ADVANCES IN CARRIER TELEGRAPH TRANSMISSION 169 



Characteristic distortion attributable to the telegraph channel filters 

 is not an important limitation at the speeds now generally used and 

 need not detain us as it has been discussed at length elsewhere.2°' ^^' ^^ 

 It might be stated, however, that while the frequency band used at the 

 present time, which provides an effective width of about 110 cycles, 

 allows some margin of transmission with present speeds, the proposition 

 of reducing the spacing of carriers is not very attractive for a number 

 of reasons, among which may be mentioned the reduction in cost per 

 cycle of band width due to the development of carrier-telephone 

 systems, the possibility that higher speed requirements may ultimately 

 make the present spacing desirable, and the greater degree of main- 

 tenance demanded by a system designed with less liberal operating 

 margins where a number of sections are operated in tandem. With 

 respect to the speed factor, it may be observed that service is already 

 being rendered commercially in several cases at 75 words per minute, 

 and still higher speeds have been used. 



Bias 



Variations in circuit loss consequent upon changes in temperature, 

 battery voltages, etc., are a major factor in determining signal distor- 

 tion, as will be seen by reference to Fig. 3 which shows graphically the 



LOSS GAIN 



DEVIATION FROM AVERAGE NET GAIN (AT 2300 CYCLES) IN DECIBELS 



Fig, 3— Probable distribution of net-gain variations at 2300 cycles. H44 circuits 

 about 1000 miles (1600 km.) long. Includes both differences between circuits and 

 variations with time. 



approximate manner in which a particular group of 19 gauge H44 

 circuits about 1000 miles (1600 km.) in length varied through a one- 

 year cycle at the frequency of channel 12 (2295 cycles). The effective 



