496 BELL SYSTEM TECHNICAL JOURNAL 



cause some characteristic distortion. Xo attempt was made to obtain an 

 ideal distortionless filter characteristic because it would have been necessary 

 also to take into account the characteristics of other parts of the circuit, 

 which would have required considerably more time than was then available. 

 Curves D and E of Fig. 3 show that the modulators caused very little distor- 

 tion when channel and low-pass sending filters were absent. 



Band Width 



Figure 4 shows a comparison of the distortion vs. speed characteristics of 

 the normal-band and wide-band frequency-shift arrangements when a relay 

 modulator was used. Curve A of Fig. 4 is the same as curve C of Fig. 3 and 

 shows the characteristic of the normal band arrangement. Curve C of Fig. 

 4 shows the characteristic of the wide-band arrangement when the low-pass 

 filters at the detector output were the same as for the normal-band arrange- 

 ment, their cut-off frequency being about 503 cycles. The distortion shown 

 in curve C is much lower than that of curve A since the width of the side- 

 bands transmitted was doubled. Curve B of Fig. 4 shows the characteristic 

 of the wide-band arrangement when the low-pass filters at the detector out- 

 put had a cut-ofif at about 58 cycles. As previously indicated, the latter 

 cut-oflf was selected in order to give the wide-band arrangement about the 

 same distortion vs. speed characteristic as the normal-band arrangement. 

 The reason for the use of the lower cut-off is explained under the heading 

 "Noise Tests". 



Compariscn of Frequency-Shift and On-Off Arrangements 



Figure 5 is a comparison of the distortion vs. speed characteristic of the 

 frequency-shift arrangement having a relay modulator (curve B), with 

 characteristics of two on-off arrangements having commercial receiving cir- 

 cuits^- '"• ". The 40B1 detector had no level compensator and included a 

 triode detector having an output vs. input characteristic which roughly 

 followed a square law. The other detector had a slow acting level compen- 

 sator" designed to eliminate receiving bias due to slow changes in line 

 equivalent. The output vs. input characteristic of this detector was much 

 steeper than that of the 40B1 arrangement at the transition points of the 

 signals. The level compensated arrangement was adjusted at each speed to 



^ B. P. Hamilton, H. Nyquist, M. B. Long and W. A. Phelps: "Voice Frequency 

 Carrier Telegraph System for Cables", Jour. A. /. E. E., \'ol. XLIV, No. 3, Mar. 1925. 



^^ A. L. Matte: "Advances in Carrier Telegraph Transmission", B. S. T. J., Vol. 

 XIX, pp. 161-208, Apr. 1940. 



" A separate paper describing a commercial system using an improved level compensator 

 is now in preparation by other Bell System authors. The function of the level compensator 

 is to vary the gain of the receiving amplifier-detector so as to automatically compensate 

 for relatively slow level changes. See also: V. P. Thorp: "A Level Comj)ensator for Car- 

 rier Telegraph Systems", Belt Laboratories Record, Vol. XVIII, No. 2, October 1939, pp. 

 46-48. 



