The Time Factor in Telephone Transmission * 



By O. B. BLACKWELL 



Until comparatively recent years the telephone engineer gave little at- 

 tention to transmission time in his problems. For all practical purposes 

 he could assume that speech was transmitted instantly between the ends of 

 telephone circuits. The rapid extension of the distances over which com- 

 mercial telephony is given and the introduction of long telephone cables has 

 changed the situation and has introduced time problems in telephone trans- 

 mission which are of large technical interest and difficulty. As a result, time 

 problems are receiving more consideration in the technical papers published 

 in recent years on transmission. The accompanying bibliography lists a 

 considerable number of such papers. There seems to be no paper, however, 

 giving a general over-all picture of this subject. The present paper gives 

 briefly such a picture. 



THE time factor introduces five different types of problems in 

 telephone transmission : 



1. A Slowing-Doitm of Telephone Communication. In talking 

 over long lengths of certain types of cable, the time interval between 

 the formation of a sound by the speaker and its reception by the listener 

 may become of sufficient magnitude to slow down conversation. This 

 is not a serious matter with the types of circuits now used in the United 

 States, even for the longest distances between points in this country. 

 It does, however, become of considerable importance when we consider 

 the joining together of long lengths of cable in this country and long 

 lengths in Europe with possibly long lengths of intervening submarine 

 cable. 



2. Delay Distortion. Difference in the speed of transmission over 

 a circuit of the different frequencies which make up speech. This may 

 introduce peculiar distortions in speech which cause considerable inter- 

 ference. 



3. Echo Effects. These arise from the fact that parts of the energy 

 transmitted over a circuit may be reflected back from points of irregu- 

 larity in it, particularly at the ends. Small amounts of the energy may 

 wander back and forth over a circuit two or more times. While these 

 echoes may affect both the talker and listener, they generally have the 

 greatest effect on the talker who may have an uneasy feeling that the 

 distant party wishes to break in on the conversation. 



4. Effects of Voice-Operated Devices. To overcome echoes, and 



* Presented at the South West District Meeting of the A. I. E. E., Kansas City 

 Mo., October 22-24, 1931. 



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