PHASE DISTORTION IN TELEPHONE APPARATUS 



507 



tion is .029 seconds. A 3000 mile length would give .174 seconds delay. 

 The difference after correction for 3000 miles between minimum and 

 maximum value of dB/dco is .007 seconds and before correction .035 

 seconds. This phase corrector (for 500 miles) consists of 12 sections, 

 8 of one kind and 4 of another. Each section contains four condensers 

 and two four-terminal inductance coils with mutual between windings. 

 Both this phase corrector and the previous one are formed by connect- 

 ing together such all-pass network sections as to give the phase char- 

 acteristic desired. In the first bridge T-sections are used and in the 

 second lattice type.^~* The former are more economical when unbal- 

 anced apparatus may be used, though similar phase characteristics 

 may generally be obtained with either. 



Fig. 15 — Schematic of the phase corrector for 500 mile length of cable for telephone 



message service. 



2. Filters. — The following factors influence the phase distortion in 

 filters: (1) The width of the frequency band transmitted, (2) the amount 

 of discrimination between transmitted and attenuated regions (cor- 

 responds to number of filter sections), (3) the rate at which the atten- 

 uation rises at the edges of the transmitting band, (4) the types of filter 

 sections used, (5) the number of filters in tandem, (6) the amount of 

 reflection due to impedance mis-match near the edges of the trans- 

 mitting bands, and (7) the amount of dissipation in the filter 

 elements. 



The insertion phase characteristics of Fig. 17 and the insertion delay 



characteristics of Figs. 18 and 19 are for two low pass filters ^^ of the 



usual type. As will be seen from their attenuation characteristics 



(insertion loss) each gives a discrimination of about 35 db, although 



the second requires an additional section in order to provide the rapidly 



'^ Nyquist, U. S. patents Nos. 1,675,460 and 1,735,052 and Zobel patent Xo. 

 1,701,552; Maximum Output Network for Telephone Substation and Repeater Cir- 

 cuits, by G. A. Campbell and R. M. Foster, Trans. A. I. E. E., Vol. 39, pp. 231-280. 

 ^^ This note explains symbols used in these three figures and also the following 

 three. Zj is the image impedance. Zo for a low pass filter is the value of Zj at zero 

 frequency and for a high pass filter at infinite frequency. Q is the ratio of the coil 

 reactance to its effective resistance. Dissipation in the condensers is considered 

 negligible. For a filter section having an attenuation peak at frequency, /«,, and a 

 cut-off at, fc, "a " is the ratio /„//c for a low pass filter and/e//„ for a high pass filter. 



