GB2146204A - FM subsidiary transmission method and system - Google Patents

Abstract

A method and system for providing multiple channels of high quality FM transmission within the allotted baseband spectrum of commercial FM stations is presented. Efficient utilization of the available spectrum is provided by use of suppressed carrier single sideband modulation for each of the individual channels of transmitted intelligence, without utilizing any of the spectrum for transmission of sideband subcarrier frequencies. The already existent stereo pilot frequency of the FM stereo transmitter is used to derive the desired sideband subcarriers in the transmitter, and similarly identical reinsertion carriers in the associated receivers. Problems associated with inexact local oscillators are thereby avoided, while the need for transmission of separate sideband subcarriers is eliminated. <IMAGE>

Description

SPECIFICATION FM subsidiary transmission method and system TECHNICAL FIELD The present invention relates in general to an improved method and system for electromagnetic signal transmission, and in particular to an improved method and apparatus for transmitting multiple channels of high quality FM subsidiary communications within the allocated baseband spectrum of standard FM stereo commercial broadcasts, by use of sideband transmission techniques utilizing a reinsertion carrier derived from the existing FM stereo pilot subcarrier.

BACKGROUND OF THE INVENTION Advances in technology and the resulting demand for near instantaneous and flexible transmission of data and entertainment programming has placed great demands upon the limited portions of the electromagnetic spectrum which are available for commercial and private use. Therefore, it has been an ongoing goal of broadcast technology to provide maximum utilization of the scarce broadcast spectrum resources available.

Existing FM SCA (Subsidiary Communications Authority) broadcasts are one example of an effort to maximize the utilization and flexibility of the available broadcast spectrum.

Although commercial FM stations in the United States are presently allocated a 100 KHz band, typical stereo transmissions require only the lower 53 KHz. The balance is therefore potentially available for alternate use. Due to inherent limitations in equipment, however, subsidiary communications were for many years limited to the narrow band between 53 KHz and 75 KHz, with a guard band extending from 75 to 100 KHz to prevent interstation distortions. In response to significant advances in transmitter and receiver design and careful frequency allocation, FCC guidelines now permit subcarrier transmissions to extend from 53 KHz to 99.9 KHz, and to utilize numerous forms of signal modulation and encoding.

The availability of FM SCA transmissions provides many important benefits to both the broadcaster and the public. Such signals are routinely used for transmission of commercial private broadcasts,such as background music transmission and paging functions, as well as for transmission of data such as market or agricultural information. Many other public services, such as foreign language transmission and reading services for the handicapped, which are not economically feasible for standard commercial broadcasters, are possible.

In addition, SCA broadcasting provides important income to the commercial broadcaster, particularly to the smaller urban broadcaster with specialized program formats. This additional income from SCA channel leasing permits higher quality programing and services which would otherwise be economically unfeasible.

Accordingly, demand for SCA channels typically outstrips the number of existing channels available, particularly in crowded urban areas.

Therefore, it has been and continues to be highly desirable to maximize the number of SCA channels provided within the allotted spectrum. Existing systems typically utilize a single frequency modulated SCA signal at 67 KHz, with numerous sidebands. For many years this was the only SCA channel available in association with a given commercial stereo station. More recently, some broadcasters have added a second SCA channel, with a frequency modulated signal at 92 KHz. Such existing systems still fail, however, to make maximum use of the available spectrum, and therefore fail to meet modern demand.

It is thus desirable to take fullest advantage of the available SCA broadcast spectrum for transmission of desired intelligence, while minimizing use of this resource for transmission of redundant or unnecessary data or carriers. One method of reducing such inefficiencies is by use of sideband transmission, and particularly single sideband transmission in which the redundant second sideband and the carrier are suppressed. Such techniques have been used for years by radio amateurs, for they provide much greater power and reduced noise when compared to standard AM transmission, and requires less bandwidth and transmitter power.

Unfortunately, there are several shortcomings with existing sideband methods and apparatus. They typically either fail to take maximum advantage of the available broadcast spectrum, or provide only limited quality of reception. Such problems arise because of the inherent requirement of a reinsertion carrier for demodulation of the received signal to recover the original modulating intelligence signal. For quality reception, the reinsertion signal must be identical to the original carrier utilized at the transmitter to generate the sideband. It is therefore necessary for the receiver to have a highly accurate reinsertion signal representative of the original subcarrier.

In order to provide an accurate reinsertion signal, many existing sideband systems broadcast a separate subcarrier along with the sideband, which then may be utilized by the receiver to provide reinsertion carriers. For example, some systems transmit a clock or synchronizing frequency which may be hetrodyned or divided to provide the desired reinsertion carriers. However, there are inherent and important shortcomings of such existing systems, for a portion of the scarce available broadcast spectrum must be utilized for the transmission of the specialized synchronizing frequency which contains no useful intelligence information. The power required to transmit such separate carriers also increases the operating expense of the transmitter, and consumes a portion of the total signal modulation available to the broadcaster.

Another method which has been used in order to avoid the need to transmit a separate reinsertion carrier involves use of a locally generated reinsertion signal. Ideally such a locally generated signal is identical in frequency and phase to the original carrier. However, hardware constraints and practical limitations prohibit such perfect operation. Even advanced and expensive crystal oscillators exhibit inherent frequency variations, as does the carrier oscillator of the transmitter itself.

Thus, it is impossible to precisely match a locally generated reinsertion signal to the original transmitter signal.

This shortcoming is particularly devastating to the quality of the received signal. Differences of only a few cycles per second between the reinsertion signal and the original carrier frequency may cause serious signal degradation to the recovered intelligence, such as a frequency shift equal to the instantaneous difference in frequency of the original carrier and the reinsertion carrier. This may result in a "Donald Duck" effect to transmitted voice or audio, and is particularly devastating to data transmissions, which require stable operation and accurate reproduction of the original signal for representation of digital data. Because loss of a single bit of such encoded digital information may prove fatal to a desired transmission, transmission speeds, efficiencies, and reliability are seriously reduced.In response, many modern SSB communcations systems utilize a very costly stable oscillator system with a vernier or "pitch" control which can be adjusted to make the received voice or audio sound natural. However, such manual adjustments are inherently imprecise, and the need for manual monitoring and adjustment, in addition to the prohibitive complexity and expense of the system, makes it unsuitable for widespread and general use by non-technical operators.

