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FM Beep Signals: Supersonic-Controlled FM for Bus and Storecasting
June 1951 Radio-Electronics

June 1951 Radio-Electronics

June 1951 Radio-Electronics Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

This news bit from a 1951 issue of Radio-Electronics magazine reports on the FCC's declaration of illegality the practice by some FM broadcasting stations of providing a means for blanking out commercials and station identification to entities willing to pay for the special receivers and pay for a subscription. Nobody I have ever known looks forward to enduring commercials on television or radio (or Internet these days). The only way most of us could listen to music without interruption was to by a record, tape, or CD. VHS tapes and DVDs provide some relief from commercials, although even though you pay for them there are typically promotions for other movies at the beginning. Commercials on radio and television (and now the Internet) have consumed a larger part of each hour of programming with each passing year. The DVD collections we have of 1960s and 1970s Prime Time TV shows average run times of about 54-55 minutes for what was an hour-long program. Nowadays, you are lucky to get 40 minutes because of all the commercials (not that I watch any of them). One familiar form of commercial-free music was MUSAK (ended in 2013), commonly referred to as elevator music. It was a subscription-based service provided by commercial broadcasters for a fee. A scheme called Subsidiary Communications Authority (SCA) permitted the addition of sub-carrier signals that could carry separate messaging that could be sold for use in restaurants, department stores, public buildings, and even elevators. Receiving the for-pay programming without permission is considered to be illegal, akin to how receiving a police radar signal with a passive radar detector in the state of Virginia is considered to be illegal.

FM Beep Signals

FM Beep Signals, used by some FM broadcast stations to mute receivers during commercials and station announcements in restaurants and other public places, are not legal, according to a ruling by the FCC. A few FM stations have been using such signals to provide "functional music" programs to subscribers as a source of additional revenue. (See April, 1951 issue, page 30, "Supersonic-Controlled FM for Bus and Storecasting.")

In a letter to four stations using the system, the FCC expressed sympathy with the aims of the broadcasters, but ruled that eliminating the station identification announcement violates the Communications Act. The commission further ruled that subscribers to such services" must be logged as sponsors, and all such programs must be "listed as commercial" time periods: The stations involved WRLD, Miami, Fla.; WACE-FM, Chicopee, Mass.; WFMF, Chicago, Ill.; and KDFC, Sausalito, Cal. were ordered to show how they intend to comply with the order by April 30.

Radio-Electronics April 1951 Cover - RF CafeSupersonic-Controlled FM for Bus and Storecasting

Radio-Electronics, April 1951, p30

The author describes the circuits used to increase volume or mute the set during FM broadcasts for store and transit systems.

By W. H. Collins

Since the advent of television, FM stations have been looking for a new source of revenue to enable them to keep operating. By carefully programming musical numbers, without vocals, some FM stations have been able to sell a music service to local merchants and factories which is unexcelled in quality and variety, and at the same time is particularly economical. By using a broadcasting station for this service, a much wider scope of operation is opened up, both from coverage and fidelity standpoints, than wired music services can provide in all cases.

S17-A-FM receiver, made by Collins Audio Products - RF Cafe

Photo A - This S17-A-FM receiver, made by Collins Audio Products, is typical of those used for FM music service. It requires only a speaker and an antenna.

Overtone type crystal tunes its oscillator to the correct frequency - RF Cafe

Photo B - An r.f. subassembly. An overtone type crystal tunes its oscillator to the correct frequency.

Subassembly is the tone plate - RF Cafe

Photo C - This subassembly is the "tone plate," which cuts the set on or off, depending on the supersonic tone signal it receives from the transmitter.

i.f. strip has three i.f., tubes, two limiters, and discriminator - RF Cafe

Photo D - The i.f. strip has three i.f., tubes, two limiters, and discriminator.

Hypothetical system shows the possibilities of FM music service - RF Cafe

Fig. 1-  This hypothetical system shows the possibilities of FM music service. Many stores and factories use it already, and its popularity is increasing.

Audio circuits of a music-service receiver - RF Cafe

Fig. 2 - The audio circuits of a music-service receiver. The 6SK7 is blocked when the mute tube is cut off, unblocked when the restore tube is cut off. High-Q tuned circuits select the tones which control the set's audio channel.

