电子信息类专业英语Unit 6 Eectronic Communications.ppt

　　The second source of information, music, originates from instruments such as the piano, violin, and flute. The note made by a musical instrument may last for a short time interval as in the pressing 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 time sequence of sounds, and a harmonic structure consisting of a set of simultaneous sounds. Like a speech signal, a music signal is bipolar. However, a music 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, music signals demand a much wider channel bandwidth than speech signals for their transmission. 　　Pictures 　　The third source of information, pictures, relies on the human visual system for its perception. The picture can be dynamic, as in television, or static, as in facsimile. Taking the case of television first, the pictures in motion are converted into electrical signals to facilitate their transport from the transmitter to the receiver. To do so, each complete picture is sequentially scanned. The scanning process is carried out in a TV camera. In a black-and-white TV, the camera contains optics designed to focus an image on a photocathode consisting of a large number of photosensitive elements. The charge pattern so generated on the photosensitive surface is scanned by an electron beam, thereby producing an output current that varies temporally with the way in which the brightness of the original picture varies spatially from one point to another. The resulting output current is called video signal. The type of scanning used in television is a form of spatial sampling called raster scanning, which converts a two-dimensional image intensity into one-dimensional waveform; it is somewhat analogous to the manner in which we read a printed paper in that the sc