PHASE DISTORTION IN TELEPHONE APPARATUS 



513 



distortion between 7500 and 8000 cycles is purposely left uncorrected 

 so that if these filters are used when a number of them are connected 

 in tandem on a line, the effective cut-off will be lowered therefore 

 eliminating the effects of any accumulating delay over this range. 

 Thus for short distances 8000 cycle quality will be realized, and for 

 very long circuits 7500 cycle quality, and for intermediate lengths 

 something in between. In any case the effects of delay distortion will 

 be negligible. Special filter sections are used in order to meet the 

 unusual delay and attenuation requirements. The same results could 



0.020 



0.016 



m 

 a 



z 



0.012 8 



m 



0.008 > 



< 



-I 



UJ 



a 



0.004 



1,000 



2p00 3,000 4,000 5,000 6,000 7,000 



FREQUENCY IN CYCLES PER SECOND 



8p00 



9,000 



Fig. 23 — Schematic, insertion delay and attenuation characteristics of twenty-five 

 5000 cycle low-pass line filters connected in tandem. 



have been obtained using the usual type sections like those used in the 

 5000 cycle filter and then correcting for phase by an all-pass structure. 

 Such a method would have resulted in a somewhat more expensive 

 filter and one giving more overall delay. 



3. Repeaters. — The chief sources of delay in telephone repeaters are 

 the transformers. However, some additional low frequency delay is 

 caused by shunt retard coils and series condensers. This is kept within 

 negligible limits by using large values of both. Conversely inductance 

 in series and capacitance in shunt cause high frequency delay but this 

 effect can easily be made negligible in any frequency range in which 

 one is interested. Fig. 25 is a schematic of the telephone repeater 