In store and transit broadcasting, the commercial announcement is reproduced at a higher volume level than the music to command the attention of the listeners. When the music resumes, the volume again drops to its previous level. When desired, the receivers may be automatically muted by the radio station and again restored to operation. Supersonic signals from 15 to 30 kc transmitted from the broadcast station control the receivers.

Fig. 1 portrays multiple setup using all the facilities of a supersonic controlled FM system. When music is transmitted, it is received by all receivers at all locations. Suppose the program schedule shows that a breakfast cereal commercial is to be made in the supermarkets. Before the commercial is made, supersonic tones must be transmitted so that the announcement is not received in the wrong locations. In this case, the bus receivers are muted by 21 kc, the department stores, smaller stores, and factory sets by 17 kc, but the same 17-kc tone boosts the volume in the supermarkets, to put across the advertising message. During the commercial, the 17-kc tone is sustained, and upon its completion, the tone is removed to return the supermarket receivers to normal volume level. But what about the receivers in the other four groups? They too must be brought back to life. A momentary 15-kc signal restores the bus sets and 26 kc restores all other locations. Then the musical program resumes.

The use, service, and maintenance of this FM equipment requires skilled technicians who must understand the operation of the system before attempting to service it. No one is more upset than a user of this equipment whose set doesn't work right or allows commercial announcements to come through, particularly when no announcement type of music service is paid for.

Special FM receivers and tuners which meet all the requirements of this work have been developed at the Collins Audio Products Company. These include a complete crystal-controlled, fixed-frequency FM tuner ; and a mobile receiver powered by batteries. The complete receiver appears in Photo A.

Most of the circuits are made up as subassemblies. For example, supersonic control frequencies vary with control station or area so that the "tone plate," as it is called, is assembled and wired separately. The same is true of the r.f. and i.f. units. The main chassis includes the output stage and power supply. This type of design also makes servicing easier.

Supersonic Control Circuits

When the supersonic tone, say 20 kc, is transmitted over the station's carrier, it proceeds through the r.f. portion and is demodulated by the detector circuit in the same way as the regular signal. Here, however, the 20-kc tone is taken directly off the discriminator cathode before de-emphasis and is amplified by the high-mu triode tube 12AX7 (see Fig. 2). The 12AX7 dual triode is used because of its high gain and its ability to amplify two separate signals. Photo C shows this subassembly.

In the plate circuits of the 12AX7 are two specially designed inductances which, when shunted with fixed capacitors, may be sharply tuned with a high-Q slug to the proper resonant frequency. The voltage developed across the coil is rectified by the 6AL5 and is impressed upon the grid of one of the 6AK6's.

The two 6AK6 tubes operate in a flip-flop circuit. At any specific moment, the plate voltage of one is much lower than that of the other because it is conducting. A negative voltage applied to the control grid of the 6AK6 which is conducting will cause the plate current to drop and the plate voltage to rise. Because the plate of each tube is cross-connected to the screen of the other, the opposite set of conditions occur to the other 6AK6 when this is done.

By alternately supplying a negative voltage to each of the 6AK6 grids, either one can be made to conduct. Plate and screen voltages for the first audio tube (6SK7) are obtained by one of the 6AK6 tubes, the one we will call the "restore" tube. When the receiver is playing, the plate voltage on the 6SK7 tube is approximately 35 volts. When the set is muted, this voltage drops to 22, which is insufficient for the tube to operate and amplify, thereby silencing the audio. Assisting this condition is an added bias on the 6SK7 cathode.

When the radio station sends the restore pulse, the restore tube is cut off and it again supplies plate voltage to the 6SK7 and amplification is resumed. The duration of tones emitted by the radio station need be only two or three seconds to achieve operation. After the momentary transmission of the mute tone, the receiver remains silent until the restore tone is transmitted by the station.

The Other Circuits

Crystal control of the oscillator circuit is almost a must, as frequency stability is paramount. The r.f. section, shown in Photo B, has plate and grid tuning for high selectivity and to avoid intermodulation of strong r.f. signals in close proximity. An overtone type crystal operates at three times its fundamental frequency in the oscillator circuit. This frequency is again tripled in the plate-coupling coil, and is injected into the grid of the converter tube, a 6AU6, through a low value ceramic coupling capacitor.

