fig2 illustrates a detailed block diagram of an ordinary switching system to which this invention is applied . in fig2 reference numeral 11 indicates a telephone set ( tel - set ); 12 designates a line circuit ( lc ); 13 identifies a central office trunk ( trk ); 14 denotes a pb receiver ( pbrec ); 15 represents a tone signal trunk ( tn ); 16 shows a time division switching network ( tdsw ); 17 refers to a speech path controller ( spc ); 18 indicates a position controller ( pos ); 19 designates a signal distributor ( sd ); 20 identifies a scanner ( scn ); 21 denotes a central processor unit ( cpu ); 22 represents a main memory ( mem ); and 23 shows a cartridge magentic tape controller ( mtc ). the illustrated switching system is a time division switching system using a stored program control system which performs a switching operation in accordance with a program . this switching system and its operation are well - known . this invention is directed to the line circuit 12 shown in fig2 but , if required , may also be applied to the central office trunk 13 . fig3 is explanatory of the construction of an embodiment of this invention as being applied to a δm time division digital switching system . in fig3 an analog signal from lines 121 including a telephone set is inputted via a loop current detector ( det ) 41 to the primary winding side of a hybrid circuit 31 , and the analog output signal from its secondary winding side is applied to a δm coder 42 which comprises a comparator 51 , a flip - flop ( ff 1 ) 52 and an integrator 53 all arranged , for example , in a transmission path 122 of the four - wire circuit ( only two wire being shown for the sake of simplicity ). the analog signal is quantized by the δm coder 42 to convert it into a digital signal , which is applied to a time division switch 16 ( fig2 ) of the digital switching system . in a receiving path 123 ( fig3 ), the digital signal transmitted from the time division switch 16 is converted into an analog signal by a δm decoder 43 composed of a flip - flop ( ff 2 ) 54 and an integrator 55 , and the analog signal is applied to the hybrid circuit ( hyb ) 31 . in this manner , speech signalling is carried out . in the present embodiment , the flip - flop ( ff 2 ) of the δm decoder 43 is employed as a circuit opening switch . in this exchanger , the loop current detector 41 of the line circuit 12 detects the on - hook or off - hook condition of the telephone by supervision or monitoring of the speech current . in other words , the loop current detector 41 always detects an impedance variation on the line . a low and high impedance detected by the loop current detector 41 are respectively applied as loop closure data and loop open data to a scanning circuit ( scn ) 20 ( fig2 ) and to the four - wire circuit . with the loop closure data , the flip - flop ( ff 2 ) 54 of the δm decoder 43 is reset to electrically open the loop of the four - wire circuit . thus , singing phenomena due to decrease of a return loss does not occur . in this instance , if the flip - flop 52 of the δm coder 42 is also reset ( via loop data -- see * in fig3 ) by the loop open data for preventing that noise generated at the time of opening the loop from being transmitted to the called party , the flip - flop 52 can be held inoperative until a channel being set is reset . for a better understanding of the present embodiment ; reference will be made to fig4 ( a ) showing a more detailed circuit diagram , fig4 ( b ) which supplements fig4 ( a ), fig5 which shows a general timing diagram in the case of communication between originating and called subscribers , and fig6 which shows a timing diagram of the delta modulator utilized in fig4 ( a ). in fig4 ( a ), parts corresponding to those in fig3 are identified by the same reference numerals . when a telephone set goes off - hook , a speech current flows , as indicated by the arrow is . this will become more apparent from the timing diagram of the subscriber ( originating ) in fig5 . that is , in an idle state , speech current flows to achieve calling . the speech current is detected by the loop current detector 41 , and its detected signal is applied to a scanner ( scn ) 20 ( fig2 ), from which the signal is transferred as off - hook data to a central processor unit ( cpu ) 21 . upon reception of the off - hook data , the central processor unit ( cpu ) 21 connects a dial tone ( dt ) to the originating subscriber . then , when the originating subscriber performs dialing , the detector 41 ( fig4 ( a )) detects the dialing as interruption of the speech current , and the resulting signal is fed as dial data to the central processor unit 21 ( fig2 ) via the scanner 20 . when the first dialing data has been received , the central processor unit 21 disconnects the dial tone . upon completion of dialing , the central processor unit 21 calls the subscriber selected by the originating subscriber . the central processor unit 21 intermittently operates a relay rl shown in fig4 ( b ) so as to open and close a ringing gate via a signal distributor which is connected to the called subscriber . since the relay rl has contacts rl 1 and rl 2 shown in fig4 ( a ), these contacts are also intermittently operated . accordingly , when the relay rl operates , the ringing gate ( the contact rl 1 ) is closed to supply therethrough a ringing current to the called telephone set for the purpose of actuating its ringer . in this case , a ring - back tone ( rbt ) is connected to the originating subscriber . upon answering of the called subscriber , that is , when the called telephone set goes off - hook , the loop current detector 41 on the side of the called subscriber detects the flowing of a dc current ( a speech current ) regardless of the operation of the relay rl , and sends ring - trip data via the scanner 20 ( fig2 ) to the central processor unit 21 . upon reception of the scanner output , which is indicative of the called subscriber having answered , the central processor unit 21 establishes a channel from the originating