496 



BELL SYSTEM TECHNICAL JOURNAL 



An important point to note here is that if a given signal has all its 

 important frequency components falling in a region between /i and /o 

 either because of the nature of the signal or as a result of attenuation 

 in the system we are only interested in the phase characteristic in that 

 region. Thus for such a signal a sufficient condition for negligible phase 

 distortion is that the phase characteristics be like those in Fig. 1 and Fig. 2 

 between /i and fx only. 



The phase characteristics actually found in telephone apparatus 

 that frequently must be considered may for convenience be classified 

 as follows: (1) those typical of the low pass filter and the loaded line, 



180 



160 



<n 



uj 140 



u 



cr 



u 120 

 C 



? 100 



I- 



- 80 



X 



en 



60 

 u 



< 



I 40 

 a. 



20 

 



0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 

 Ff 



"REQUENCvf— j 



Fig. 3 — Image transfer phase characteristics of typical low-pass filter sections. 

 Same as insertion phase characteristic except in neighborhood of fe. Curve C 

 resembles closely the phase characteristic of a loaded line. 



(2) those of band pass filters, (3) those of high pass filters, and (4) those 

 of all pass networks.^ The attenuation of all of these networks is 

 fairly constant and small in the transmitting range. 



The solid curves of Fig. 3 show the image transfer phase characteris- 

 tics of typical low pass filter sections having a cut-off frequency fc. 

 The significance of these different type sections will be discussed later. 

 The curve, C, resembles closely the phase characteristic of a loaded 



* An interesting type of insertion phase is that obtained when these four types of 

 apparatus are terminated over a considerable portion of their transmitting range in 

 impedances that differ radically from their image impedances. The wa\-y phase 

 curves obtained may be said to be chiefly responsible for the so-called reflection ef- 

 fects thus obtained though here too the attenuation plays an important part. If the 

 attenuation in the network is large as in a line this waviness due to miss-match tends 

 to disappear and the phase characteristic resembles closely in general shape that which 

 would be obtained if no miss-match occurred. 



