156 BELL SYSTEM TECHNICAL JOURNAL 



magnitudes. Occasionally one cycle of the be^t tone will be some- 

 what upset by a sudden change in the amplitude but in general no 

 changes which consistently distort the wave form were observed. 



The slow fading may be considered as a modulation and on this 

 basis the received signal is seen to be composed of the original con- 

 stant carrier frequency accompanied by very narrow side bands 

 occupying at best perhaps a fraction of a cycle. 



The next progressive step in the radio transmission studies is 

 naturally from a single frequency to two or more frequencies trans- 

 mitted simultaneously. By the use of two crystal oscillators at the 

 transmitter two separate and distinct radio frequency signals were 

 transmitted simultaneously. These crystals were ground by the 

 Bell Telephone Laboratories to oscillate at 610,000 cycles and 609,750 

 cycles. The amplitudes of these signals at the transmitter were 

 controllable so that it was possible to make them equal, or one larger 

 than the other, equivalent to the relative magnitudes usually found 

 for the carrier and single side-band transmission case. Records 

 were obtained of the variation of these radio signals, but none is 

 reproduced here since the information shown by them can be just as 

 easily obtained from the triple frequency records shown below. 



Radio transmission on three frequencies is readily obtained by 

 modulating the carrier with an audio frequency tone, and observing 

 the three frequencies separately at the receiver. 



If the modulating tone is 



sin (vt-\-<i)) 

 and the carrier signal 



A sin pt, 



the transmitted signals are 



Aa 



+ -^ cos [(p+v)t+(t)] (upper side band) 



-fyl sin pt (carrier) 



Aa 



and ^ cos \{p — v)i — ^\ (lower side band). 



where a is a constant proportional to the percentage modulation. 



These three frequencies are not merely a mathematical fiction 

 b ut are physically existent as three separate waves bound together 

 only at their point of origin. 



To adequately record them separately by means of the oscillograph 

 advantage was taken of the fact that a group of frequencies beaten 