In view of the foregoing, it becomes an object of the present invention to provide an improved method and system for allowing transmission of multiple FM SCA subchannels within the allocated portion of the commercial FM broadcast spectrum, along with the normal stereo signal. A specific object is to provide such a method and system capable of transmitting five or more independent SCA subchannels within the band extending from 53 to 99.9 KHz.

A related object is to provide a system which will provide additional SCA channels without affecting existing FM stereo or standard 67 KHz SCA transmissions.

An important object of the present invention is to maximize the efficient utilization of the available SCA broadcast band and more fully utilize the available broadcast resource it provides. Therefore, it becomes a specific object to permit such SCA transmissions and multiple subchannel SCA transmisions without utilizing any portion of the SCA broadcast band for transmission of redundant and unnecessary information, or for transmission of separate sideband subcarriers.

A further object of the present invention is to provide such multi-channel utilization while simultaneously assuring high quality in the resulting received signals, such that the individual subchannels may be utilized for quality transmission of numerous forms of audio and data intelligence without sacrifices in quality.

An additional object is to provide such high quality reception without the need for technical adjustments or compensation by the operator.

Yet another object is to provide such an improved method and system which is compatible with existing commercial stereo broadcast transmitters, and which is in full compliance with all FCC (U.S. Federal Communications Commission) guidelines and rules for FM subcarrier operations. A further object is to provide such an improved system in a manner which may be utilized in conjunction with existing stereo broadcast equipment as an "add on" device, without substantial modification of the existing equipment.

Finally, it is an object of the present invention to provide such an improved transmission method and system in an economical fashion such that the substantial benefits offered by the invention are maximized and are economically available to broadcasters, to industry, and to the public.

These and other advantages, objects, and features of the present invention will become apparent in light of the present specification and accompanying drawings.

BRIEF SUMMARY OF THE INVENTION In order to achieve the aforementioned objects and to overcome the shortcomings and problems associated with the prior art, the present invention provides a FM subcarrier broadcast method and system which permits the broadcast and high quality reception of multiple SCA channels within the allotted spectrum. Use of sideband transmission minimizes the spectrum utilization of each channel, while the shortcomings typically associated with reception of sideband transmissions are avoided by deriving the subcarrier frequencies for each channel directly from the available 1 9 KHz stereo pilot, both at the transmitter and at the associated individual receivers.

One embodiment of the present invention provides these benefits by utilizing a sup pressed carrier single sideband modulation scheme, with the associated subcarrier suppressed by at least 60 db. Only one sideband is transmitted, which is displaced from the suppressed subcarier by the highest instantaneous modulating frequency of the intelligence signal to be transmitted. Suppression of the carrier avoids the difficulties of broadcasting a separate subcarrier, which may otherwise use up to 10% of the main channel modulation.

In order to avoid the signal distortions resulting from imperfectly matched subcarriers at the transmitter and the receiver, and to avoid the problems associated with local generators, the present method and system provides for utilizing the available 1 9 KHz stereo pilot frequency as a reference carrier or clock frequency from which the desired subcarriers are then derived at both the transmitter and the receiver. Because the frequencies and phases of the derived subcarriers at the transmitter are therefore related to the frequency and phase of the transmitted stereo pilot, the present invention permits identical reinsertion subcarriers to be likewise generated at the receiver from the received pilot, which are accordingly of identical frequency, and of equal phase relative to the sideband modulation signals, to the transmitter subcarriers.No separate subcarriers need to be transmitted, yet multiple perfect reinsertion signals for multiple channel sideband operation result.

At the transmitter, this 1 9KHz carrier may be either provided directly to the SCA system, or may be filtered from the standard composite stereo signal. The pilot may then be processed to derive a set of subcarier frequencies. In one embodiment, this is accomplished by dividing the resulting carrier by two, resulting in a 9.5 KHz reference frequency. Multiple subcarriers are derived from this signal by shaping the output to produce harmonics evenly spaced at 9.5 KHz intervals, including signals at 57 KHz, 66.5 KHz, 76 KHz, 85.5 KHz, and 95 KHz. These frequencies all are within the available SCA baseband, and may therefore be utilized to provide five independent SCA subcarriers.

In one embodiment, individual phase locked loops are utilized to lock onto the desired individual subcarriers, which are then fed tobalanced modulators to produce a suppressed carrier dual sideband modulated signals from the individual intelligence signals. These signals may then be filtered to remove the undesired upper or lower sidebands to produce a single sideband signal with no carrier.

In similar fashion, the corresponding receiver processor for use with the present method and system may comprise a standard wideband FM receiver with a baseband response of 30 Hz-100 KHz. The 1 9 KHz stereo pilot carrier is filtered from the received signal, and stripped of all stereo modulation components, resulting in a 1 9 KHz pilot signal identical to the pilot present in the transmitter. This signal is then processed to produce the desired subcarriers. In one embodiment corresponding to the previously discussed transmitter, the basic frequency is divided by two and shaped to provide harmonics of the resulting 9.5 KHz signal. A phase locked loop may then be utilized to lock onto the desired harmonic, which becomes the sideband carrier for reinsertion into the receiver demodulator.

In this manner, the reinsertion carrier will be precisely equal in frequency, and in phase relative to the sideband modulation signals, to the carrier of the transmitter modulator. The resulting sideband system may therefore be utilized to provide high quality audio reception.

