INDUCTIVE LOADING FOR TELEPHONE FACILITIES 



175 



The attenuation-frequency distortion in long loaded cables can be reduced 

 by raising the theoretical cut-off frequency. As a secondary factor, the re- 

 duction of core losses in the loading coils is advantageous. 

 (c) Velocity Distortion: This became noticeable as peculiarly disturbing, 

 transient distortion in the intervals when spoken syllables were building 

 up or dying down, prior to, or after, then* steady-state transmission. It is 

 most disturbing at the upper speech-frequencies where the steady-stage 

 velocity of wave propagation varies at an accelerating rate as the cut-off 

 frequency is approached. It can be particularly disturbing in very long 



0.60 



0.87 



1000 1500 2000 2500 3000 



FREQUENCY IN CYCLES PER SECOND 

 Fig. 6— Attenuation frequency characteristics toll cable loading. 



3500 



circuits where repeaters are used to reduce the over-all loss, and in extreme 

 cases it could make the circuits unusable for commercial service. These 

 transient distortion impairments may be reduced by raising the loading 

 cut-off frequency, and by increasing the velocity of wave propagation. The 

 loading design changes that were made in these features gave a satisfactory 

 control of the velocity distortion in the longest loaded cables used commer- 

 cially, without requiring the use of velocity-distortion corrective networks 

 in the lines or at repeater stations. 



(d) Echoes: Echo effects also were found to be potentially limiting factors 

 in providing satisfactory transmission-performance over long loaded cable 