The crystal frequency for a given station frequency is:

Crystal frquency - RF Cafe

If a station is operating at 100.7 mc, the crystal frequency is obtained in this manner:

FM station frequency - RF Cafe

The antenna coil is tapped, which allows a low impedance lead-in to be used such as 50to 72-ohm coaxial cable. A 300-ohm ribbon line may also be used with slight modification.

The intermediate-frequency amplifier has a three-stage circuit for high gain. Following this are two pentode limiters and then a conventional dual diode demodulator. This is built up in a complete unit as shown in Photo D. A.V.C. is fed back to the first i.f. amplifier tube (6BA6) as well as to the first r.f. stage. This avoids overloading from very strong signals but allows full amplification on weak signals.

The audio circuit of this receiver has a 6SN7-GT phase inverter and two 6V6-GT's in push-pull arrangement. Fig. 2 also shows this circuit. The power output is 6 to 10 watts which is more than sufficient for a half dozen speakers in a typical installation. For requirements where more power is required, the R-12-A tuner is recommended, which may be used with any conventional amplifier.


The requirements for general radio listening are not critical from a transmitted power viewpoint, and a momentary break in the program due to atmospherics is quickly forgotten. With a music system, however, a strong signal must always be present at the receiver antenna for consistent results. It is not wise to work out too far from the station where the signal is only 25 to 50 microvolts. Even though a receiver has a sensitivity of 5 microvolts, a 50microvolt signal contour can easily drop to zero momentarily during a storm or extreme atmospheric pressure changes. If one of the control pulses is being transmitted during such a period, the set cannot respond, and a commercial will be received or the set will be left muted, as the case may be. This is one reason why a good antenna installation is so important.

A very rough rule-of-the-thumb estimate is that about 2,G00 watts of power is required for each 5 miles from the station. For example, if consistent operation is desired at 35 miles, 14,000 watts should be available, remembering, of course, that the curvature of the earth seriously hampers FM reception beyond about 50 miles. Working at a distance of 50 miles from the station requires a special receiving antenna and a power contour of the transmitted pattern.

Due to the high sensitivity of the set, almost any antenna will work in locations close to the transmitter. The one possible exception occurs if there are "dead spots" where there is a high order of man-made static or interference.

If there is a good signal of several hundred microvolts over a radius of 15 to 20 miles, a short length of wire may be used with the receiver. However, the performance of the set will dictate how long or high an antenna is required. Do not discount the possible advantage of a good antenna installation where the power of the transmitter is low or interference is high. The terrain is another thing to consider. If there are mountains, valleys, or tall buildings between the transmitter and the receiver, the signal may be reflected or altered in its characteristic and cause inconsistent operation. Under such circumstances, the higher and more elaborate the antenna system, the better. To overcome as much as possible the chance of. noise pickup in the lead-in wire, the antenna input circuit has been designed for use with 50to 72-ohm coaxial shielded cable.


Although the receivers are aligned, tuned, and adjusted at the factory for maximum performance at the designated frequency, certain field adjustments are necessary at the time of installation. This is because the test equipment cannot exactly match the operating frequency of the station. These adjustments include peaking the antenna circuit to compensate for antenna length and position. The discriminator must be balanced to place it exactly on frequency. The discriminator circuit is rather broad and considerable detuning or out-of-balance condition can be tolerated and still provide acceptable reception. But operation of the supersonic control Circuits depends entirely on the amount of energy developed at supersonic frequencies, so that precise setting of the discriminator is mandatory.

The r.f. unit and i.f. amplifier are peaked with a vacuum-tube voltmeter connected between the a.v.c. return of the first limiter tube and ground. With the set receiving the station, the tuning slugs are adjusted for maximum deflection of the meter.

After these adjustments have been made, the receiver is ready to be left on location if the supersonic control is working. The installer should remain with the set until he is certain that this most important part of the receiver is operating satisfactorily.  



Posted June 3, 2020

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