METHOD FOR ECHO CANCELLOR TO DETECT CAS SIGNAL AND TELEPHONE SYSTEM DEVICE

摘要

PROBLEM TO BE SOLVED: To improve the reliability of a communication system by connecting a CAS service signal detector between an echo cancellor and a voice output device when the telephone system device is set on-line so as to avoid a danger of erroneous detection in the case that a detection criterion not so tight is employed. SOLUTION: A call alert signal CAS detector 20 that detects a CAS service signal which is used to inform a user having been busy of it that other party is going to make a call to the user connects between an echo cancellor 13 and a speaker 12. A 2/4-line converter 10 couples with a microphone 11 via an outgoing transmission channel A and couples with the speaker 12 via a return channel R. The echo cancellor 13 controls an adaptive filter 14 so as to cancel an echo signal on an output of a subtractor element 16 to the utmost. When a called party temporarily interrupts a voice channel, since caller information by FSK modulation is sent to an FSK detector 21, the 21 connects to an upper-stream of the 2/4-line converter 10.