244 BELL SYSTEM TECHNICAL JOURNAL 



roughly summed up as follows. For a line to transmit signals without 

 distortion it would be necessary for all frequency components of the 

 signals to be attenuated to the same degree and also for the delay or 

 time-lag of transmission to be the same for all these frequencies. 

 Legible signals can, however, be received if the attenuation of the 

 combination of cable and apparatus increases with frequency, pro- 

 vided the increase in attenuation up to a certain value of frequency 

 is not too great. Frequencies higher than this value are not required 

 to form the received signal. For example, in the case of cable-code 

 operation, a legible signal will be received if the attenuation at 1.5 

 times the fundamental or dot frequency is as much as 5 or 10 times 

 the attenuation at the lowest frequencies involved in the signal, 

 and if still higher frequency components are reduced to an inappreciable 

 amplitude as a result of transmission through the system. The dots 

 and dashes of the received signal will in this case be recorded as 

 rounded but readily recognizable humps or depressions in the line 

 traced on the siphon recorder strip. 



Now the attenuation of the cable for the various frequency com- 

 ponents is not uniform but increases rapidly with frequency. For 

 example, on a particular transatlantic non-loaded cable a frequency of 

 2 cycles per second is received from the cable with one seventieth the 

 amplitude which it had at the sending end, whereas for 4 cycles per 

 second the received amplitude is one four hundredth of the sent 

 amplitude and for 8 cycles per second it is only one five thousandth. 

 The function of the distortion-correcting networks and apparatus is 

 to attenuate the lower frequencies more than the higher ones so that 

 the combination of cable and terminal apparatus will attenuate all 

 frequencies up to a certain value approximately alike. The process 

 of signal shaping may thus be regarded as one of attenuation equaliza- 

 tion for a limited frequency band extending upward from zero. With 

 the networks and apparatus employed on non-loaded cables the same 

 means which serve approximately to equalize attenuation serve also 

 to equalize time-lag. For frequencies higher than those required to 

 form legible signals it is desirable to reduce the received current to as 

 low a value as possible, since at such high frequencies the currents 

 induced by sources of interference are usually stronger than those 

 which belong to the signals. Accordingly the exclusion of these high 

 frequencies makes the received signals more legible in being less 

 affected by external disturbances. 



The electrical networks for correcting distortion may be applied at 

 either the sending or receiving end of the cable, or may be divided 

 between the two ends. In ordinary cable practice it is common to 



