1318 THE BELL SYSTEM TECHNICAL JOURXAL, NOVEMBER 195-t 



to use in the design of single frequency guard type receivers. In observa- 

 tions on many thousands of calls it was noted that vowel type sounds 

 were the predominant cause of signal imitations, with all except a few 

 being formed by female speech. At the highest frecjuency tested (2,600 

 cycles) over 90 per cent were caused by the long e vowel sound (as in 

 jeet). At the intermediate frequencies 1,350 and 1,800 cycle) most ^'owel 

 sounds were noted, while at 800 cycles signal imitations were caused 

 principally by two sounds, namely (as in hole') which accounted for 

 about 50 per cent of the total and aw (as in awX) and similar sounds like 

 ah as in father. 



Signal imitations from vowel sounds are to be expected because of 

 their relatively large energy and sustained nature. For instance it is well 

 known that a sustained long e sound can have a large component in the 

 high frequency range with very little energy in the range from 500 to 

 2,000 cycles where the guard action is effective. Likewise a sustained 

 long sound can have a large peak in the 500-cycle to 1,000-cycle range 

 with little energy in the 1,000-cycle to 3,000-cycle range where the guard 

 action is effective for the 800-cycle receiver. 



Speech formed currents are not the only source of signal imitation. In 

 one series of observations using 2,600-cycle receivers in\'ol\'ing circuits 

 from New York to a number of other cities including Toronto, Boston, 

 Baltimore, Washington and Miami a total of 69 signal imitations were 

 observed. In each case an attempt was made to determine the sound 

 that caused the false operation, with the results given in Table IV. 



NOISE CONSIDERATIONS 



Noise affects the signaling system in a \^ariety of ways depending upon 

 the nature of the noise and upon the particular signaling function being 

 performed. When tone is first applied it is of course desired that the re- 

 ceiver operate. However at this time the "guard" circuit is functioning 

 because it is also desired that the receiver be non responsi\-e to speech. 

 Noise, which acts on the guard circuit like speech, will therefore tend to 

 prevent operation of the receiver. If the noise is steady and large enough 



