This system, which enables FM broadcasting stations to transmit simultaneously two or more different programs, is the most recent development based on principles which were described by Dr. Armstrong in 1935. The new development, however, employs a number of improvements created since World War II. The result, according to Dr. Armstrong, will be to double, at least, the effective function of the FM transmitter, "with obvious far-reaching effects on the methods of broadcasting radio communications of all kinds."
Several new types of service are made possible by this new system. An FM station, for example, can transmit a musical program on one channel, and send out on a second channel a voice program such as a speech, news bulletins or an advertising program; or it can transmit a single program stereophonically on the two channels. It can also act to transmit two independent musical programs so as to relay along a program to another station which it itself is not carrying on the main channel. All of these methods of operation have been successfully tested at Dr. Armstrong's Alpine FM transmitter, KE2XCC, at Alpine, N.J.
Dr. Armstrong added that, of course, the system would not be limited to two channels only.
The present-day FM set is quite unaffected by the presence of the second channel, Dr. Armstrong said. Listeners on present-day FM sets, in fact, are unaware that there is a second program from Alpine that they are missing. Such two-channel transmissions have passed unnoticed for several years from the Alpine station. In order to receive the second channel, a modified form of FM receiver will be necessary, but the number of additional parts required will not unduly increase the cost of the set.
While multiplex operation of FM in the communications field has been in practical use for some years, its use in broadcasting presented problems not important in the communications field. These problems have been studied and successfully disposed of in several years of quiet work in the Marcellus Hartley Research Laboratories at Columbia and in experimental tests at the Alpine transmitter, with the result that the system has been brought to a practical entity, according to Dr. Armstrong. The credit for the engineering of the system, he said, belongs to Mr. Rose who, with Perry Osborn, chief engineer of the Alpine station, directed the experimental work.
He added that the real significance of the new system is that FM will prevail as the final aural system. It would be completely impossible to multiplex on any AM system, either on the standard band, or on a very high frequency (v.h.f.) band of the same frequency as the FM stations, and a station operator who can sell time on two channels obviously will outsell eventually the station operator with only one. The cost of equipping a transmitter for multiplex operation is not high. Transmitters already equipped with Serrasoid modulators which are employed in the new system are most readily susceptible to change, he said. Receiver prices will doubtless follow the course they have always taken since the superheterodyne receiver was introduced, producing a set ultimately fitting the capabilities of everyone's pocket-book.