498 BELL SYSTEM TECHNICAL JOIRXAL 



The noise is increased (over llie 71-db value) by the compandor only in the 

 presence of speech and then only in proportion (roughly) to the amplitude, 

 and so becomes masked by the speech. The masking is sufficient to make 

 impairment of medium and loud speech imperceptible provided that the ratio 

 of speech power to noise power is greater than about 22 db. Under these 

 conditions we are justified in defining the equivalent signal-to-noise ratio in 

 terms of the low level noise. 



For compandors with a more drastic characteristic, yielding more low- 

 level improvement, the high-level noise increase is enhanced and the limit 

 to this enhancement is controlled by the "uncompanded" signal-to-noise 

 ratio (the ratio without companding.) Thus the amount of low-level im- 

 provement that is permissible from the standpoint of high-level performance 

 is determined by the uncompanded signal-to-noise ratio. Experiments 

 have shown that the permissible low-level improvement increases several db 

 for each db increase in uncompanded signal-to-noise ratio. Another way of 

 putting it is that the value of the equivalent signal-to-noise ratio in the speech 

 channel determines the amount of compandor advantage which may be 

 invoked to attain that ratio, and that the permissible compandor contribu- 

 tion increases nearly as fast as the equivalent signal-to-noise ratio. The 

 uncompanded signal-to-noise ratio is thus required to increase only slightly. 



For the 45 db uncompanded signal-to-noise ratio of Fig. 2 the compandor 

 could have been designed to yield more than the 26 db low-level improvement 

 shown without impairing high-level performance. In the time-division 

 systems of message grade with which we will deal later, a 22 db compandor 

 advantage is assumed." 



In the quantized systems included in the PCM headmg the instantaneous 

 compandor advantage applies to the granularity, or quantizing, noise in the 

 same way as to the common kinds of noise which plague other systems. The 

 compandor of Fig. 2 was actually used in an experimental PCM system.^- 

 A discussion of quantizing noise appears in Appendix I and a more compre- 

 hensive treatment appeared in the Bell System Technical Journal recently.'^ 



In transmitting frequency divided groups of channels by pulse methods" 



" This is the maximum compandor advantage permissible for a circuit equivalent to 57 

 db signal-to-noise ratio. We will use this figure in connection with power requirements for 

 circuits whose signal-to-noise ratio is intended to be equivalent to 60 dl) but since we pre- 

 sume that interference or crosstalk ma\- be present in an amount equal to noise and since 

 the compandor acts on interference as on noise we must protect against high level impair- 

 ment on the basis that the noise is 3 db greater. 



12 L. A. :Meacham and E. Peterson, ".An Experimental Multichannel Pulse Code Modu- 

 lation Svstem of Toll Qualitv", Bell Sys. Tech. JL, Vol. 27, pp. 1-43, Jan. 1948. 



" \V.' R. Bennett, "Spectra of Quantized Signals," Bell Sys. Tech. Jl. Vol. 27, pp. 446- 

 472, July, 1948. 



'^ If the group occupies a frequency range extending from zero to Fb, the minimum 

 sampling rate is well known to be 2Fb. If the group range does not start at zero frequency 

 the minimum sampling rate is not twice the highest frequency of the group but lies be- 

 tween two and four times the width of the band depending on the location of the band. 

 This matter is treated in Appendix V. 



