STUDIES IN RADIO BROADCAST TRANSMISSION 145 



Such a modulated high-frequency wave drawn out in the familiar 

 graphical representation is a comparati\'ely simple-looking thing, but 

 analyzed into its elements and studied in detail it is revealed as being 

 an intricate fabric of elemental waves so interwoven with each other 

 that no one of them can be disturbed without changing in some 

 degree the complexion of the whole. For perfect results the whole 

 band must arrive at the receiver with an amplitude continuously 

 proportional to that leaving the transmitter, or the inflections or expres- 

 sion of the speech or music will not be correctly reproduced. All the 

 component frequencies within the band must be unchanged in their 

 relative amplitudes lest the character of the sounds be altered. Even 

 the relative phase relations of the various frequencies must be preserved 

 or, as will be shown later, the interaction of the two side bands in the 

 receiving detector will result in the partial loss of some of the frequency 

 components. 



It is not long since the time when radio was supposed to be the 

 perfect medium for voice transmission it being presumed that since the 

 ether of space (if there be such a thing) was substantially perfect in 

 its electrical characteristics it must transmit frequency bands carrying 

 telephone channels without distortion of any kind. This may be true 

 theoretically of a pure ether but in fact, the ether used for radio com- 

 munication is filled with a number of things ranging from gaseous 

 ions down to the solid bed rock of the earth. It is rather to be ex- 

 pected that these will affect the progress of electromagnetic waves 

 and we know from experience that they do. Diurnal variations of 

 attenuation, fading, directional changes, dead spots and the like 

 are already well known phenomena resulting from the complexity 

 of our transmission media, although no entirely adequate explana- 

 tions of their causes have been certainly established. One of the 

 most recent manifestations of the effects of irregularities in trans- 

 mission through space is in the distortion of the quality of telephone 

 signals. This was perhaps first noticed in the use of short waves for 

 broadcasting it being found that frequently the transmission was so 

 distorted that after detection the signals such as speech and music 

 were in severe cases almost unrecognizable. 



Preliminary Investigations 



For some time after quality distortion was recognized as a character- 

 istic of existing short wave transmissions, it was thought that for the 

 lower broadcasting frequencies at least, it was present only at night 

 and at relatively very great distances from the transmitter. However, 



