378 BELL SYSTEM TECHNICAL JOURNAL 



however, that for a standard set, to be used on all loop condi- 

 tions, a response designed solely for the long loop, with its large 

 loss at high frequencies, would be distinctly "tinny" and dis- 

 agreeable in quality on average or short loops. 

 3. The materially smaller losses in the transmitted band for the aver- 

 age telephone connection than for the orthotelephonic system. 

 Even for the long loop conditions the losses are no greater than 

 for this orthotelephonic system up to about 2,200 cycles. In 

 other words, for a large majority of telephone calls the received 

 speech level will be higher for the same talking level than in the 

 case of direct conversation at one meter distance. The desira- 

 bility of this in a monaural system of limited frequency range 

 has already been indicated. 



Combined Effects of Circuit and Instruments 



The part played by the anti-sidetone circuit in permitting better 

 utilization by the subscriber of the capabilities of the telephone system, 

 in increasing the level of received speech, and minimizing the effect of 

 noise is summarized by the illustrative power level diagrams in Fig. 8, 

 using typical values of room and line noise and other conditions. 



The data shown were obtained by objective measurements with the 

 artificial mouth, sound level meter, volume indicator, and artificial 

 ear. Frequency weightings appropriate to the levels involved were 

 employed in the sound level measurements. 



Figure 8a shows, for the anti-sidetone set, the losses between the 

 talker and listener under average room noise and circuit conditions. 

 Figure 86 gives corresponding information for the sidetone set. For 

 comparison, an approximate level diagram is shown also for direct 

 speech. The relative speech levels at the transmitter for the two 

 telephone conditions were adjusted to give volume indicator readings 

 on the line in accordance with the results of service observations. The 

 upper curves in each drawing are for speech and the lower for noise 

 picked up by the transmitters. 



At the transmitting end the lower sidetone of the anti-sidetone set 

 results in higher talking levels, with about 4 db higher speech level 

 at the input to the circuit. In the overall circuit this gain is increased 

 by the receiving-end effect of the anti-sidetone circuit in minimizing 

 the effect of room noise. 



The total noise in the telephone ear, as shown on the drawing, has 

 as its principal contributing factors: 



1. Leakage of noise under the receiver cap. 



