COMMON FREQUENCY BROADCASTING 579 



isochronism I'' and this has been sufficient to permit a satisfactory ser- 

 vice to be rendered to the territories immediately adjacent to each sta- 

 tion. As will be shown in detail later there is a substantial difference 

 between the service range of a station operating in almost perfect iso- 

 chronism with the other stations in the common frequency broadcast 

 system and that of a station which is more than a small fraction of a 

 cycle per second out of isochronism. In this country "matched crys- 

 tals" and other means of independent frequency control have been tried 

 but the frequency stability of the best equipment available in the past 

 has fallen far short of that required for the satisfactory operation of the 

 stations on a common frequency. 



In the spring of 1930 the Central Broadcasting Company of Iowa 

 found itself in the possession of a concrete example of the need for the 

 simultaneous operation of two stations on a common frequency in that 

 its stations WHO and WOC were compelled to divide time equally on 

 1000 kc. so that the Davenport and Des Moines areas each received ser- 

 vice from its local station but half the time. These stations are 153 

 miles apart and either could be depended upon to render a high-grade 

 service only within a radius of about fifty miles of the station. It was 

 felt that with the simultaneous operation of both stations, each of these 

 areas would receive full time service from its local station. 



The Central Broadcasting Company presented its problem and 

 asked for equipment capable of maintaining the carriers of these two 

 stations within the limits of isochronism required for their simultaneous 

 operation. Bell Telephone Laboratories therefore undertook the 

 necessary development work. 



The degree of isochronism required for the various conditions ex- 

 isting under the different types of common frequency broadcast sys- 

 tems is in fact a fundamental question that must be answered before 

 any logical delineation of the problem can be attempted. Unfortun- 

 ately there exists no similar condition in ordinary human experience 

 from which a valid analogy can be drawn, so that the a priori assump- 

 tions which have been used in the preliminary theoretical discussions of 

 the various phases of this problem have of necessity been based pri- 

 marily upon personal opinion and the resultant conclusions have quite 

 naturally varied between extremely wide limits. 



The problem had been studied intensively during the preliminary 



field tests of common frequency broadcasting which were made in the 



fall of 1929 in cooperation with the Columbia Broadcasting System us- 



^^ The term "isochronous" has been used instead of "synchronous" in order to 

 exclude the concept of identity of phase which is usually included in the meaning 

 of the latter together with the meaning of identity of frequency which is common to 

 both words. 