subscriber to the called subscriber . after the channel has been established , a voice signal from the telephone set appears across a resistor r3 via the hybrid circuit 31 . for the analog - to - digital and the digital - to - analog conversion of the voice signal , the embodiment of fig4 ( a ) employs a linear delta modulator 42 having a sampling rate of 800 khz . the sampling rate need not always be limited specifically to 800 khz but may also be selected as desired . the analog - to - digital conversion is carried out by the δm coder 42 , which comprises a comparator 51 , a sampler ( that is , a flip - flop ( ff 1 ) 52 ), and an integrator 53 . the comparator 51 compares the voice input signal level with an analog signal reproduced by the integrator 53 and outputs a high or low level depending upon whether or not the voice input signal is higher in level than the analog signal . the flip - flop 52 is a so - called d - type flip - flop , and therefore , samples the output signal from the comparator 51 at the time of rising of a clock pulse of 800 khz and , as a result , provides a sampled output δm digital signal . the output of the flip - flop 52 is subjected to level conversion by a transistor q 1 and then applied to the integrator 53 . the transistor q 1 outputs v h ( approximately + 12 vdc ) or v l ( approximately 0 vdc ) depending upon whether the output data from the flip - flop 52 is low or high in level . the integrator 53 is preferably a miller integrator using an operational amplifier . the δm digital signal converted in level by the transistor q 1 is applied to the integrator 53 -- which is a negative integrator -- so as to be regenerated as an analog signal . the output from the integrator 53 is voltage - divided by resistors r 1 and r 2 and then applied to the reference input of comparator 51 . the comparator 51 compares the voice signal and the voltage - divided integrator ( 53 ) output with each other so that the integrator output signal may follow up the voice input signal . the δm coder 42 provides a δm signal amplified ( 1 +( r 1 / r 2 ) times the voice input signal . the digital - to - analog conversion is achieved by the δm decoder 43 , which comprises a flip - flop 54 and an integrator 55 . the δm signal received from a time division switching network ( tdsw ) 16 ( fig2 ) is latched by the d - type flip - flop 54 ( fig4 ( a )). the flip - flop 54 holds the data , for example , for 1 . 25 μs , and then supplies the data to a transistor q 2 for level conversion . the data is provided to the integrator 55 after the level conversion by the transistor q 2 . the integrator 55 is identical in construction with the integrator 53 of the coder 42 and reproduces exactly the same analog signal as the integrator ( 53 ) output of the coder 42 of the terminating subscriber . the analog signal thus reproduced is applied via the hybrid circuit to the telephone set . a better understanding of the operation of the delta modulator will be obtained by referring to fig6 which illustrates the signals appearing at various positions depicted in fig4 ( a ). for example , after the originating subscriber has been released as described above , the channel is reset after a certain period of time , as indicated by time t ( see ff out ) in fig5 . if the central processor unit immediately resets the channel upon receipt of on - hook data , speech cannot be continued even in the case of an erroneous instantaneous on - hook condition , and it is impossible for the subscriber , by hooking , to talk with a new party that is , a party other than the party connected -- until this certain period of time has passed . the time t is usually selected to be several hundred msec . accordingly , singing is very likely to occur in this period . in the above embodiment , however , the flip - flops 52 and 54 ( fig4 ( a )) are turned off by the detecting signal from the loop current detector 41 so as to cut off the four - wire loop , so that no singing phenomenon occurs . the flip - flops 52 and 54 can be turned off by inputting the detecting signal from the detector 41 to their reset or set terminals . further , it is also possible to turn off either one of the flip - flops 52 and 54 . these flip - flops 52 and 54 and the detector 41 are included in the delta modulator 42 and the line circuit 12 ( fig2 ), and are not specifically provided for this invention . accordingly , the part of the delta modulator to be changed for this invention is very small . when a predetermined period of time has elasped after occurrence of the on - hook condition for the originating subscriber , the central processor unit 21 ( fig2 ) assumes that the originating subscriber has released . if the called subscriber is still in an off - hook condition at that time , the central processor unit 21 achieves the busy tone connection to urge the on - hook condition , as seen from fig5 . upon on - hook of the called subscriber , the idle state again occurs , and one speech cycle is completed . as has been described above , according to this invention , an impedance fluctuation on the two - wire side of a two - wire - to - four - wire converting hybrid circuit is detected to open a switch inserted in a receiving path of a four - wire loop , by which occurrence of singing resulting from decrease of a return loss of the hybrid circuit is completely prevented so as to enable speech of good quality . the opening of the four - wire circuit for preventing singing exerts a bad influence on an amplifier , but this invention settles this problem by the electrical opening of the four - wire circuit . further , although this invention has been described in connection with a subscriber &# 39 ; s circuit , the invention is also applicable to a trunk circuit , as referred to at the beginning of this specification . moreover , the foregoing description has been given with regard to a digital switching system , but this invention can be applied to an analog switching system . it will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of this invention .