一、引言自从1962年第一套商用的PCM电话系统问世以来,到1985年国际上已有50％以上的公用电话网实现了PCM传输和交换。目前很多国家正在加紧研究将电话、慢扫描电视、传真,计算机数据等信号都归一化为PCM形式进行传输和交换,使传统的公用电话网发展为 I. INTRODUCTION Since the first commercially available PCM telephone system in 1962, more than 50% of public telephone networks in the world have achieved PCM transmission and exchange by 1985. At present, many countries are stepping up research to normalize the signals such as telephones, slow-scan television, fax, computer data and the like into PCM for transmission and exchange so that the traditional public switched telephone network develops into