| 摘要 |
PURPOSE:To eliminate the need for the no-hook operation by detecting a transmitted busy tone to discriminate the end of a call when the other party of the call rings off first and performing the processing of the end of the call by detection of the busy tone. CONSTITUTION:In a busy tone detecting part 6, the busy tone is extracted by a band-pass filter 61, and it is detected by a comparator 62 that its level is higher than a certain value, and a one shot multivibrator 63 is triggered by the detection signal to send a busy tone detection signal S6 to an input/ output part 11 through an AND gate 64. If a hook switch 9 is set to the off- hook state, the AND gate 64 of the busy tone detecting part 6 permits the detection signal to pass through because the output of an inverter 10 goes to the high level; but if the hook switch 9 is set to the on-hook state, the AND gate 64 inhibits the detection signal from passing through. When on-hook of a subscriber of the other party is detected, a microcomputer sends a hook signal S5, which indicates on-hook of its own terminal, to an exchange through the input/output part 11 and a radio machine to disconnect a telephone line. |
