United States Patent
`
`[19]
`
`[11] Patent Number:
`
`4,660,193
`
`Young et al.
`
`[45] Date of Patent:
`
`Apr. 21, 1987
`
`[54] DIGITAL MODULATION METHOD FOR
`STANDARD BROADCAST FM SUBCARRIER
`
`[75]
`
`Inventors:
`
`Jing J. Young, Indianapolis, Ind.;
`Richard L. Stuart, Columbia, Md.
`
`[73] Assignee:
`
`Regency Electronics, Inc.,
`Indianapolis, Ind.
`
`Appl. No.: 540,518
`Filed:
`Oct. 11, 1983
`Int. Cl.4 ................................................ 1104.1 9/00
`U.S. Cl. ........................................ 370/11; 455/45;
`331/4
`Field of Search ...................... 370/11, 122,.11o.4,
`370/76; 381/4; 375/39; 455/45
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`2,578,714 12/1951 Martin
`2,709,254
`5/1955 Halstead
`3,160,812 12/1964 Scantlin .
`3,714,375
`1/1973 Stover .
`4,3 79,947 4/ 1983 Warner .............
`4,476,573 10/1984 Duckeck
`4,517,562
`5/1985 Martinez
`OTHER PUBLICATIONS
`
`............. 455/45
`,.'....... .. 370/122
`
`370/11
`. 455/45
`......... 370/ll
`
`Request for Proposal for SCA—FM Receivers, Mul-
`ticomm Project, Mutual Broadcasting System, Broad-
`cast and Communication S., Arlington, VA 4/22/83.
`Request for Proposal for Phase II SCA—FM Receivers,
`
`Multicomm Project Mutual Broadcasting System,
`Broadcast & Comm. Serv., Arlington, VA 12/28/82.
`Feher, Digital Communications: Microwave Applica-
`tions, Prenctice—I-lall,
`Inc., Englewood Cliffs, NJ.,
`1981, pp. 122-124.
`
`Primary Examiner—-Douglas W. Olms
`Assistant Examiner—M. Huseman
`Attorney, Agent, or Firm—Woodard, Weikart, Emhardt
`& Naughton
`
`[57]
`
`ABSTRACT
`
`A digital transmission system using standard FM broad-
`cast stations employs an SCA subcarrier which is ampli-
`tude modulated with a second subcarrier modulated to
`have different phase orientations representative of digi-
`tal data. Bits are separated from a digital data stream in
`groups and are differentially encoded into one of at least
`four phase orientations of the second subcarrier. The
`amplitude modulated SCA subcarrier is combined with
`the stereo signal output of a stereo multiplexer to form
`the modulating signal for the station FM modulator.
`The receiver includes a differential phase detector for
`reconstruction of the differentially encoded digital data.
`A system which additionally includes means for modu-
`lating the SCA subcarrier with a composite signal hav-
`ing an audio signal in addition to the differentially en-
`coded second subcarrier is disclosed.
`
`16 Claims, 6 Drawing Figures
`
`DIFFERENTIAL
`40AM
`DETECTOR
`
`Aruba Networks et al. Exhibit 1008 Page 00001
`
`

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`U. S. Patent Apr. 21, 1987
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`Sheetl of2
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`4,660,193
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`U. S. Patent Apr. 21, 1987
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`1
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`4,660,193
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`DIGITAL MODULATION METHOD FOR
`STANDARD BROADCAST FM SUBCARRIER
`
`FIELD OF THE INVENTION
`
`The invention relates to a method for modulating a
`subcarrier with digital signals in connection with a stan-
`dard FM broadcast station.
`BACKGROUND OF THE INVENTION
`
`In the past, some standard broadcast FM stations
`(operating from 88.1 to 107.9 MHz) have incorporated
`a subcarrier at, for example, 67 kHz which was fre-
`quency modulated to incorporate audio information,
`such as background music (Muzak). This subcarrier has
`sometimes been referred to as the SCA subcarrier (sub-
`sidiary communications authorization). Additionally,
`digital systems utilizing the SCA subcarrier were devel-
`oped which transmitted data by frequency shift keyed
`modulation of the subcarrier. Since the frequency swing
`of the composite signal had to be restricted to avoid
`adjacent channel
`interference,
`the amplitude of the
`subcarrier has been required to be restricted to a level
`which would produce not more than 7.5 kHz deviation.
`Several systems have been proposed to use the SCA
`subcarrier for both digital and audio use. One such
`system, proposed by Mutual Broadcasting, frequency
`modulated the SCA subcarrier with a composite signal.
`This composite signal included an audio signal plus a
`signal obtained from digital information modulating a
`second 4QAM (four-level quadrature amplitude modu-
`lation) modulated subcarrier. While this sytem was an
`improvement over earlier systems,
`it was unable to
`accommodate the digital baud rate desired.
`SUMMARY OF THE INVENTION
`
`Standard broadcast (88.1 to 107.9 MHz) FM transmit-
`ters conventionally carry an audio signal (either monau-
`ral or, if stereo, the sum of right and left audio channels
`for stereo) and, if stereo, a multiplexed signal represent-
`ing the difference between the right and left channel.
