PRIVATE LINE DATA TRAXSMISSIOX 14()1 



between 1 and volt, a sudden drop of 6 db (without compensating 

 change in the "sHcing" or critical level) causes error. 



In the two level signal between 1 and 0.25 volt, the permissible drop 

 is reduced to 4 db (ratio of 0.625/1). 



In the three level signal, the permissible drop is reduced to 1.8 db 

 (ratio of (0.625 + 0.1875) /I). 



Automatic gain control and corresponding adjustment of the slicing 

 level ameliorate these conditions to some extent, but the problem is still 

 a serious one; a sudden rise is also serious. 



The use of a compandor in the transmission facility exaggerates the 

 situation. Some discussion of the action of a compandor to improve the 

 signal-to-noise ratio for speech is given in Section 3.1. For the moment 

 it can be said that, at the transmitting end, the compandor compresses 

 the range of amplitudes in the speech signal at approximately a syllabic 

 rate. At the receiving end an expansion restores the original amplitude 

 relationships in a complementary manner. 



The compression and expansion are matched almost perfectly as far 

 as the human ear is concerned. However data transmission is more vul- 

 nerable to short time level irregularities. This allows small imperfections 

 in the amplitude restoration to impose a further penalty in the form of 

 error hazards. 



2.1.4 Other Considerations 



Present ideas call for the acceptance of the signal by the telephone 

 company, and redelivery to the subscriber not in the form indicated 

 by Fig. 1, but in the form of the three separate components listed in 

 2.1.2. This presents some problems regarding the transmission of such 

 signals over the connecting loops between the subscriber and the tele- 

 phone office. These are not, however, germane to the general transmis- 

 sion problem and will not be considered further here. 



2.2 Bell Telephone Laboratories and Lincoln Laboratory Vestigial Side- 

 band Systems 



The first of these represents an unpublished exploration, particularly 

 bj^ C. B. H. Feldman and A. C. Norwine, of the possibilities of trans- 

 mitting moderately high speed data pulses over telephone facilities. 



The exploratory system used a carrier at 2,200 cycles; a vestigial band 

 extended up to 2,600 C3^cles; the nominal effective band extended down 

 to 1,375 cycles; and a roll-off band continued down to 1,100 cycles. In 

 order to reduce the quadrature component resulting from the ^'estigial 

 sideband the spacing signal was made equal to one-third the marking 



