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BELL SYSTEM TECHNICAL JOURNAL 



The 4000-cycle channel spacing would permit obtaining a narrower 

 band width with some simplification in the selecting circuits. With 

 further development in selecting circuits, it is believed that it would 

 permit obtaining a somewhat wider band or, if desired, a reduction 

 in the cost of apparatus, maintaining the same band. 



The band chosen initially for the new systems is believed to be a 

 desirable and forward-looking step in the direction of improving the 

 quality of speech transmission, a continuing trend which is as old as 



u 



I 1000 2000 3000 



FREQUENCY-CYCLES PER SECOND 



Fig. 1 — Transmission frequency characteristics of broad-band systems. 



telephony itself. Figure 2 shows typical band characteristics which 

 mark the progress of transcontinental telephony since 1915. For 

 shorter distances, the band widths have, of course, generally been 

 wider than indicated on this series of curves. In the case of carrier 

 systems the band depends on the number of links. The curve shown 

 for 1937 is for the broad-band systems, estimated on the basis of a 

 three-link connection. 



The increase in band width is achieved without material increase in 

 cost, since in situations which favor their use, broad-band systems 

 provide circuits which are substantially more economical than other 

 alternatives, and the improvement can therefore be obtained by giving 

 up only a small portion of the savings which the systems themselves 

 make possible. If, as in some older types of systems, it had been 

 chosen to maintain a standard of 250 to 2750 cycles for a single-link 

 connection in the broad band systems, this could have been accom- 

 plished by the use of a channel frequency spacing of about 3000 cycles. 

 The wider transmission band is therefore obtained by a sacrifice in 