Alternative algorithms for deriving reinsertion carrier frequencies from the 1 9 KHz stereo pilot frequency at the transmitter and at the receiver are also possible. For example, the pilot frequency may be divided by a greater number and thereafter multiplied to generate desired subcarrier frequencies for channels spaced at closer intervals. Such a system could provide yet additional SCA channels within the broadcast baseband, although of more limited bandwidth. Yet another embodiment may provide for multiplying the stereo pilot frequency to a high resulting value, and thereafter dividing to obtain the desired individual reinsertion carrier frequencies. In either alternative, however, care must be taken to assure that the total injection of all resulting SCA channels is within the 10% maximum established by FCC (U.S. Federal Communications Commission) rules.Such limitation may be accomplished by time division, assuring that the resulting subcarriers are interleaved or of differing phase, or may alternatively be assured by additionally providing fast response AGC circuitry responsive to the total signal from all SCA channels.

It is seen, therefore, that the present invention provides a system and method of SCA transmission that will permit numerous subchannels to be broadcast within the available SCA baseband spectrum by use of efficient sideband techniques, while simultaneously providing previously unavailable quality in the resulting reception.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of a preferred embodiment of an SCA transmitter associated with the present invention.

Figure 2 is a block diagram of a preferred embodiment of an SCA receiver associated with the present invention.

Figure 3 is a block diagram of an alternative embodiment of the subcarrier frequency processor.

Figure 4 depicts the baseband spectrum of a standard existing FM stereo and 67 KHz SCA broadcast channel.

Figure 5 illustrates the baseband spectrum of an FM stereo and five channel FM SCA broadcast channel according to the preferred embodiment of the present invention.

Figure 6 illustrates the baseband spectrum of a standard existing FM stereo and 67 KHz SCA channel, with additional channels provided according to the present invention.

Figure 7 is a block diagram of a preferred embodiment of an automatic gain control for use with the present invention.

Figure 8 is a circuit diagram of a preferred embodiment of the positive rectifier illustrated in Fig. 7.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The remaining portions of this specification will describe preferred embodiments of the invention when read in conjunction with the attached drawings.

Fig. 1 illustrates a basic functional block diagram of a preferred embodiment of the present invention for use in conjunction with existing FM stereo and FM SCA broadcast equipment. The stereo generator 100 of such standard FM stereo broadcast equipment produces a stereo composite signal 101 for broadcast, which comprises in a manner known in the art and illustrated in Fig. 4, the 1 9 KHz stereo pilot 400 and signals representative of the sum 401 of the left and right audio channel information and the difference 402 of the left and right channels.

In a preferred embodiment designed for add-on use in connection with existing FM stereo transmitters, the 1 9 KHz stereo pilot is isolated from this stereo composite signal 101 by use of high quality factor filter circuitry 110. Filter 110 strips the pilot of all stereo modulation, resulting in a recovered pilot signal 111 identical in frequency and phase to the stereo pilot which is transmitted with the stereo composite signal.

Alternatively, the necessary pilot may be obtained directly 11 9 from the stereo pilot generator 1 29 of the stereo generator 100.

However, use of filter 110 permits the preferred embodiment to be utilized in conjunction with equipment wherein direct access to this oscillator output 11 9 is not available, without requiring intrusive modification of the existing equipment.

Finally, the preferred embodiment further includes a 1 9 KHz redundancy generator 11 7 for providing a necessary backup pilot signal 1 22 when the stereo pilot generator 1 29 of the stereo generator 100 fails. Automatic switch-over means 118 automatically connects the redundancy generator 11 7 when the standard FM stereo pilot signal is absent, and provides the backup pilot 122 to the FM transmitter for simultaneous transmission, so that the back-up 1 9 KHz signal 1 22 will be available to associated receiver apparratus for necessary processing in accordance with the present invention. Manual switch selection may alternatively be provided.

When the redundancy generator 11 7 is utilized as discussed, its injection is preferably limited to approximately 2%, in contrast to the 8-10% injection level of the normal stereo pilot. This reduction in injection levels does not affect performance of the sensitive assoiated SCA receivers and processors described herein, yet it prevents erroneous operation of normal stereo receivers, which are typically sensitive only to stereo pilot modulation levels of 5% or greater. In addition, this redundancy pilot 1 22 may be transmitted and utilized in conjunction with monophonic FM broadcasts, providing an extended SCA baseband spectrum for multiple channel use extending from approximately 20 KHz to 99.9 KHz.

According to the method of the present invention, the 1 9 KHz stereo pilot is applied to a subcarrier generator 128, which derives one or more subcarrier signals 1 32, 1 33 from the pilot for use in generating the sideband modulated signals 162, 1 63 for transmission.

In the preferred embodiment, subcarriers generator 1 28 includes a frequency divider 11 5 which divides the stereo pilot by two, resulting in a 9.5 KHz reference signal 116. The lower frequency reference signal 11 6 is then further processed to gnerate desired multiples of the 9.5 KHz frequency. In the preferred embodiment, this function is performed by a harmonics generator 1 20 which shapes the input reference signal 11 6 in a manner known in the art to create an output signal 121 including even and odd harmonics of the input 11 6. These harmonics represent integral multiples of the input frequency 116.

As illustrated in Fig. 5, certain harmonic multiples of this reference frequency 11 6 fall within the FM SCA baseband spectrum. Specifically, the sixth, seventh, eighth, ninth and tenth harmonics of the 9.5 KHz reference signal 116 are located at 57 KHz, 66.5 KHz, 76 KHz, 85.5 KHz, and 95 KHz, respectively.

Accordingly, these select harmonics, or a subset therefrom, may be utilized as subcarriers for generation of sideband modulated signals 162, 1 63 for broadcast within the FM SCA baseband. For optimum operation of the preferred embodiment illustrated, it has been found preferable to generate even and odd harmonics extending to the twelfth harmonic of the 9.5 KHz reference frequency 11 6.

