578 BELL SYSTEM TECHNICAL JOURNAL 



In one case, a control frequency has been transmitted either by wire 

 Hne or radio to each station and a frequency multiplier used to develop 

 directly the carrier frequency which was to be transmitted from the 

 station. This method has met with some success both here and abroad. 

 It was used in this country for the commercial operation of WBZ- 

 WBZA ^ and in Germany the Postal authorities have operated several 

 stations experimentally with equipment developed by the Telefunken 

 G.m.b.H. and the C. Lorenz A.G.^^ Both the WBZ-WBZA and the 

 Telefunken systems used a high control frequency which was particu- 

 larly suitable for transmission over open wire lines while the Lorenz 

 system used a lower control frequency which was suitable for transmis- 

 sion over cable circuits as well. Three stations located at Berlin, Stet- 

 tin, and Madgeburg, respectively, are now in commercial operation on a 

 common frequency using control equipment manufactured by the 

 Lorenz firm.*' In Sweden the postal authorities have developed a sim- 

 ilar system of frequency control capable of using either a high or low 

 standard frequency interchangeably. This system was used in placing 

 the broadcast stations at Malmo and Halsingborg in commercial 

 operation on a common frequency in the latter part of 1929.'' Inten- 

 sive development work on similar systems is under way in the United 

 States. The National Broadcasting Company has in operation in its 

 network, two groups of two stations each, which are being operated 

 synchronously using a standard reference frequency transmitted be- 

 tween stations over telephone circuits. The Bell System has developed 

 a common frequency broadcast system using a standard reference 

 frequency suitable for transmission over telephone circuits. This sys- 

 tem has been given a practical test in cooperation with the Columbia 

 Broadcasting System. It will shortly be commercially available. 



The other method of attack has been to derive the carrier frequency 

 at each station from an independent oscillator. In England,^' ^ electri- 

 cally driven tuning forks have been used to supply an audio frequency 

 of high stability from which the carrier frequency has been derived by 

 means of frequency multipliers. With this equipment it has been pos- 

 sible to maintain the derived carrier within a few cycles per second of 



3 Frank B. Falknor, "A History of Synchronization," Citizens Radio Call Book 

 Magazine and Technical Review, 12, 38-40; March, 1931. 



4 W. Hahn, Funk, 35, 247-248, 1928. 



B W. Hahn, Die Sendung, 5, 430-432, 1928. 



6 F. Gerth, "A German Common Frequency Broadcasting System," Proc. I. R. E., 

 18, 510-512; March, 1930. 



7 Erik Esting, Elektroteknik, pp. 109-112, June 7, 1930. 



8 P. P. Eckersley, "The Operation of Several Broadcasting Stations on the Same 

 Wave-length," Jour. I. E. E., 1929. 



" P. P. Eckersley, "The Simultaneous Operation of Different Broadcast Stations 

 on the Same Channel," Proc. I. R. E., 19, 175-194; February, 1931. 



