Radio broadcast .. (1922-30)

A FILTER THAT SAVES 5 KILOCYCLES This picture shows part of the speech input, modulating, filter and amplifying apparatus at the Rocky Point station of the Radio Corporation of America, where radiotelephonic transmission to Europe is carried on. The filter suppresses the carrier wave and one of the side-bands, thus halving the width of the channel necessary for transmission Channels By Albert F. Murray THE progress of radio in the past has been so rapid, due to scientific genius and manufacturing skill, that a large part of the American public is now in the frame of mind in which it believes that almost anything is possible in the way of future developments. This atti- tude is held not only by the lay en- thusiast for radio, but by some serious experimenters and technically minded men. New developments are expected to be realized almost by magic, and tech- nical problems of the most difficult nature immediately solved by some new product of the inventor's skill. So it is that when- ever some advance is made in radio, the press and the public immediately greet it as the herald of a new radio Utopia, with- out consideration of what technical, prac- tical, or manufacturing obstacles are in the way. We can understand then, in view of this attitude, the great stir of interest which some time ago was occasioned by press reports in which a well-known radio engineer outlined a "double tuning" sys- tem which was said would multiply each 'T'HK solution of the problem of broadcast congestion is -*• perhaps the most pressing of the needs of the broadcast listener to-day, and many people seem only too willing to believe that the solution will come, mysteriously, from some new invention. And so, when Dr. Lee Def-'orcst some time ago made the statement that some 500,000 radiotelegraph stations could be disposed on the short-wave channels from 10 to 200 meters by means of a system n! "double modulation," the press and general public be- lit'-.'ed that the long expected solution of the broadcasting problem was solved, and "double modulation" would soon multiplex all radio channels. Mr. Murray considers the multiplexing of radio chan- nels by means of "double modulation" from a technical and practical angle, shows how the system works, exactly what advantages may be realised from it, and what defects have kept it from being used so far. A method of mul- tiplying channels — that of "single side-band" trans- mission — is also examined in a similar way. It is this second method which Mr. Murray considers most possible, its present practicality being limited by the need of skilful operation and precision instruments. —THE EDITOR. 245 of the present radio channels by 100, thus making room for all those who are clamor- ing for space in the crowded ether. Con- currently, the question of finding space in the short-wave bands for the radio traffic of many corporations and private enter- prises engaged the attention of the Fed- eral Radio Commission. The natural result is that many of the radio public are now asking. "Is there really anything in these new radio systems?" Regarding the proposed "double tun- ing" system, suppose we consider these questions: Is it a new or novel system? Will it multiply the existing radio chan- nels? Are there good chances of this sys- tem being used in our country? How can more radio channels be made available? IS THE SYSTEM NOVEL? F'IRST, the system referred to in the press reports as "double tuning" is usually more accurately described by the name of " double modulation" or " multi- plex radio telephony." In radio, "modu- lation" means the " moulding" of the radio wave at the transmitter by voice frequen-