It is necessary to isolate the individual harmonics corresponding to the desired sideband subcarriers. In the preferred embodiment, a phase locked loop 1 30 is utilized to lock onto the desired harmonic frequency 1 32. Of course, it is understood that alternative methods known in the art for signal isolation may similarly be utilized in connection with the present invention, although it has been found that superior and economical performance is provided by use of phase locked loops.

The resulting desired subcarrier 1 32 is next applied to sideband modulation circuitry 141 for generation of sideband modulated signal 1 62 in a manner known in the art. Specifically, the preferred embodiment of sideband modulation circuitry 141 includes a balanced modulator 140 which accepts as an input one of the desired subcarriers 1 32 and a modulating intelligence input signal 142. The balanced modulator 140 suppresses the subcarrier 1 32 by at least 60db in a manner known in the art, resulting in a dual sideband suppressed carrier output 145. Output 145 therefore contains both the upper and lower sidebands representing the sum and the difference of the input intelligence signal 142 and the subcarrier 132, respectively.

In order to maximize the utilization of the available SCA baseband spectrum, a preferred embodiment next includes a single sideband filter 1 50 for removing the redundant lower sideband from signal 145 in a manner known in the art. Because the suppressed carrier single sideband modulation signal 1 52 contains all of the desired intelligence for transmission, it is seen that multiple single sideband channels generated from the selected harmonics 121 of the 9.5 KHz reference signal 11 6 may be simultaneously utilized for transmission of up to 5 independent intelligence signals within the available bandwidth of the FM SCA baseband spectrum, as illustrated and further discussed in connection with Fig. 5.

As further illustrated in Fig. 1, when more than one channel of sideband modulated SCA intelligence is desired, additional phase locked loops 131 are utilized to lock onto other of the harmonic frequencies 1 21 that lie within the FM SCA baseband spectrum as discussed.

The resulting additional subcarriers 1 33 are then applied to additional sideband modulation generating circuitry 1 51, along with additional intelligence inputs 143, to generate additional sideband modulated signals 1 63 for transmission.

Each individual sideband modulation circuit 141, 151 may further include a buffer amplifier 1 60, 1 61 for processing the single sideband signals produced and generating sideband modulated output signals 162, 163 which are delivered to the standard FM exciter of the existing FM transmitter for transmission along with the composite signal.

It is understood that alternative embodiments are likewise possible. For example, frequency multiplying circuitry known in the art may be utilized in place of harmonics generator 1 20. Further, although use of single sideband modulation techniques as described provides a greater number of SCA channels, dual sideband transmissions are also possible, either alone or in conjunction with remaining single sideband channels. For example, a dual sideband channel may be broadcast utilizing the 66.5 KHz harmonic as the sideband subcarrier, with single sideband signals utilizing the 76 KHz, 85.5 KHz, and 95 KHz harmonics, respectively. Although fewer SCA channels are thereby available, dual sideband channels having twice the broadcast power provide an improvement of 3 db in signal to noise ratio. Further alternatives are discussed in the following.

Fig. 2 illustrates a preferred embodiment of an FM receiver and signal processor for receiving one of the FM SCA channels previously discussed. The broadcast signal is received by antenna 201 and applied to a standard FM receiver and demodulator 200, which has a baseband response in the preferred embodiment of 30 Hz-100 KH. The resulting demodulated signal 202 comprises the standard stereo composite signal, including the 1 9 KHz stereo pilot frequency, and the transmitted sideband modulated SCA signals.

According to the method of the present invention, the 1 9 KHz stereo pilot is isolated from the demodulated received signal 202 by filter 210, which strips the pilot of all stereo modulation components. The resulting pilot signal 211 is then processed by reinsertion subcarrier generator 280 to generate the desired reinsertion carrier for demodulation of the received sideband signal. Specifically, in the preferred embodiment illustrated, the filtered pilot 211 is divided by two by frequency divider 220, resulting in a 9.5 KHz reference signal 221.This reference signal 221 is then processed to derive a desired multiple of the 9.5 KHz frequency which lies within the FM SCA baseband spectrum and which is equal to the frequency of the selected subcarrier 1 32 utilized in conjunction with the input intelligence signal whose reception is desired. In the preferred embodiment, harmonics generator 230 is utilized in a manner known in the art to shape the reference frequency 221 to generate even and odd harmonics 231 thereof, while phase locked loop 240 is utilized to lock onto and isolate the desired harmonic frequency, resulting in reinsertion carrier 241.

Because the desired reinsertion carrier 241 is therefore derived directly from the received stereo pilot 211, and because this received pilot 211 is identical in frequency, and in phase relative to the transmitted sideband modulation signal, to the transmitted stereo pilot present in the transmitter 100, it is seen that the desired reinsertion carrier 241 will be identical in frequency, and phase relative to the sideband modulation signal, to the corre sponding carrier 1 32 of the transmitter. Thus, although no sideband carrier signal has been independently transmitted, a signal may be derived in the receiver for reinsertion which is precisely identical to the carrier applied to the sideband modulation circuitry of the transmitter.

The resulting reinsertion carrier 241 is then applied, along with the received sideband modulated signal 203, to a balanced sideband demodulator 250, which extracts the intelligence signal 251 therefrom in a manner known in the art. This resulting intelligence signal 251 may then be amplified 260 and further processed in manners known in the art to result in output intelligence signal 261.

As previously discussed in connection with the transmitter illustrated in Fig. 1, it is understood that alternative embodiments are likewise possible. For example, signal isolating means known in the art may be utilized in place of phase locked loop 240. Further, other frequency multiplying techniques known in the art may be utilized in place of harmonics generator 230. It should also be noted that the receiver of Fig. 2 may be modified to provide a multiplicity of phase locked loops 240 and associated demodulators 250 to permit reception of more than one of the transmitted SCA channels.

Alternative algorithms for generating the desired transmitter and receiver subcarriers from the 1 9 KHz stereo pilot frequency are also possible. For example, as illustrated in Fig. 3, the 1 9 KHz pilot 300 may first be multiplied by frequency multiplier 310, resulting in a high frequency reference signal 311.

