124 BELL SYSTEM TECHNICAL JOURNAL 



operating level is then established at a suitably lower value, frequently 

 6-10 db. Recently, by modulating at low power levels, transmitters 

 have been produced which are capable of 100 per cent modulation 

 without noteworthy distortion. It is obvious that, if a transmitter 

 of this latter type is employed and the same margin is observed in 

 determining the average audio input level, the resulting sidebands will 

 be twice the amplitude of those produced by a transmitter whose 

 modulation capability is only 50 per cent. To produce equivalent 

 sidebands with a transmitter capable of but 50 per cent modulation 

 requires that the carrier amplitude be doubled or the power output 

 multiplied by four. In other words, insofar as signal-to-noise ratio is 

 concerned, which is the factor that usually determines the coverage 

 of a broadcasting station, the increase in modulation capability men- 

 tioned results in an improvement that in the older type of apparatus 

 could only be had by quadrupling the rated output of the transmitter. 

 From the coverage standpoint, the night range of a given station can 

 be approximately doubled in this manner. Since this is accomplished 

 without increase in the carrier power, the outlying zone in which the 

 station may produce serious beatnote interference with others as- 

 signed to the same channel will not be extended. Accordingly, the 

 use of transmitters capable of a high degree of modulation is a notable 

 contribution toward the more effective utilization of the medium, 

 which is the outstanding technical problem in American broadcasting 

 today. 



. Another important factor, from the standpoint of intensive devel- 

 opment of the available frequency band, is frequency maintenance. 

 In a system involving so many stations as are now operating in the 

 United States, accurate maintenance of the assigned frequencies pre- 

 sents a very difficult problem. The maximum deviation permitted 

 by the existing government regulations (± 500 cycles) is somewhat 

 beyond the capabilities of the ordinary wavemeter and difficulty has 

 been experienced in obtaining a satisfactory substitute. In the ab- 

 sence of adequate frequency control apparatus, very serious beatnote 

 interference has been of frequent occurrence. During the past year, 

 however, considerable improvement has been brought about by the 

 extensive adoption of piezo-electric reference oscillators and automatic 

 piezo-electric control. Equipment for the latter purpose capable of 

 a relatively high standard of performance is now being offered com- 

 mercially and it is probable that apparatus of this type will be installed 

 in the near future by the majority of stations. Its use is expected 

 to avoid entirely heterodyne interference on the "cleared" channels, 

 where the beatnotes are those produced between the carriers of sta- 



