694 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1957 



The recognition that use of B = 100 rather than a value approaching 

 1,000 may imply a change from six to eight digits per code group (e.g., 

 for ju = 1,000), representing an increase of 33 per cent in the required 

 bandwidth in the transmission medium as well as a significant increase 

 in the complexity of the multiplex terminal eciuipment, provides the 

 proper perspective for competent appraisal of the cost of improving 

 gate circuitry to the point where B would approach 1,000. These con- 

 siderations might be of crucial importance in the planning of actual 

 PCM systems. 



Finally these results also show that caution is required in attempting 

 to determine an adequate nimiber of digits and/or degree of compression 

 from listening tests employing preliminary experimental equipment. If 

 the conditions of the test do not duplicate exactly the expected behavior 

 of the channel gates to be used in the final system, the transition from 

 the laboratory to practice might lead to an embarrassing disappearance 



40 

 38 

 36 

 34 

 32 



SIGNAL POWER IN DECIBELS BELOW FULL LOAD SINUSOID 

 5 10 15 20 2S 30 35 40 45 50 



cr 30 



? 



O 

 Q. 



CH 



o 

 a. 

 a: 



LU 

 IS 



z 

 N 22 



28 



26 



24 



Z 

 < 



a 



< 



z 

 (J 

 in 



20 



18 



16 



14 



12 



4 6 8 10 



20 40 60 100 



C 



200 400 600 1000 



Fig. 18 — Signal to quantizing error power ratios (in db) as a function of rela- 

 tive signal power for companding corresponding to /x = 1,000. Symbols have the 

 same significance as in Fig. 15. B = co throughout. 



