TRANSOCEANIC RADIO TELEPHONE DEVELOPMENT 563 



has radically changed the character of the physical plant and invest- 

 ment necessary to the employment of directivity in short-wave 

 transmitting and receiving. 



In Hawaii and the Philippines on circuits to the United States the 

 "diversity" method of reception is used wherein three individual 

 separated antennas and receivers with interlocked automatic gain 

 controls are combined to produce a common output having less distor- 

 tion and noise than a single receiver. 



The effects of distortion in short-wave circuits are avoided to some 

 extent by an arrangement called a "spread sideband system," which 

 has been used on circuits between Europe and South America. By 

 raising the speech in frequency before modulation the speech sidebands 

 are displaced two or three kilocycles from the carrier and many of the 

 product frequencies resulting from intermodulation fall into the gap 

 rather than into the sidebands. 



On the Holland-Java route a system is being used whereby more than 

 one sideband is associated with a single carrier or pilot frequency, each 

 such sideband representing a different communication. 



An improved signal-to-noise ratio is given by a device called a 

 "compandor" ^ employed on the New York-London long wave circuit. 

 It raises the amplitude of the weaker parts of the speech previous to 

 transmission. In depressing these raised parts to their proper relative 

 amplitude, after reception, the compandor also depresses the accumu- 

 lated radio noise. 



Present Outlook 



The foregoing makes it evident that many fundamental engineering 

 problems have been solved and that the pioneering stage of the service, 

 when its possibility of continued existence might reasonably have been 

 in doubt, has definitely been passed. In looking toward the future we 

 find that the greatest needs are for improvement in reliability and in 

 grade of service, accompanied by reduced costs. 



Improving the reliability struggles against the fact that short wave 

 transmission varies through such a wide range of effectiveness, and 

 seems to be so much influenced by the sun. We have not only a daily 

 cycle in the transmission of a given frequency but also an annual cycle 

 and beyond this an eleven-year cycle associated with the change in 

 sunspot activity. Superimposed upon these are erratic and occa- 

 sionally large variations associated with magnetic storms. 



^ The synthetic word "compandor" is a contraction of the compound word "com- 

 pressor-expander," which describes the effects the device has on the volume range 

 of speech. 



