514 



BELL SYSTEM TECHNICAL JOURNAL 



used in the cable circuit previously referred to for program transmis- 

 sion. A repeating coil and input transformer at its input and the out- 

 put transformer at its output are shown. These will be discussed in 

 the next paragraph. 



4. Transformers. — As an example of phase distortion in transformers 

 we shall consider those shown in the telephone repeater of Fig. 25. 

 Here, the phase shift of a small number of transformers would be of 

 little importance but where a large number are connected together as 

 in long toll circuits their effect cannot be overlooked. The delay 

 caused by a transformer is similar to that of a high-pass filter. The 



140 



120 



100 





80 



2 

 O 



< 



Z 

 u 



I- 



60 



40 



20 



0.028 



0.024 



0,020 



If) 



Q 



Z 



o 



0.016 o 



0.012 



>- 

 < 



0.006 



0.004 



UJ 



O 



1000 



2000 3000 4000 5000 6000 7000 



FREQUENCY IN CYCLES PER SECOND 



8000 



9000 



Fig. 24 — Insertion delay and attenuation characteristics of twenty-five 8000 cycle 

 low-pass line filters connected in tandem. 



insertion phase characteristics of these three transformers between 



impedances for which they were designed are shown in Fig. 26 and their 



insertion delay characteristics, dB/dco in Fig. 27. It will be noticed 



that practically all of the delay occurs below 100 cycles. The three 



together give at 40 cycles a value of dBjdi^ of .0008 seconds. 25 sets 



would give .020 seconds delay. Experience has shown that this amount 



of delay for speech at low frequencies is negligible whereas at high 



frequencies such would not be the case.-'^ 



5. Attenuation Equalizers. — Attenuation equalizers introduce some 



phase distortion but the amount is so small that it can generally be 



-- At high frequencies {dB/dw)aiax. — {dB/dui)^^. must generally be kept under 

 .005 to .010 seconds if its effect can be neglected for speech. 



