JOIST DEVELOPMENT AND NOISE FREQUENCY INDUCTION 201 



effects of noise. As previously pointed out, present telephone trans- 

 mitters materially amplify the jiower received from the voice so that 

 the electrical power on the telephone line is some hundreds of times 

 greater than the acoustic power applied. In development work on 

 telephone transmitters, telephone engineers are proceeding on the 

 basis that more is to be gained by improving the frequency response of 

 the transmitter than can be gained by mere increase of power. This 

 line of development has, of course, the effect of raising power levels at 

 frequencies where they have been relatively low. 



Two proposals for application to the toll telephone plant were 

 studied. One would involve changing the repeaters now in use at 

 terminals and at intermediate points to a more powerful type and 

 equipping all toll circuits with terminal repeaters of this same type. 

 This would permit raising the power levels without altering the relative 

 levels of the various telephone circuits and thus would not change the 

 crosstalk. Another would involve such changes only on certain long 

 toll circuits, leaving the remainder of the circuits at their present 

 levels. As the result of a trial installation, it was found that to realize 

 any appreciable change in level on these circuits, very extensive changes 

 would be required to avoid crosstalk from the higher level circuits to 

 the remaining ones which were not changed. 



The levels employed in carrier telephone circuits, while somewhat 

 lower than those used on voice-frequency open-wire telephone circuits 

 at the receiving end. are higher at the sending ends than the corres- 

 ponding voice-frequency levels. Since the power system harmonics 

 in the carrier-frequency range normally are small as compared to those 

 in the voice-frequency range, carrier-frequency open-wire systems 

 experience considerably less noise from power systems. 



Effects of Noise. — The actual voice power level on telephone circuits 

 varies over a wide range, depending upon the particular user, his 

 distance from the telephone central office, and by the transmission loss 

 in the connection between the two subscribers. Impairment caused 

 by a given amount of line noise on the circuit may also vary over a 

 considerable range, depending upon the voice power level and the noise 

 in the room where the telephone is being used. The method in use 

 by the Bell System for an engineering basis in considering the effects 

 of noise on telephone conversation is to substitute for the noise in- 

 creases in the transmission loss of the circuit. Thus, the circuit with 

 its actual loss and noise is represented by a circuit of lower noise and 

 increased transmission loss. These added losses are known as Noise 

 Transmission Impairments and are abbreviated N. T. I. The N. T. I.'s 

 were determined from articulation tests and judgment tests made 



