RELIABILITY OF RADIO TELEPHONE CIRCUITS 191 



no significance except as an arbitrary reference point on this scale. 

 A vertical line is erected through this point as shown by g-h. If, on 

 a certain short-wave circuit, conditions are unsatisfactory 50 per cent 

 of the time, the effect of 10 db improvement upon this percentage 

 may be determined by shifting the curve a-c-d-b of Fig. 5 ten db to 

 the left and reading the percentage value on the vertical scale opposite 

 the intersection of the vertical reference line. It will be remembered 

 from previous discussion that such a shift of the 6-A noise curve 

 toward lower values accompanies a corresponding db transmission 

 improvement. If 50 per cent of the time conditions were unsatis- 

 factory in the former case, they would be unsatisfactory only some 25 

 per cent of the time for the same tolerable noise condition and 10 db 

 improvement. That is, the lost circuit time has been reduced from 

 50 to 25 per cent by 10 db transmission improvement. 



By shifting the curve a-c-d-b of Fig. 5 various amounts to the right 

 and left and tabulating the percentages obtained as described above, 

 a generalized "reliability" curve may be plotted which shows the 

 transmission improvement required to reduce the lost circuit time by 

 any desired amount. Similarly, if we know the percentage lost time 

 on two circuits their transmission performance may be compared on a 

 decibel basis by determining the horizontal db separation between 

 these two lost time values on the "reliability" curve. 



A "reliability" curve of the kind described above is shown in Fig. 6. 

 Although it is obviously unsafe to conclude on the basis of the data 

 presented that this curve is accurately representative of all long- 

 range short-wave circuits and circuit conditions, it serves to indicate 

 the order of service improvement that will be afforded within the 

 practical range of transmission improvement. For example, to reduce 

 the lost or unsatisfactory circuit time from 50 per cent to 25 per cent 

 appears to require about 10 db transmission improvement on any 

 long-range short-wave circuit. Starting with a 50 per cent lost time 

 condition and applying improvements in 10 db steps the successive 

 percentages of lost circuit time would be roughly 25, 10, 2.5, 0.7 and 0.1. 



Changes in the standards of tolerable service may be treated as 

 equivalent to a change in the effectiveness of transmission as described 

 earlier. Thus in terms of a high grade service the lost circuit time 

 might for example be 50 per cent. For a grade of service 10 db lower 

 than this the lost circuit time would be reduced to 25 per cent. The 

 effect is equivalent to improving the transmission 10 db for the same 

 standard of service. 