This high frequency signal 311 may then be divided by one or more frequency divider circuits 320, 321 to generate one or more subcarrier frequencies 322, 323 as desired.

Although frequency multiplier 310 as illustrated provides a multiplication factor of 10, it is understood that numerous schemes of multiplication and subsequent division to generate one or more resulting subcarriers 322, 323 are likewise possible utilizing frequency multipliers and dividers known in the art.

It should be noted, however, that with respect to the preferred embodiments discussed it is important to maintain time divsion or interleaving of the various multiple subcarriers for proper operation of the present invention, such that each is of independent phase with respect to the others. Such interleaving permits use of the preferred phase locked loop circuits previously discussed for isolating the desired subcarriers, while maintaining the total injection level of the multiple SCA channels at or below the 10% maximum established by FCC (U.S. Federal Communictions Commission) rules. For example, when only two of the available channels are in use, time division interleaving is sufficient to maintain the resulting modulation within the required 10% maximum, for inter-channel effects are negligible.Interleaving techniques may similarly be used with three or more channels.

However, as the number of channels increases, precise phasing becomes critical to prevent the possibility of inter-channel effects, such as addition between signals. Suseptibility of the critical phasing to multipath effects presents additional problems. Therefore, although the interleaving technique is acceptable for providing two SCA channels where phasing is less critical, it has been found to be preferable to provide a rapid response automatic gain control (AGC) limiter for use with embodiments providing greater numbers of SCA channels.

A preferred embodiment of an AGC circuit for use with the previously described five channel SCA generator is illustrated in Fig. 7.

In operation, a signal representing the sum of all sideband modulation signals is fed to a very fast attack, slow release peak amplitude limiter. The limiter operates to positively prevent either one or more of the sideband modulation products from exceeding a desired RMS output, so that the total injection level is maintained at or below the prescribed 10% maximum modulation.

Specifically, in the preferred embodiment illustrated, one or more inputs 704a-704e corresponding to the individual SCA sideband subchannels 162, 163 of Fig. 1 are applied to a summing amplifier comprising individual input resistors 701a-701e, and operational amplifier 700. Because of the very high input impedance of op amp 700, buffered output signal 703 will correspond to the sum of all individual input channels, in manners known in the art. Operation with greater or fewer than five inputs is likewise possible.

Under normal operating conditions, the RMS amplitude of summing amplifier output 703 will remain fairly constant. For example, if only two inputs with appropriate interleaving are applied, little or no RMS addition between channels will occur. However, when for example all 5 channels are in simultaneous operation, RMS addition may occur, whereby the resulting total signal exceeds the desired RMS levels.

The fast attack, slow release peak amplitude limiter is provided to respond to this condition of undesirably high signals. In the preferred embodiment illustrated, the amplitude limiter comprises an operational amplifier 710 whose gain may be rapidly varied. This rapid gain variation is accomplished by providing a variable resistance gain cell 71 5 in the negative feedback circuit of amplifier 710, in parallel with feedback resistor 711. The instantaneous resistance of gain cell 715 is dependent upon a control bias voltage 722 in manners known in the art. Both the cp amp 710 and gain cell 715 may be provided in integrated circuit form, such as by use of a NE570N integrated circuit which combines the two into a single package.

Amplifier 710 receives the output signal 703 of summing amplifier 700, which contains all of the multiple channel SCA information whose transmision is desired. Gain adjusted output 712, also representative of the sum of all desired SCA subchannels, is then supplied to the FM exciter of the FM stereo transmitter, through blocking capacitor 71 3 and resistor 731. In addition, variable resistor 732 may be provided to permit selection and adjustment of the level of signal 733 provided to the FM exciter, so that the precise desired injection level may be selected.

In order to provide for the desired automatic gain control, amplitude comparator 720 is further provided. Amplitude comparator 720 accepts the output 71 2 of operational amplifier 710 as an input, and compares the peak output level of signal 712 to preselected maximum positive and negative threshold voltages in manners known in the art. Whenever the negative or positive peak output voltage of signal 71 2 exceeds the preselected threshold of comparator 720, a charging voltage is supplied 721 to a positive voltage rectifier 730. The rectified output 722 is then applied to capacitor 725, which is preferably selected to be of a suitably low value of capacitance that, in conjunction with a low charging impedance of rectifier 730, it may charge to a high control voltage within approximately 1 5 microseconds.The smoothed control voltage 722 is then utilized to control the internal resistance of gain cell 715 by reducing its instantaneous resistance, thereby reducing the total effective feedback resistance of op amp 710 and correspondingly reducing its gain.

Accordingly, a control circuit with fast attack results which controls the gain of operational amplifier 710 to prevent output 71 2 from exceeding a preselected desired RMS range, thereby preventing either one or more of the sideband modulation products from exceeding a desired total injection level.

In order to prevent undesirable oscillation or repetitive activation of the control circuitry described, which may result in an objectionable "pumping" noise, the decay time of capacitor 725 is selected in manners known in the art to be considerably longer than the attack time, preferably approximately 300 milliseconds. For example, variable bleed resistor 726 may be provided in parallel with capacitor 725 to provide a controlled discharge path for the stored charge of capacitor 725.

A preferred embodiment of the fast attack, slow decay positive rectifier and capacitor combination is illustrated in Fig. 8. PNP transistor 810 is nornally biased "off" by pull-up resistor 801 attached to the positive supply voltage Vcc. When a positive or negative peak exceeding the threshold of comparator 720 is detected, the comparator output 721 goes low and turns transistor 810 "on" through current limiting resistor 802. During the time that transistor 810 is "on" a high voltage equal to the supply voltage Vcc less the emitter to collector drop of transistor 810 is supplied to rectifying diode 811, which is therefore forward biased to supply a charging current to a first capacitor 830 through limiting resistor 812. A portion of the charging current is, however, shunted by decay control variable resistor 813.

