TRANSMITTER FREQUENCY RANGE 



485 



per cent, for the 3,000-cycIe point is 87 per cent and for infinity is 97 

 per cent. Increasing the cutoff from 2,000 to 3,000 cycles gives a 

 growth in articulation which is 17/27, or .63, of the total increase in 

 articulation which would be obtained in going to a cutoff of infinity. 

 The values for the other curves of Figure 2 are obtained in a corre- 

 sponding manner, it being appreciated that the articulation values with 

 commercial instruments are lower than those for the Master Reference 

 Circuit. This method of plotting the results has the advantage of 

 showing the rate of growth of articulation for the three kinds of circuits 

 on a comparable basis. 



1.0 

 09 

 OB 

 07 

 0.6 

 Ob 

 0.4 

 0.3 

 02 



0.1 



2000 2400 2800 3200 3600 4000 



'.UTOFF FREQUENCY OF LOW PASS FILTER IN CYCLES PER SECOND 



Ordinate is: 



Af - A: 



2000 



Am - Aa 



Where Af = the syllable ar- 

 ticulation with a low pass 

 filter of cutoff frequency f. 



A2000 = the syllable articula- 

 tion with the 2000 cycle 

 low pass filter. 



Am = the syllable articulation 

 obtained with no filters. 



Fig. 2 — Syllable articulation of telephone systems at optimum received volume 



under quiet conditions. 



It is seen from the curves of Figure 2 that raising the upper fre- 

 quency limit from 2,000 to 2,500 cycles gives about one-half of the 

 total increase which would be obtained in going to an infinite cutoff 

 and raising to 2,750 cycles gives for the commercial instruments about 

 two-thirds of the increase in articulation which would be obtained in 

 going to an infinite cutoff. These curves do not indicate any particular 

 cutoff frequency as a stopping point for commercial circuits but it is 

 considered that going as far as about 2,750 cycles is justified. While 

 there is some articulation advantage in going further, observations of 

 the number of repetitions occurring in conversations over circuits 

 having different cutoff frequencies have indicated but little reduction 

 in repetitions by going beyond about 2,750 cycles with commercial 

 types of terminal sets. 



For the lower end of the range, the lower cutoff frequency curve of 

 Figure 1 shows little effect on articulation of cutoffs below 400 cycles. 



