Patent ID: 6631190
Filing Date: 2003-10-07
Classification: H04M

Abstract:
A telephone system comprisinga rectifying unit for rectifying an alternating current incoming through a telephone line; a relay circuit for capturing a line; a voltage dividing resistor for dividing a line voltage; a second A/D converter; a digital low pass filter; a fast Fourier transformer for frequency analysis; and a central processing unit for processing a signal from each circuit, wherein in a state where the relay circuit for capturing a line captures a line, a voltage change between a tip and a ring of the telephone line is dropped by the voltage dividing resistor and then inputted to the second A/D converter; and the central processing unit detects the voltage change based on a digital bell signal outputted from the second A/D converter to recognize that a telephone system connected in parallel is put into an off-hook state.