In addition, when transistor 810 is conducting, a current is established through the voltage divider comprising resistors 803 and 804. Accordingly, a positive voltage less than the collector voltage of transistor 810 is supplied to the base of NPN transistor 820, which is thereby turned "on". When transistor 820 is conducting, its emitter voltage is less than the collector voltage of transistor 810 by an amount equal to the voltage drop across resistor 803 of the voltage divider plus the base to emitter voltage drop of transistor 820. This lesser voltage is supplied through rectifying diode 821 and limiting resistor 822 to a second capacitor 831, which is similarly charged to a determined voltage less than the stored voltage of capacitor 830.

Finally, a portion of the emitter current of transistor 820 is conducted by biasing resistor 805, which in the preferred embodiment is approximately equal to the total resistance of the series combination of resistors 803 and 804 in the collector circuit of transistor 810.

It is seen, therefore, that when a peak is detected, capacitors 830 and 831 are quickly charged by the supplied currents. In the preferred embodiment, the limiting resistors 812 and 822 are low valued, preferrably 10 ohms, while the capacitors are preferably high quality 2.2 microfarad tantalum capacitors. Very rapid charging times for the respective capacitors are therefore provided, corresponding to the desired fast attack in control signal 722.

When the peak ha subsided, transistors 810 and 820 will both return to their "off" or non-conductive states, causing diodes 811 and 821 to become reverse biased. Because of the high impedance of reversed biased diodes 811 and 821, as well as the high impedance of the gain cell input to which output 722 is connected, the predominant discharge path for capacitors 830 and 831 is provided by decay resistor 813, which may preferably be an adjustable resistor of 600 kilohms total resistance. Alternatively a high valued fixed resistance may be used to establish a desired decay time. In either alternative decay resistor 81 3 is chosen to be of a sufficiently high value to provide the slow decay desired as previously discussed.

Because capacitor 831 is charged to a lower voltage than capacitor 830 as discussed, diode 832 will initially be reversed biased such that the output 722 will be predominantly determined by the voltage across capacitor 831. However, as capacitor 830 discharges through resistor 813, diode 832 will eventually become forward biased such that capacitor 831 will discharge along with capacitor 830. Thus, the preferred embodiment illustrated provides the desired fast attack, while providing a maintained output for a determined period of time after removal of the undesired peak, followed by a smooth exponential decay.

Although the foregoing describes a preferred embodiment for an automatic gain control circuit for use in connection with the present invention to improve the overall performance of the invention when three or more channels are simultaneously present, thereby eliminating the need for precise and costly phasing and switching techniques, it is understood that other automatic gain control circuits known in the art may similarly be employed.

Utilization of the allotted FM SCA baseband spectrum to provide multiple channel operation as discussed may be better understood with relation to Figs. 4 through 6. Fig. 4 illustrates a typical FM stereo and FM SCA broadcast prior to the recent FCC (U.S. Federal Communications Commission) rules extending the SCA baseband spectrum to 99.9 KHz. As seen, the composite stereo signal comprises the 1 9 KHz pilot 400, and signals representative of the sum 401 of the left and right stereo channels and the difference 402 of these -channels. The entire commercial FM stereo signal therefore lies within that portion of the broadcast spectrum extending from 0 to 53 KHz.

Under the previous FCC (U.S. Federal Communications Commission) rules and as illustrted, FM SCA subcarriers 410 were permitted within that portion of the baseband spectrum extending from 53 KHz to 75 KHz, with a guard band 420 extending from 75 KHz to 100 KHz, Typical SCA systems in existence, therefore utilize a single FM modulated signal 411 with multipe sidebands located at 67 KHz, with approximately 10% modulation.

Fig. 5 illustrates the present FCC (U.S.

Federal Communications Commission) alocation for FM SCA subcarriers extending to 99.9 KHz, and use of this available SCA baseband spectrum for transmission of five SCA channels in accordance with the preferred embodiments of the present invention previously described. Specifically, it is seen that by utilizing the upper sideband only of sideband modulation signals generated from subcarriers located at 57 KHz, 66.5 KHz, 76 KHz, 85.5 KHz and 95 KHz, up to five individual channels are available. Such maximum utilization of the available baseband spectrum is possible because, by generating the necessary reinsertion signals from the available 1 9 KHz stereo pilot 400, the present invention avoids the need to utilize a portion of this spectrum for subcarrier transmision.

Although bandwidths of up to 9.5 KHz are available for channels 1-4 shown in Fig. 5, channel 5 is restricted by the 99.9 KHz limit of allotted spectrum to 4.9 KHz bandwidth.

However, it has been found desirable to limit the bandwith of channels 1-4 to approximately 8 KHz, leaving 1.5 KHz guard bands as separating buffers to avoid possible interchannel interference, such as from vestigal sidebands. Finally, injection of the resulting SCA channels must be limited to approximately 5% modulation due to present FCC (U.S. Federal Communications Commission) rule limitations when used with an existing 67 KHz carrier.

It is not necessary however to utilize all of the five available subcarriers provided by the preferred embodiment as shown in Fig. 5. For example, as discussed in connection with Fig.

4, many commercial FM stations presently employ FM SCA transmission at 67 KHz. The present method and system may be utilized in connection with such existing systems to provide up to three additional channels in that portion of the SCA baseband spectrum extending beyond the previously allotted 75 KHz. Specifically, as illustrated in Fig. 6, additional channels generated utilizing subcarriers at the 76 KHz, 85.5 KHz and 95 KHz harmonics of the 9.5 KHz reference signal of the preferred embodiment may be used in conjunction with existing 67 KHz SCA systems, without significant modification of the existing system. However, due to the FCC (U.S. Federal Communications Commission) rules limitations previously discussed, it is necessary to limit the injection of both the 67 KHz SCA signal and the additional SCA signal described to approximately 5% modulation each.