`The invention relates to a system having a standard
`broadcast FM tranmitter and receiver which addition-
`ally utilizes an SCA subcarrier (above 60 kHz) by am-
`plitude modulating and demodulating the subcarrier
`(such as with full amplitude modulation or with single-
`sideband or double-sideband suppressed-carrier) with a
`signal which includes a second subcarrier which has
`been modulated to have different phase orientations for
`digital data.
`Preferably the second subcarrier incorporates 4-
`phase quadrature phase-shift-keyed modulation or the
`equivalent 4-level quadrature double-sideband sup-
`pressed-carrier amplitude modulation. Alternatively,
`higher order modulation techniques may also be used,
`such as 8-phase PSK or l6-ary QAM. Also, preferably,
`an audio signal additionally amplitude modulates the
`SCA subcarrier directly.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a block diagram of a digital transmission
`system using standard FM broadcast stations according
`to the preferred embodiment of the present invention.
`FIG. 2 illustrates "the phase coding relationship for
`the differential encoding and decoding of data as per-
`formed by the system according to the preferred em-
`bodiment of the present invention.
`FIG. 3 is a graph of the baseband spectrum of the
`composite signal comprised of 4QAM data and auxil-
`
`2
`iary audio found at the input to the AM modulator of
`FIG. 1.
`'
`
`FIG. 4 is a graph of the spectrum of the single-side-
`band, suppressed-carrier modulated signal at the output
`of amplifier 40 of FIG. 1.
`FIG. 5 is a block diagram of a receiver according to
`the present invention.
`FIG. 6 is a block diagram of an alternative embodi-
`ment of the present invention in which two SCA sub-
`carriers are employed.
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT
`
`Referring to FIG. 1, a digital transmission system
`using standard FM broadcast stations is shown in block
`diagram form. Digital data at 9600 baud is received on
`input line 10, and an auxiliary audio signal is received on
`line 12. The digital data on line 10 is clocked into differ-
`ential 4QAM (4-level quadrature amplitude modula-
`tion) modulator 14 under control of a clock signal gen-
`erated therein and supplied to the data source (not
`shown) on line 15. 4QAM modulator 14 is a phase ori-
`ented modulator which removes data from the digital
`data stream in pairs of bits and differentially encodes
`each pair
`into one of four quadrature-amplitude-
`modulated signals. Differential encoding of the data
`conveys the information by carrier phase changes from
`the previously sent phase, rather than by the absolute
`phase of the carrier with respect to a reference signal.
`The phase change of the carrier is determined by
`pairs of bits which are sequentially taken from the digi-
`tal data stream. FIG. 2 shows the phase change relation-
`ship for differential encoding of received bit pairs, in
`tabular and graphical form. As an example, if the bit
`pair 10 is received by differential 4QAM modulator 14,
`the 4QAM signal generated by modulator 14 shifts in
`phase by 180 degrees with respect to the phase of the
`previous 4QAM signal, as indicated at 21 in FIG. 2.
`Differential 4QAM modulator 14 operates in conven-
`tional fashion and the details of its operation are not
`shown. In general, however, it phase-divides the 9.6
`kHz subcarrier signal received on line 16 into an in-
`phase (I) subcarrier and a quadrature (Q) subcarrier
`shifted 90 degrees with respect to the I subcarrier, in
`conventional quadrature modulation fashion. The I and
`Q subcarriers are supplied respectively to I and Q AM
`modulators each of which also receives one bit of a
`modulating signal bit pair as a modulating signal. The
`current modulating signal bit pair is different from the
`previous modulating signal bit pair by an amount depen-
`dent on the bit pair received on line 10. The in-phase
`and quadature modulated signals are added to produce
`a 4QAM signal.
`It will be appreciated that the four possible 4QAM
`signals are separated in phase from each other by an
`integral multiple of 90 degrees. Further, it will be un-
`derstood by those skilled in the art that a 4QAM signal
`as described hereinabove is identical to that produced in
`a 4-phase phase-shift-keying (4PSK) modulator. Refer-
`ence to one form of modulation herein is intended to
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`4QAM modulator 14 includes a raised cosine band-
`pass filter, with a passband from 6.0 to 13.2 kHz, to
`shape the 4QAM signal spectrum prior to transmission.
`4QAM modulator 14 also has an internal scrambler of
`conventional design which introduces changes into the
`bit stream to guarantee phase changes of sufficient fre-
`
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`4,660,193
`
`3
`quency in the transmitted signal to enable the receiver
`to recover a clock signal from the transmitted signal.
`The 4QAM output signal of modulator 14 is coupled
`to summing amplifier 24. The audio signal input on line
`12 is limited to frequencies below 5 kHz by low pass
`filter 26, and the high-amplitude excursions of the fil-
`tered signal are compressed in compressor 28. The com-
`pressed audio signal and the 4QAM signal are combined
`in summing amplifier 24 to form a composite signal
`having audio and digital components in adjacent por-
`tions of the baseband spectrum, as illustrated in FIG. 3.
