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很多接在电话线上的产品(例如调制解调器)都有一个“呼叫过程监听”(CPM)功能,CPM线路就像人一样监听电话线路上的情况,并根据所听到的信号,作出相应的反应.比方说,你在没有听到拨号音之前,不应拨号.对计算机来说,也是如此.当人监听呼叫过程的信号时,信号频率的准确性并不那么重要.但如让计算机来完成这一任务,则必须对音频作准确的规定以免解释错误.因此,国际电话电报咨询委员会(CCITT)颁布了“北美可听音调计划”.下面表中的资料就是从该委员会的绿皮书中摘录的(第Ⅳ期第4卷).
Many products connected to the telephone line, such as modems, have a Call Process Monitor (CPM) feature that monitors the telephone line just like a human and responds accordingly to the signals heard For example, you should not dial before you hear the dial tone, and so on your computer, the accuracy of the signal frequency is not as important when one listens to the signal from the calling process, but if the computer does it This task must be accurately defined for the audio to avoid interpretations of errors, so the International Telegraph and Telephone Consultative Committee (CCITT) enacted the “North American Audiobooks.” The information in the following table is based on the committee’s Green Paper Excerpts (Volume IV, Volume 4).