634 BELL SYSTEM TECHNICAL JOURNAL 



energy levels. Signal components ranging in frequency from 10 to 

 20,000 cycles must be transmitted with as little discrimination with 

 respect to either amplitude or phase as reasonable design practices 

 will permit. It is required that a suitable radio system be designed 

 to afford satisfactory transmission between terminals when operated 

 under prevailing conditions with respect to static, other radio traffic, 

 and local electrical disturbances. The maximum allowable "noise" 

 level is probably somewhat arbitrary but it has been found that if the 

 ratio of signal to interference current is 10:1 the results are satisfac- 

 tory. The variation of amplitude with frequency should probably 

 not exceed ± 2 TU at any point in the required signal band. The 

 equivalent of the circuit must be substantially constant; in other 

 words, no fading effects can be tolerated. In this respect a variation 

 of perhaps 3 TU is the maximum allowable." 



2. For synchronizing purposes, a second channel must be provided 

 to transmit 17.7 and 2125 cycles, the impedances and the signal 

 energy levels at both ends of the circuit being known. The grade of 

 transmission required in this case is probably considerably lower 

 than that needed for the picture circuit but stable operation must 

 be assured. 



3. Arrangements must also be made for a high quality telephone 

 channel to transmit speech and music for loud speaker reproduction. 



4. All of these channels must, of course, be capable of operating 

 simultaneously without mutual interference and without effect on 

 established radio services. 



Preliminary Survey 



In the vicinity of New York, an assignment of this type is sur- 

 rounded with unusual difficulty due to the serious congestion which 

 exists in the ether. Operations were started, therefore, by under- 

 taking a survey of available frequency bands at periods of the day 

 during which transmission might be required. 



The pioneering nature of the project and the character of the 

 apparatus available led to an early decision to base the system on 

 the transmission of the carrier and both sidebands. Since the upper 

 limit for the signal was specified as 20,000 cycles, an interference-free 

 band somewhat greater than 40,000 cycles in width was, therefore, 

 required. The unusual width of this band indicated the desirability 

 of fixing upon a relatively high carrier frequency. No readily available 



^ Definitely agreed on limits were essential to proper coordination of the various 

 development activities and figures of the order mentioned were assumed for design 

 purposes. 



