NEGATIVE FEEDBACK 



A21 



\ 



The region embracing the appearance of the above phenomenon is 

 also characterized by a marked reduction in signal level. At high 

 modulation levels "cracking" begins at a somewhat lower disturbance 

 level than is necessary to initiate it in the absence of modulation. The 

 initial effect is to impart a roughness to tone modulation. Further 

 increase in disturbance level produces a rapid depression of the signal, 

 so that it soon becomes submerged in noise. The manner in which this 

 depression takes place is shown in Fig. 14. The signal, produced by 

 1000-cycle modulation was measured by means of a highly selective 



O t5 



5 10 15 20 25 



R. M.S. CARRIER-DISTURBANCE RATIO IN DB 



Fig. 14 — Depression of output signal by the disturbance. Modulation: ± 7 

 kilocycles for no feedback, ± 78.5 kilocycles for 21 -decibel feedback, and ± 124.5 

 kilocycles for 25-decibel feedback. Points P and Q denote incidence of crackling in 

 the absence of modulation, for 21- and 25-decibel feedback, respectively. 



analyzer so that observations could be carried well below the general 

 noise level. 



The point at which depression of the signal begins coincides with the 

 appearance of roughness in the output tone resulting from the momen- 

 tary suppression of the signal by the higher noise peaks. A further 

 increase in disturbance level increases the number of peaks per second 

 which rise above the critical value, and the energy content of the signal 

 is rapidly diminished. The point at which faint crackling could first 

 be detected in the absence of modulation is indicated on each curve. 



The signal-to-noise ratios obtained with zero and with 25 decibels of 



