272 BELL SYSTEM TECHNICAL JOURNAL 



The establishment of this frequency range played a part not only 

 in the design of circuits, but also in guiding the evolution of the 

 transmitter and receiver. The transmitter last referred to meets this 

 requirement very well. In fact, its efficiency is fairly uniform for a 

 frequency range extending beyond 4000 cycles. 



The next step in the process was to improve the performance of 

 the receiver. A pronounced resonance at 1000 cycles was no longer 

 necessary since means had been found to improve the efficiency of 

 instruments in other ways than by concentrating all the resonances 

 at one frequency. The importance of the higher frequencies in trans- 

 mitting and reproducing the transient sounds characteristic of the 

 consonants in speech led to placing more emphasis on these frequencies 

 and attempting to produce more uniformly the band of frequencies 

 which was set as a limit for circuits. This has now been accomplished 

 in a practical fashion in the receiver which is being introduced in 1937. 



The effect of this evolution in the design of station instruments may 

 be brought out by a comparison of the overall response characteristic 

 — that is, the relation of the sound delivered to the ear to the sound 

 available at the transmitter — for a typical telephone connection having, 

 in one case, both terminal instruments of the 1920 type and, in the 

 other case, the terminal instruments of the coming new 1937 type. 

 In this typical circuit, the trunk has been taken as free from dis- 

 tortion so that its effect will not influence the indicated performance 

 of the instruments, although the circuit does include two 22-gauge 

 loops each three miles long. 



At the resonance point of the old instruments, just over 1000 cycles, 

 the overall response in going to the new instruments is reduced by 

 almost 30 db while the response in the range from 2000 to 3000 

 cycles is increased by over 20 db. In the frequency range from 500 

 to 2000 cycles, the circuit employing the older instruments shows a 

 variation of overall response of over 30 db. For the new type, the 

 variation for this same frequency range is reduced to 15 db, and, 

 furthermore, this variation of 15 db applies approximately for the 

 range of 250 to 2750 cycles which was mentioned as the transmission 

 range requirement for the design of new circuits. In regard to the 

 variation of 15 db in this frequency range, there is good indication 

 that this response is more desirable than one of no variation, from the 

 standpoint of having the telephone performance approach that of 

 direct air transmission. 



In addition to these improvements in frequency response and 

 efficiency, the intensive development program on these instruments 

 has improved materially the stability of the carbon transmitter under 



