508 BELL SYSTEM TECHNICAL JOURNAL 



however, that at any frequency where amplification occurs, the gains 

 and phase shifts of the two 21 -type circuits should be equal. This 

 insures that the echoes will balance out and that the maximum output 

 power will be directed toward the listener. To accomplish this it is 

 merely necessary to make the corresponding parts of the two circuits 

 alike within the allowable tolerance. Filters, condensers and other 

 devices may be used as required provided that the corresponding parts 

 in the two 21 -type circuits are nearly enough alike. 



Referring to Fig. 15, the series 21 -type circuit is coupled to the line 

 inductively by the line hybrid coil and the shunt circuit is connected 

 through condensers. This arrangement makes it possible for the line 

 conductors to be joined through the windings of the twin 21-type cir- 

 cuit without a conductive bridge across the line, and so provides the 

 desired path for d-c. impulses or low frequency alternating current. 



The inductance of the line hybrid coil cannot, of course, be infinite. 

 Practically, it must be a compromise between the opposing require- 

 ments that it shall be low enough not to interfere seriously with the 

 transmission of low frequency signaling impulses or transfer too much 

 of their energy to the series 21-type circuit and that it shall be high 

 enough to prevent too great a transmission loss at the lower frequencies 

 of the voice range. Due to this finite inductance, the amplified voice 

 currents from the series 21-type circuit will be shifted in phase at the 

 lower frequencies. 



The capacitance of the condensers in series with the bridged 21-type 

 circuit is similarly limited, and shifts the phase of the amplified voice 

 currents from that circuit. These two shifts are in the same direction 

 which makes it possible to keep the amplified currents from the two 

 21-type circuits in phase and prevent the production of echoes. 



The transmission loss and phase shift due to the finite inductance 

 of the line hybrid coil will be approximately equal to the loss and phase 

 shift due to the condensers when 



in which 



L\ = Inductance of the whole line winding of the line hybrid coil, 

 with the drop open. 



Ci — Capacitance in series with the bridged circuit. 

 L = Inductance per unit length of the line. 

 C = Capacitance per unit length of the line. 

 R = Nominal impedance of the line. 



