COMPANDORS ON NARROW BAND SPEECH CHANNELS 1219 



form of transient response indicates that the width of pulses (and hence of 

 echoes) received over a 4 kc band, is such that they will be within 65% of 

 their peak amplitude for a full 125 microseconds. Thus such echoes will cause 

 at least 65% of their peak interference to at least one subsequent pulse. 

 This illustrates why it is so difficult to control intersymbol interference in 

 pulse systems operating under minimum bandwidth conditions. 



Assuming from this argument that the interference from echoes should 

 be taken at their peak values, the tolerable phase deviations from linearity 

 must be measured in tenths of a radian in this case. On ordinary speech 

 channels the tolerable phase deviations from linearity are measured in 

 radians, which represents a difference of one or two orders of magnitude. 



Another estimate of the allowable intersymbol interference may be ob- 

 tained by comparing it to quantizing noise on a PCM system. A 5-digit 

 PCM system has 32 quantizing levels, and the average uncertainty in the 

 recovered pulse amplitude is one half of a quantum step, or approximately 

 1.6%. The 10% intersymbol interference requirement chosen above repre- 

 sents approximately 6 times as much deviation in recovered pulse amplitude. 

 Again only subjective measurements can tell whether intersymbol inter- 

 ference in this case is six times more tolerable than quantizing noise. How- 

 ever, a 5-digit PCM system is not a high quality circuit by Bell System 

 standards. 



The distortion effects due to lack of synchronization of the transmitting 

 and receiving samplers have been ignored in the discussion so far, on the 

 assumption that it would not prove too difficult in practice to make it a 

 relatively negligible source of intersymbol interference. However, it may not 

 prove to be a negligible factor from the economic standpoint, when an at- 

 tempt is made to prove in a system of this type. 



Multichannel Aspects 



In the case of multichannel time division systems, the addition of in- 

 stantaneous campandors seldom requires an increase in the transmission 

 requirements of the medium. In multichannel PAM and PPM systems, for 

 example, intersymbol interference causes intelligible crosstalk between chan- 

 nels, and the requirements on such crosstalk usually calls for the intersymbol 

 interference to be some 60 db down in the recovered speech. In such cases 

 the addition of an instantaneous compandor can serve to reduce this re- 

 quirement on the line to some 40 db, through the so-called "Compandor 

 Advantage"^ It is fair to point out, however, that such systems are seldom, 

 if ever, operated as minimum band pulse systems. 



*€. O. Mallinckrodt, "Instantaneous Compandors," B.S.T.J., Vol. XXX, No. 3, 

 July 1951. 



