SECOND DIAL TONE DETECTION SYSTEM

摘要

PURPOSE:To surely detect a dial tone and to control operations so as to be ready for the tone by detecting a frequency component inherent in the dial tone, making a transmitter inoperable when a detection output is available and detecting a 2nd dial tone when the inherent frequency component is again detected. CONSTITUTION:A frequency detection circuit EDT 8 detects the frequency component inherent in the dial tone. At the time of obtaining the detection output Df, the transmitter is made inoperable. When the frequency component inherent in the dial tone is detected and obtained again, a control part CNT 7 detects the 2nd dial tone. Namely if sound or noise is detected as a dial tone from the transmitter, similar detection is made after the transmitter is made inoperable. Thus the 2nd dial tone can be surely detected.