512 



BELL SYSTEM TECHNICAL JOURNAL 



is that the straight portion of the phase curve may if extended intersect 

 the vertical axis at any point and does not like the low pass filter pass 

 always through Ntv. 



As an example of a condition where it has been found necessary to 

 take phase into consideration in designing low pass filters let us con- 

 sider the case of line filters used in open wire circuits transmitting 

 simultaneously both programs for broadcasting and carrier telephony. 

 Here as many as 40 or 50 filters may be used in tandem. 



Fig. 23 shows the measured delay and attenuation characteristics 



i 



3 

 "olx 



>- 

 < 



u 



50 



45 



40 



35 



30 



25 



20 



15 



10 



5 







z 

 o 



1- 

 < 



D 

 Z 



UJ 



I- 

 I- 

 < 



0.6 



0.8 



1.0 



1.2 1.4 



FREQUENCY 



1^^) 



1.6 



1.8 



2.0 



2.2 



Fig. 22 — Insertion delay and attenuation characteristic for filter B of Fig. 20. 



for 25 of the present 5000 cycle quality line filters now in use, these 

 filters being connected in tandem. The circuit is so designed that it 

 practically equalizes up to 5000 cycles for the attenuation distortion. 

 When a number of these filters are used in tandem as in the longest 

 "hook ups" the phase distortion of these filters is somewhat noticeable 

 but not seriously so and their effective cut-off is appreciably lowered 

 because of this phase distortion.'^ 



Fig. 24 gives the calculated delay and attenuation of twenty- 

 five 8000 cycle low pass line filters connected in tandem. This 

 filter is being considered for use in place of the 5000 cycle line 

 filter of Fig. 23. The delay for 25 of these filters in tandem is ap- 

 proximately constant up to 7500 cycles (within .001 seconds). The 

 attenuation is also constant up to this frequency. The attenuation 



-' J. C. Steinberg, loc. cit. 



