PRIVATE LINE DATA TRANSMISSION 1469 



The conclusion is reached that there is hardly any excess conservatism 

 in the 43A1 system. Some confirmation of this is indicated in a paper 

 by Jones and Pfleger.^ Fig. 4 reproduces some curves presented in that 

 paper. The curve at (B) (from Fig. 5 of that paper) shows that the FS 

 telegraph rises rapidly in distortion above the 100 word/min speed. The 

 curve at (A) (taken from the same Fig. 5) for level-compensated AM 

 shows a substantially broader and somewhat lower curve in this region. 

 From curve (C) (taken from Fig. 3 of the paper) for diode modulated 

 signals, it is seen that the sharp rise in distortion for FS is even more 

 accentuated than for the relay modulator. This indicates how much 

 more characteristic distortion FS exhibits than AM because of the 

 sharper roll-off of its signal bands within the confines of the same 170- 

 cycle channel spacings. Of course, as the word speed is raised beyond 

 the practically usable values, either type of modulation leads to so much 

 power in the filter cutoff regions that the characteristic distortions be- 

 come more or less indistinguishable. 



2.5 "Polytonic" System 



This is a frequency discrimination system experimentally proposed 

 for toll and local signaling.^ It works on 5 channels at speeds of 100 and 

 300 decimal digits per second (or the equivalent of some 330 to slightly 

 under 1,000 bits per second). It has some similarities to an earlier multi- 

 frequency system,'" but is faster. 



The distinguishing characteristic of the polytonic system lies in the 

 mathematical theory which has been followed to reduce interchannel 

 interference. This analysis makes use of the theory of orthogonal func- 

 tions, and is similar to that used in the computation of Fourier com- 

 ponents. The mathematical analysis leads to an ideal receiver design 

 for minimum interchannel interference. The ideal detector in this 

 receiver closely resembles the conventional homodyne detector. The 

 detector actually UvSed, however, represents a practical simplification of 

 the latter. A complete description cannot be given here, but it may be 

 noted that the theory leads to a need for synchronization of the signal 

 elements in the five channels, and to the setting of an exact timing 

 instant for the sampling of the received wave to obtain minimum inter- 

 channel interference. The indication for this instant is obtained from 

 the use of a sharp wave-front pulse in the marking channels. 



Tests with the 100 decimal digit per second system (100 decimal 

 digits normally correspond to 332 binary digits) indicated that it gave 