Finally, it is understood that numerous other combinations are likewise possible. For example, as previously discussed, two or more of the available subchannels may be utilized to provide a single transmission of greater bandwidth or with a better signal-to-noise ratio. In all events, it is seen that the present method and system will permit transmission of multiple SCA channels within the allotted spectrum, while providing for very high quality reception by utilization of a precise reinsertion carrier derived directly from the already transmitted stereo pilot, and identical to the generating carrier of the transmitter.

The foregoing description and drawings merely explain and illustrate the invention.

The invention is not limited thereto, except insofar as the appended claims are so limited, for those skilled in the art who have the disclosure before them will be able to make modifications and variations therein without departing from the spirit and scope of the present invention.

Claims (36)

1. A method for providing one or more communications channels within the allocated baseband of a commercial FM stereo broadcast comprising the steps of: isolating the stereo pilot produced by the stereogenerator of the FM stereo transmitter; processing said stereo pilot to generate one or more individual transmitter reinsertion carriers therefrom whose frequency and phase are dependent upon the frequency and phase of said stereo pilot, and whose frequencies fall within the allotted FM baseband; applying said transmitter subcarriers and corresponding intelligence signals whose transmission is desired to means for generating suppressed carrier sideband modulated signals therefrom; causing said sideband modulated signals and said FM stereo pilot to be.radiated; causing said radiated FM signal, including said stereo pilot and said sideband modulated signals, to be received;; separating said FM stereo pilot from said received signal; processing said received FM stereo pilot to generate one or more desired individual receiver subcarriers therefrom, said desired receiver subcarriers corresponding in frequency to the transmitter subcarriers utilized in generating the transmitted sideband modulated signals corresponding to the intelligence signals whose reception is desired; wherein the frequency and phase of said receiver subcarriers are dependent upon the frequency and phase of said received FM stereo pilot; and applying said desired receiver subcarriers and said received sideband modulated signals to means for extracting the desired intelligence signals from said sideband modulated signals.

2. A method for generating signals for RF transmission of one or more desired intelligence signals within the allocated baseband of a commercial FM stereo broadcast, comprising the steps of: obtaining the stereo pilot produced by the stereo generator of the FM stereo transmitter; deriving one or more sideband subcarriers from said stereo pilot, wherein the frequencies and phases of said subcarriers are related to the frequency and phase of said stereo pilot; processing said one or more intelligence signals in conjunction with said one or more sideband subcarriers to produce one or more corresponding suppressed carrier sideband modulated signals; and delivering the resulting sideband modulated signals to the RF transmitter of the commercial stereo transmitter for transmission with said stereo pilot.

3. The method of Claim 2 wherein the step of deriving one or more subcarriers fromsaid stereo pilot further comprises the steps of: dividing said stereo pilot frequency by 2 to create a lower reference freuency; and processing said lower reference frequency to generate harmonics thereof, wherein said sideband subcarriers comprise selected resulting harmonic frequencies located within the allotted FM baseband spectrum.

4. The method of Claim 3 further comprising the step of isolating said or more sideband subcarriers from a set of said generated harmonics by utilization of phase locked loops.

5. The method of Claim 2 wherein said step of obtaining the stereo pilot comprises filtering said stereo pilot from the composite stereo signal generated by said commercial FM stereo transmitter.

6. The method of Claim 2 further including the steps of testing for the presence of said stereo pilot, and providing a backup redundant pilot for processing and transmission when said stereo pilot is absent.

7. The method of Claim 2 wherein said suppressed carrier sideband modulated signals comprise suppressed carrier single sideband modulated signals.

8. The method of Claim 2 further comprising the method of providing automatic gain control responsive to the levels of said one or more suppressed carrier sideband modulation signals whereby the total injection level of the sum of said one or more signals delivered to the RF transmitter of the commercial stereo transmitter is maintained within a desired range.

9. A method for processing a received commercial FM stereo and FM RF signal including a stereo pilot and one or more suppressed carrier sideband modulated intelligence signals. to extract desired intelligence therefrom, comprising the steps of: separating said stereo pilot from said received FM RF signal; processing said stereo pilot to derive one or more sideband reinsertion subcarriers therefrom. wherein the frequencies and phases of said resulting reinsertion subcarriers are related to the frequency and phase of said received stereo pilot; and utilizing said subcarriers as reinsertion signals for processing said received sideband modulated intelligence signals to extract said desired intelligence therefrom.

10. The method of Claim 9 wherein the step of processing said stereo pilot comprises the steps of: dividing said stereo pilot by 2 to create a lower reference frequency; and processing said lower reference frequency to generate harmonics thereof, wherein said reinsertion subcarriers comprise selected resulting harmonic frequencies located within the allotted FM baseband spectrum.

11 The method of Claim 10 further comprising the step of isolating said one or more reinsertion subcarriers from a set of said generated harmonics by utilization of phase locked loops.

12. The method of Claim 11 wherein the step of separating the stereo pilot comprises filtering said stereo pilot from the received FM signal.

1 3. The method of Claim 9 wherein said suppressed carrier sideband modulated signals comprise suppressed carrier single sideband modulated signals.

1 4. A single sideband generator providing one or more channels for broadcast of one or more intelligence signals within the allotted FM baseband spectrum of a commercial FM stereo and FM broadcast, said generator comprising: means for isolating the stereo pilot of said FM stereo transmitter; means attached to said isolating means for receiving said stereo pilot and generating therefrom one or more sideband subcarriers whose frequencies and phases are related to the frequency and phase of said stereo pilot, the frequencies of said one or more subcarriers being located within the FM baseband spectrum;; means attached to said subcarrier general ing means for receiving said subcarriers and the corresponding intelligence signals whose transmission is desired, and producing therefrom one or more corresponding suppressed carrier sideband modulated signals; and means for supplying said sideband modulated signals to the FM transmitter for transmission with said stereo pilot.

