APPLICATION OP HIRE TRANSMISSION TO RADIO 139 



band required for transmission and would double the message-carrying 

 capacity of the ether were no frequency range required to space the 

 channels apart. This advantage of the present method is likewise of 

 special importance in long-distance-long-wave transmission. 



Quality of Transmission 



We have spoken above of factors concerned with the volume of 

 transmission and only incidentally of that other requisite of trans- 

 mission, namely, good quality. Without going into this matter in 

 much detail, it will be well to make note of the several factors in- 

 volved in obtaining good quality, as follows: 



1. It is important that a substantial band of voice frequencies be 

 transmitted. Of course, distorted talk can be transmitted on a 

 relatively narrow band, but commercial transmission has been found 

 to require a single side-band width of the order of 2,000 or more 

 cycles, the band width increasing with the quality desired, up to 

 about 5,000 cycles. 



2. It is necessary that the distortion which is due to non-linearity 

 of transmission with respect to amplitude, be avoided. This is 

 equivalent to saying that there should not be permitted to take place 

 self-modulation between the components of the side-band, nor the 

 too close cutting-ofT of the peaks of the telephone waves due to satura- 

 tion effects. 



3. The transmission must be kept free from interfering noises. 

 The ratio between interfering noise current and voice currents of the 

 order of 0.1 is regarded as large in wire practice. While this amount of 

 interfering current will not prohibit service, it does seriously impair the 

 effectiveness of transmission and annoys the listener. In radio the 

 ratio of static noise to signal strength is very often much greater than 

 this value. As the radio art progresses it will be necessary to work 

 toward standards more nearly in keeping with those which have been 

 found necessary for wire service. 



The writer wishes to express his indebtedness to the following of 

 his associates for helpful suggestions and assistance — Messrs. J. R. Car- 

 son, Ralph Bown, and D. K. Martin. 



APPENDIX 



The curves of Figs. 2 and 3 are based upon the following equations 

 and data: 



