PURPOSE: To detect a minimum input signal continuation time accurately for optional echo signal length and pause time between signal digits, by adding a detecting function for the leading edge of a signal digit to a conventional variable threshold type circuit.
CONSTITUTION: A multifrequency signal is inputted to one output of a group L or H through a band-pass filter. One input signal is inputted to a comparing circuit CMP through a fixed threshold circuit Fth, and the other is supplied to the comparing circuit CMP through a rectifying circuit Vth, a switch, and a level holding cirucit LHD; and the output of the comparing circuit CMP is supplied to a rectifying circuit REC with a prescribed time constant. The output of the rectifying circuit REC opens a gate together with a voice guarding signal and at the trailing edge of the REC output, a flip-flop F/F is set to turn off a switch SW by the set output and also to drive a monostable multivibrator MM, thereby putting an echo eliminating circuit in operation. The output of the monostable multivibrator MM is used to reset the F/F.
| JPH08288997 | DTMF SIGNAL GENERATOR |
HASHIZUME YUKINAO
FUJII TERUJI
