prevent any signals from the drop passing through the transmitting portion of the central office channel during test in addition the network 44 is connected to lowpass filter 42 via paths 46 and 48 as will be shown hereinafter the connection of network 44 causes lowpass filter 42 to oscillate at a predetermined frequency and amplitude first audio test signal providing an output signal in the form of square waves the output signal from lowpass filter 42 oscillator is applied to the modulator 52 and as previously described passes out in modulated form through transformer 62 along the path 8 to the receiving terminal if the integrity of the transmission path to the receiving terminal is intact the presence of this signal having a predetermined frequency and amplitude will operate to turn on the station oscillator at the station end and thus transmit this station carrier signal modulated by the audio test signal generated by lowpass filter oscillator 42 as will be described in more detail below referring now to fig3 the manner in which the incoming signals cause the transmission of information back to the transmitting terminal may be seen the received signals on path 32 pass through carrier line transformer 90 and path 92 to bandpass filter 94 where only those signals associated with this channel are selected these selected signals are passed to demodulator detector 96 lowpass filter 98 vf amplifier 104 path 106 to the vf hybrid 108 and path 110 to the subscriber 39 s telephone station these signals from the output of lowpass filter 98 on path 100 are also applied to the input of threshold detector 122 via path 102 the threshold level of threshold detector 122 is set so as to.