1 5. The invention of Claim 14 wherein said isolating means comprises a filter functionally connnected to the stereo generator of the FM stereo transmitter, said filter isolating said stereo pilot from the composite stereo signal generated by said stereo generator.

16. The invention of Claim 14 further comprising: backup pilot generator; means for detecting the absence of said stereo pilot; and means responsive tosaid pilot detecting means for connecting said backup pilot generator to said single sideband generator in lieu of said stereo pilot, and for supplying said backup pilot to said transmitter for transmission. to permit continued operation of said single sideband generator when said stereo pilot is absent.

17. The invention of Claim 14 wherein said subcarier generating means comprises means operably attached to said isolating means for receiving and dividing said isolated pilot to generate therefrom a lower reference frequency; and means operably attached to said dividing means for receiving said lower reference frequency and generating therefrom one or more subcarriers whose frequencies are selected real number multiples of said lower reference frequency.

1 8. The invention of Claim 1 7 wherein said multiples generating means comprises means for generating harmonics of said lower reference frequency.

19. The Invention of Claim 18 wherein the frequency of said stereo pilot Is approximately 1 9 KHz. said dividing means dividing said 1 9 KHz stereo pilot frequency by two to generate a lower reference frequency of 9.5 KHz, and wherein each of said one or more sideband subcarriers Is a selected harmonic of said lower reference frequency having a frequency of 9.5n KHz. wherein n Is 6. 7. 8. 9.

or 10.

20. The invention of Claim 1 9 wherein said subcarrier generating means further comprises phase locked loops. said phase locked loops isolating those harmonics corresponding to the selected sideband subcarrier frequencies.

21. The invention of Claim 14 wherein said subcarrier generating means comprises means operably attached to said isolating means for receiving and multiplying said isolated pilot to generate therefrom a higher reference frequency; and means operably attached to said multiplying means for receiving and dividing said higher reference frequency to generate therefrom one or more subcarriers.

22. The invention of Claim 14 wherein said sideband modulated signal generating means comprises means for generating suppressed carrier single sideband modulated signals.

23. The invention of Claim 14 wherein said means for supplying said sideband modulated signals to the FM transmitter for transmission further comprises means for automatially controlling the gain of said sideband modulated signals, said automatic gain control means responsive to the peak signal level of said one or more suppressed carrier sideband modulated signals for maintaining the total injection level of the sum of said one or more sideband modulated signals at or below a preselected desired maximum.

24. A single sideband processor for permitting reception of one or more sideband modulated intelligence signals transmitted over one or more channels within the spectrum of a commercial FM broadcast, said processor operating in conjunction with an FM radio receiver and demodulator to extract the desired intelligence from the received sideband modulated signals, said processor comprising: means for isolating the received stereo pilot of a received FM stereo transmission; means attached to said isolating means for receiving said received stereo pilot and generating therefrom one or more reinsertion subcarriers whose frequencies and phases are related to the frequency and phase of said stereo pilot, the frequencies of said one or more reinsertion subcarriers being located within the FM baseband spectrum; and means for utilizing said reinsertion subcarriers as sideband reinsertion signals for extracting the desired intelligence from said received sideband modulated intelligence signals.

25. The invention of Claim 24 wherein said isolating means comprises a filter functionally connected to the demodulator of the FM receiver, said filter isolating said stereo pilot from the received composite stereo signal.

26. The invention of Claim 24 wherein said reinsertion subcarrier generating means comprises: means operably attached to said isolating means for receiving and dividing said isolated received pilot to generate therefrom a lower reference frequency; and means operably attached to said dividing means for receiving said lower reference frequency and generating therefrom one or more subcarriers whose frequencies are selected real number multiples of said lower reference frequency.

27. The invention of Claim 26 wherein said multiples generating means comprises means for generating harmonics of said lower reference frequency.

28. The invention of Claim 27 wherein the frequency of said received stereo pilot is approximately 1 9 KHz, said dividing means dividing said 1 9 KHz stereo pilot frequency by two to generate a lower reference frequency of 9.5 KHz, and wherein each of said one or more reinsertion subcarriers is a selected harmonic of said lower reference frequency having a frequency of 9.5n K-Hz, wherein n is 6, 7, 8, 9, or 10.

29. The invention of Claim 28 wherein said reinsertion subcarrier generating means further comprises phase locked loops, said phase locked loops isolating those harmonics corresponding to the selected reinsertion subcarrier frequencies.

30. The invention of Claim 24 wherein said reinsertion subcarrier generating means comprises means operably attached to said isolating means for receiving and multiplying said isolated received pilot to generate therefrom a higher reference frequency; and means operably attached to said multiplying means for receiving and dividing said higher reference frequency to generate therefrom one or more subcarriers.

31. The invention of Claim 24 wherein said sideband modulated intelligence signals comprise suppressed carrier single sideband modulated signals.

32. A method for providing one or more communications channels within the allocated baseband of a commercial FM stereo broadcast substantially as described with reference to the accompanying drawings.

33. A method for generating signals for RF transmissions of one or more desired intelligence signals within the allocated baseband of a commercial FM stereo broadcast substantially as described with reference to Fig. 1 and Figs. 5 and 6 of the accompanying drawings.

34. A method for processing a received commercial FM stereo and FM RF signal including a stereo pilot and one or more suppressed carrier sideband modulated intelligence signals, to extract desired intelligence therefrom substantially as described with reference to Figs. 2 and 5 and 6 of the accompanying drawings.

35. A single sideband generator providing one or more channels for broadcast of one or more intelligence signals within the allotted FM baseband spectrum of a commercial FM stereo and FM broadcast substantially as described with reference to Fig. 1 of the accompanying drawings.

36. A single sideband processor for permitting reception of one or more sideband modulated intelligence signals transmitted over one or more channels within the spectrum of a commercial FM broadcast, substantially as described with reference to Fig. 2 of the accompanying drawings.