BACKGROUND NOISE IN BROADCASTING 337 



should reduce the flutter effects, since it would iron out the variations 

 in carrier amplitude impressed upon the detector. However, it is 

 evident that this stabilization of the carrier will be exactly offset by 

 the variation imposed upon the sideband amplitudes, and that conse- 

 quently the flutter effects should be as evident when a normally func- 

 tioning automatic volume control is used as they are in the case of 

 manual control. 



A perfectly functioning automatic volume control should make 

 flutter effects approximately independent of the type of detector em- 

 ployed when the beat frequency is of the order of 2 or 3 cycles. How- 

 ever, at some of the higher frequencies, of the order of 20 to 40 cycles, 

 the control will function with reduced efficiency, and at still higher 

 frequencies will not function at all. Consequently, in this intermediate 

 range the gain control may have some special effect and may make the 

 flutter either worse or better than it would be with the same type of 

 detector and manual control. 



Experimental Studies 

 Equipment Used in the Study of the Effects of a Noise Background 



A laboratory investigation was made of the interference between 

 two waves of slightly different carrier frequency. A block schematic 

 of the equipment used is shown in Fig. 1. 



A modulated signal could be received from Station WABC, or, by 

 throwing the switch S, it was possible to obtain an unmodulated carrier 

 from a Western Electric No. 700A Oscillator, which is of very great 

 frequency stability.^ Whichever signal was used was fed through 

 an impedance matching transformer to a radio frequency attenuator. 

 The output of this attenuator was fed into the grid of one tube of a 

 mixing amplifier. As indicated in the drawing, this amplifier consists 

 merely of two shield grid tubes having a broadly tuned common plate 

 circuit load. 



The other tube of the mixing amplifier was energized, through a 

 second radio frequency attenuator, by an unmodulated carrier derived 

 from a crystal controlled laboratory oscillator of the same type as that 

 which served as an alternative to WABC. This oscillator was part of 

 a Western Electric No. 1 A Frequency Monitoring Unit.^ The monitor 

 includes arrangements for measuring frequency differences between the 

 oscillator included within it and an external source. In this case the 

 external source was WABC, or the alternative carrier. The energy 



3 0. M. Hovgaard, "A New Oscillator for Broadcast Frequencies," Bell Labora- 

 tories Record, 10, 106-110, December, 1931. 



■• R. E. Coram, "A Frequency Monitoring Unit for Broadcast Stations," Bell 

 Laboratories Record, 11, 113-116, December, 1932. 



