UnitDigitalCommunicationSystem.ppt

* 一般说来，凡是原文结构接近汉语，照译时符合汉语语法的修辞习惯的，都可以而且应该照译。 * The unit which performs the pulse code modulation and demodulation of the signals in a digital transmission system is called a coder-decoder, or, in abbreviated form, a codec. demodulation ：解调, 反调制, 检波 coder-decoder ：编/解码器 abbreviated ：省略的, 缩写的 codec ：数字信号编解码器 在数字传输系统中，对信号进行脉码调制和解调的单元被称作编/解码器，或缩写为codec. In early digital systems, the switching was done in an analog form, using such analog devices as the electromechanical crossbar mechanism. electromechanical ：电动机械的,机电的 crossbar mechanism ：纵横机械组装 在早期的数字系统中，通过使用像纵横交换机这样的模拟设备，在模拟形式中进行交换. Then, the analog voice signal channels were time-multiplexed into groups of 24 or 30 channels. Each group was next converted into digital form by a single high speed codec. time-multiplexed ：时分复用 然后，将单个的模拟语音话路时分复用为24路或30路的组合。接下来，通过一个高速的数字信号编解码器将每一组合转化为数字形式。 This arrangement resulted in some savings in terms of the number of components; however, the design of the shared codec with a sufficiently low line-to-line crosstalk and noise, as well as the required high speed in a fully integrated form represented a very difficult design problem. line-to-line crosstalk ：线间串话 integrated ：集成的 这种排列导致了器件数量的一些减少，但是，具有足够低的线间串话和噪声,同时又具有所需很高速度的全集成(各路)共用编解码器的设计代表了一个很难的设计问题。 In addition, any failure of the codec resulted in the loss of service to all lines serviced by that unit. Also, the system required a large number of analog switches, which were bulky and slow. failure ：故障 bulky：庞大的, 体积大的, 笨重的, 占地方的 此外，编解码器的任何故障都会导致这个单元所服务的所有线路丢失其所需的服务。而且，这种系统需要大量笨重且低速的模拟转换器。 Finally, the analog multiplexing needed was more difficult to perform and less flexible than digital multiplexing would have been. analog multiplexing ：模拟多路复用 flexible ：灵活的, 柔软的 digital multiplexing ：数字多路复用 最终，所需的模拟多路复用与数字多路复用相比,在实现上更为困难,灵活性亦更差 With the recent availability of low-cost and high-performance integrated circuits, it became feasible to allocate one codec to each voice channel, rather than to a group of channels. availability ：可用性,