to determine the dc voltage across tip and ting the determination of the voltage provides a measurement of the total resistance of the loop according to ohm 39 s law the typical current supplied is approximately 30 m amp a scaled version v tr of the measured dc voltage across tip and ting appears at an output conductor 21 of the line feed circuit this scaled voltage is coupled through a resistor r 3 to the negative input of an amplifier 23 whose positive input is grounded the amplifier feedback loop comprises a parallel connection of a resistor r 1 and capacitor c 1 the combination of amplifier 23 resistors r 1 and r 3 and capacitor c 1 comprises a filter 22 which serves to filter out any ac signal induced onto the incoming signal from adjacent power lines the filtered voltage v tr is coupled to the negative input of a comparator 24 whose positive input is coupled to ground through a capacitor c 2 the output of the comparator 24 is coupled to an input of a standard microprocessor 25 by means of conductor 26 an output of the microprocessor 25 is also coupled via conductor 27 through r 2 to one plate of capacitor c 2 further outputs of the microprocessor 25 are provided on conductors 28 29 30 and 34 to an analog digital converter codec 31 with programmable gain typically conductor 28 serves as a chip select lead since there may be more than one codec in each circuit conductor 29 could be a clock lead while conductors 30 and 34 constitute the data input and output leads respectively the codec also has coupled thereto a conductor 35 which transmits analog signals thereto from.