COMMON BATTERY ANTI-SIDETONE SUBSCRIBER SET 255 



to provide sidetone balance does depend upon the values of the coup- 

 lings of C, neither the attainment of the balance in eq. (4) nor the 

 impedance of N required to provide it depends upon the balance in 

 eq. (2) being met. In other words, the neutralizing balance expressed 

 by eq. (2), and the sidetone balance expressed by eq. (4), are mutually 

 independent; either may be attained without the other. 



Practical Considerations 



With the simple types of coil and network permitted by economic 

 and space limitations, the balances upon which the above performance 

 of the anti-sidetone circuit depends can be obtained exactly only with 

 a given line and at a single frequency. For practical purposes, how- 

 ever, exact balances are needless. Sound leakage under the receiver 

 cap and conduction through the head structure fix a limit beyond which 

 further reduction in sidetone is not of value. Actual designs, therefore, 

 aim at the best compromise in reducing sidetone over the voice range 

 and the range of line impedances important in practice, as judged by 

 the resulting effective transmission performance obtained with the 

 instruments employed. Under typical plant conditions, designs now 

 in service reduce the volume of sidetone with present instruments to a 

 level averaging around 10 to 12 db below that of the complementary 

 sidetone sets. 



APPENDIX 



Algebraic Solution of Circuit Equations 



Referring to Fig. 10, the following circuit equations may be written : 



Fig. 10 — * The poling of windings A and B is series aiding. Winding C is 

 poled in series opposition to windings A and B. 



{Zl-\-Za-\-Zt)Iia-\- {Zt — Zab)Jia— ZacIza=Ei 



\Zt~Zab)I\a-\-{Zt-\-Zb-\'Zr-\-Z s)l2A-\- {Zr-\-Zbc)I^a =E<2, 

 — ZacIia-\- {Z r-\- Z bc) l2A-\- {Z r-\- Z c-\- Zn) I ZA = 



(5) 



