmitter will be employed at one end and a receiver at the other. Figure 1 simple communication system The source or information signal can be analog or digital. Common examples are analog audio, video signals and digital data. Sources are often described in terms of the frequency range that they occupy. Telephone-quality analog voice signals, for instance, contain frequencies from 300Hz to 3kHz, while analog high-fidelity music needs a frequency range of approximately 20Hz to 20kHz. Digital sources can be derived from audio or video signals can have almost any bandwidth depending on the number of bits transmitted per second, and the method used to convert binary ones and zeros into electrical signals. A communication channel can be almost anything: a pair of conductors, an optical fiber or a free space that we live. Sometimes a channel can carry the information signal directly. For example, an audio signal can be carried directly by a twisted-pair telephone cable. On the other hand, a radio link through free space cannot be used directly for voice signals. Such situation require the use of a carrier wave will be altered, or modulated m, by the information signals in such a way that the information can be recovered at the destination. When a carrier is used, the information signal is also known as the modulating signals. Technology is at the core of many new and emerging digital information products and applications that support the information society. Such products and applications often require the col