196 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



speed Multiplex circuits where the line frequency reaches 66 cycles per 

 second on certain letter combinations. 



The amplitude-modulated system still had difficulties inherent in 

 this type of operation. Any variation in the received signal strength 

 had to be compensated for by similar changes in the relay bias circuit. 

 Your amplitude-modulated radio suffers from the same types of 

 interference. 



Continual research by our Carrier System Development Group pro- 

 duced the first frequency-modulated terminal in 1937. A series of 

 imrovements led to the installation of the first type "E" carrier system 

 between Dallas and Los Angeles in the fall of 1942. This was a two- 

 wire system, using frequency-modulated terminals and provided one 

 voice channel in each direction of transmission. The frequencies of 

 the nine telegraph channels in the west-bound voice band range from 

 750 cycles to 3,150 cycles and the east-bound band from 4,050 cycles to 

 6,450 cycles. 



The frequency-modulated telegraph channel will continue to deliver 

 a perfect signal even though the input power to the receiving channel 

 amplifier may drop to one three-hundredths of the standard value. 

 The FM telegraph channels have contiuued to work without inter- 

 ruption when the physical wires over which they were working were 

 so badly weather-bound by heavy fog that Morse signals could not be 

 read between Chicago and Indianapolis. This system has also con- 

 tinued to work when one wire of the pair was broken and both ends lay 

 on the ground. 



The type "F" is also a two-wire system which provides two voice 

 bands in each direction with a top f reqency of 16 kilocycles. The two- 

 wire type "G" system provides four voice bands in each direction with 

 the top frequency of 30 kilocycles. 



Nine wide-band FM channels may be placed on one voice band. 

 These channels will carry signaling frequencies up to about 75 cycles 

 per second. Sixteen narrow-band FM telegraph channels may also be 

 placed on a voice band. The narrow-band channel is designed for a 

 top frequency of about 33 cycles per second and will be largely used 

 for Teleprinter operation. 



Even with all the margin provided by FM operation, interruptions 

 to the service were caused by weather conditions over which we had 

 no control. Early in the spring of 1946, ice formed on the wires in the 

 vicinity of Sidney, Nebr., until they became 3 inches in diameter. 

 The weight of this much ice will break the wires in many places and 

 if there is a slight wind, the ice-covered wires will start to swing and 

 break off many poles, sometimes every pole between sections that are 

 not extra-heavily braced. 



Terminal equipment appeared satisfactory, but trouble-free trans- 

 mission was required between terminals in order to render the best 



