MULTIPLEXED FM BROADCASTING

IDEAS FOR MULTIPLEX AUDIO OR FACSIMILE OPERATION
THAT OPEN NEW SOURCES OF REVENUE FOR FM BROADCAST STATIONS

In its initial stages, FM broadcasting was considered as a directly competive system to AM, offering advantages of freedom from interference, superior audio quality, and greater primary service range. Subsequent work by research development engineers has disclosed at the characteristics which afforded the advantages just enumerated make possible additional, special services which cannot be provided on AM.

This is due to the fact that control signals, or even a second audio program can be transmitted by an FM station without any interference with its primary modulation up to 15,000 cycles. Thus, with two multiplexed channels available, the following combinations are available FM broadcasters:

   1.  Channel A can be used for regular programs and commercial announcements, and channel B for the same program but 
        different commercials, or no commercials at all. 

   2.  The two channels can be used to carry entirely different entertainment programs, or for a mobile conmmnication service on 
        channel B. 

   3.  Channel A can be used for regular programs, and channel B for facsimile news transmission. 

Multiplex Operation:

It is now possible for any FM broadcaster to purchase and install at his transmitter a relatively simple piece of equipment, about the size of a single amplifier panel, which in effect gives his station two completely independent channels. The normal FM channel can be used in the regular way to transmit aural programs with complete freedom from interference between that transmission and audio or facsimile modulation on the second channel.

The result is much as though the broadcaster were assigned an additional transmitting frequency. However, the added B channel does not require any additional space in the radio spectrum.

Neither does it call for a second transmitter and tower, nor an increase in operating personnel or costs. The only additional expenses are those of installing a multiplex amplifier, and of preparing and selling the programs that are to be transmitted over channel B.

Any FM receiver can be modified to receive the B channel programs by merely adding a single-tube adapter-amplifier. This unit is cheap and small, and its installation in the receiver is simple. The set can be arranged to receive channel B only, or either channel A or channel B. Of course, if the user of the receiving set wants to reproduce the programs going over both channels at the same time, he will need a second audio amplifier and a second loudspeaker or a facsimile recorder.

Uses of Multiplexing:

How would the use of this multiplex system affect the operation of an FM station engaged in transit radio, for instance?’ The only changes necessary would be installation of the multiplexer panel at the transmitter and an adapter amplifier in each bus receiver, so that it would respond to channel B signals instead of those of channel A. The supersonic control circuit and relay could be removed, and this source of possible failure eliminated, for with multiplex operation the relative volume of music and commercials on channel B can be controlled from the studio without changing the levels on channel A.

The station can transmit the same music and the same commercials on both channels, or it can use the same music for both and insert different commercials in the transit program. Or, by using a somewhat more elaborate studio setup, two entirely different programs can be broadcast, one designed to please regular home listeners and the other planned to be most effective as heard in the buses or trolleys. The advantages of such a new facility for the FM station are quite apparent.

The same flexibility of operation furnished by multiplexing obviates the need for program or announcement compromises between a home service and either storecasting or functional music. Multiplex operation unties the hands of the FM broadcaster who is interested in any of these three supplementary aural services, by letting him plan two programs in such a way as to give optimum performance to each group of listeners.

Facsimile Service:

Furthermore, through the use of multiplex. facsimile transmission can be handled in a more efficient and effective manner than is possible with ordinary simplex operation. Although provisions have been made in the FCC’s Rules and Regulations for simplex transmission of facsimile, its use is generally limited to experimental or test work. There are at least two reasons for this: 1) It can only be broadcast when the station is carrying no sound program, and 2) the FCC allows only one hour of simplex facsimile between 7:00 A. M. and midnight.

On the other hand, as a result of a recent amendment of FCC Rules and Standards, facsimile may be multiplexed with sound programs without any time limitations, provided there is no degradation of the aural program below 15 kc. This advantage, plus the practicality of simultaneous transmission of facsimile and aural programs open new opportunities to FM broadcasters who are interested in developing facsimile service.

Network Multiplexing:

A recent development is the practical demonstration, by full-scale field tests, of the feasibility of networking multiplexed sound and facsimile programs.

This was proved out in cooperation with Major Armstrong who made his Alpine FM station available for feeding the first experimental programs into the Rural Radio Network. Subsequently, WOR-FM became the starting point for a four-station FM multiplex network extending from New York City to Ithaca, by way of the RRN (affiliated) stations at Poughkeepsie and Scranton.

In a recent demonstration, the network was operated to deliver some eighty pages of facsimile daily to those attending the observation of Farm and Home Week at Cornell University in Ithaca.

Shorter daily programs are being continued, and it is hoped that the present facility will be extended to furnish facsimile news, weather and crop reports, agricultural information, and similar data over all the stations of the Rural Radio Network from facsimile editorial offices at Columbia or Cornell.

Future Possibilities:

Consider what could be done by using a multiplexed FM radio relay network for handling two sound programs. One can easily visualize a new and entirely independent broadcast system, perhaps even national in scope, which could relay audio programs from one FM station to another. These programs would be transmitted on channel B and would be available to all stations in the chain.

However, if for any reason one station did not want to broadcast the channel B network program on its A channel, it could relay it on channel B and carry on its own local origination. Hence the would be no necessity for breaking chain.

FM-TV, the Journal of Radio Communications/June 1951
John V.L. Hogan/Hogan Laboratories, 155 Perry St, New York, NY, Consultant, stations WQXR and WQXR-FM