564 BELL SYSTEM TECHNICAL JOURNAL 



A statistical study of the data secured from operation of transoceanic 

 radio telephone circuits over the past several years has given valuable 

 help in engineering circuits to meet a given standard of reliability. 

 This study has shown that the percentage of lost time suffered on a 

 circuit appears to follow a probability law and that its relation to the 

 transmission effectiveness of the circuit in decibels is given by a straight 

 line when plotted to an arithmetic probability scale. Such a relation 

 tells us, for example, that if a circuit as it stands suffers 15 per cent lost 

 time, the lost time can be reduced to a selected lower value, say 5 per 

 cent, by improving the circuit a definite amount, in the assumed case 

 10 decibels. It then becomes possible, by making engineering cost 

 studies of the various available ways of securing the necessary number 

 of decibels improvement in performance, to choose the most economical 

 one. This approach is being applied to study of the radio telephone 

 circuits extending outward from the United States. Some of the tech- 

 nical possibilities which are being considered for improving these cir- 

 cuits are discussed below. 



The performance of a radio telephone circuit may be changed by 

 dynamically modifying the amplification or other characteristics of 

 the circuit in accordance with the speech transmitted. The compandor 

 already mentioned is an example of this kind of improvement on long 

 waves. Further developments particularly suited to the vagaries of 

 short-wave transmission are possible. 



The operation of the vodas, or voice-operated switching device 

 linking the wire and radio circuits, is adversely affected by noise. 

 Methods are being investigated for using single frequencies, called 

 "control tones," transmitted alongside the speech band and under the 

 control of speech currents, to give more positive operation of the 

 switching devices and reduce the adjustment required. 



The transmission improvement of about 9 decibels (about 10 : 1 in 

 power) offered by single-sideband suppressed-carrier transmission has 

 been delayed in its application to short-wave transmission partly be- 

 cause of the high degree of precision in frequency control and selectivity 

 necessary to its accomplishment. In recent years successful apparatus 

 has been developed and proved satisfactory in trials. The introduction 

 of single sideband into commercial usage is already in progress. 



Turning now from the transmitting to the receiving end, one funda- 

 mental way to reduce noise in radio telephony is to employ sharper 

 directivity. It has been found by observation that there is a limit to 

 which directivity, as ordinarily practiced, can be carried to advantage. 

 It is easy to design antennas so sharp that at times very large improve- 

 ments in signal-to-noise ratio are secured. But it is found that at other 