`The composite signal is supplied to AM modulator 30
`which effects the modulation of the 61 kHz subcarrier
`signal received on line 31. AM modulator 30 partially
`suppresses the carrier, 61 kHz notch filter 36 provides
`further carrier suppression, and bandpass filter 38 elimi-
`nates the lower sideband, resulting in a single-sideband,
`suppressed-carrier signal having a spectrum bandwidth
`from 61 kHz to 74.2 kHz, as shown in FIG. 4. After
`further amplification in amplifier 40, the composite AM
`signal is supplied to summing amplifier 41, where it is
`added to a stereo signal generated by stereo multiplexer
`43. Stereo multiplexer 43 operates in a conventional
`manner, generating the sum of left and right channel
`audio signals as well as the difference between those
`same signals, and multiplexing the sum and difference
`signals into one stereo signal. The output signal from
`summing amplifier 41 is the FM modulating signal
`which is fed to FM modulator 44. FM modulator 44
`generates a frequency-modulated signal onta selected
`carrier frequency in the standard broadcast FM band of
`88.1 MHz to 107.9 MHz. The resulting FM signal is
`amplified in RF amplifier 46 and transmitted from an-
`tenna 48.
`The voice and digital information carried on the SCA
`subcarrier may be detected using a receiver such as that
`shown in FIG. 5. Referring now to FIG. 5, an incoming
`RF signal is received on antenna 100 and supplied to
`receiver 102 which is tuned to the appropriate main FM
`channel frequency. Receiver 102 demodulates the re-
`ceived FM signal and produces modulated SCA subcar-
`rier as well as any multiplexed stereo or other audio
`signal which may be present on the main channel fre-
`quency. Bandpass filter 104, with comer frequenices of
`60 kHz and 98 kHz, provides attenuation of stereo sig-
`nals, including the 19 kHz and 38 kHz reference signals,
`which may be 20 db higher than the SCA signal. The
`output spectrum of bandpass filter 104 is substantially
`the same as that shown in FIG. 4.
`The 61 kHz subcarrier is reinserted at SSB demodula-
`tor 106 which detects the audio and digital modulation
`on the 61 kHz subcarrier. The output spectrum of SSB
`demodulator 106 includes the original baseband spec-
`trum shown in FIG. 3 as well as a subcarrier frequency
`component at 61 kHz and a higher-frequency band at
`‘ 122 kHz.
`The recovered composite signal is applied in parallel
`to audio and digital signal recovery circuits. Low pass
`filter 108, with a corner frequency of 5 kHz, filters out
`the digital data portion of the spectrum, and expander
`109 provides nonlinear gain inversely corresponding to
`that of compressor 28 (FIG. 1), thereby restoring the
`original relative amplitudes in the auxiliary audio signal.
`Expander 109 supplies the recovered auxiliary audio
`signal on line 110 for audio amplification and connec-
`tion to speakers (not shown). Similarly, bandpass filter
`112, with a passband from 6.0 to 13.2 kHz, rejects the
`audio signal portion of the spectrum and passes the
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`4QAM signal to differential 4QAM detector 114. Dif-
`ferential 4QAM detector 114 operates in conventional
`fashion and the details of its operation are not shown. In
`general, however, it decodes phase changes in the re-
`ceived waveform into one of four levels according to
`the relationship shown in FIG. 2. 4QAM detector 114
`incorporates a descrambler in conventional form corre-
`sponding with the scrambler in 4QAM modulator 14.
`Continuing with the same example as before, when
`4QAM detector 114 detects the phase change of 180
`degrees in one interval with respect to the previous
`interval, it decodes the change into the original bit pair
`10. 4QAM detector 114 outputs the decoded bit pair on
`the data line.
`Differential phase detection is preferred over fixed-
`reference phase detection because the latter is suscepti-
`ble to phase jitter and other channel disturbances (such
`as those resulting from multipath radio interference).
`These disturbances create the problem of both establish-
`ing the fixed reference and maintaining it in its fixed
`phase. Once the receiver loses synchronization, the time
`required to reestablish synchronization is often much
`longer than the duration of the disturbance. In contrast,
`with differential phase detection, an error burst affects
`only data decoded during the error burst and one inter-
`val following the burst. That is, if a channel disturbance
`results in a phase shift during one interval, the detected
`phase changes between that interval and the previous
`interval and between that interval and the following
`interval will both be incorrect. However, the second
`decoded bit pair following the error burst will be cor-
`rect, because by that time two 4QAM signals will have
`been correctly received.
`A Gray code, as shown in FIG. 2, is preferred for
`differential encoding to minimize bit errors due to phase
`shifts caused by channel disturbances. Phase errors of
`90 degrees are more likely than errors of 180 degrees,
`thus with the Gray code errors are more likely to effect
`only one bit per bit pair. When used with an appropriate
`error-correcting code, such as the Golay (23, 12) code,
`in which errors of up to three bits in two consecutive bit
`pairs of a 23-bit word can be corrected, the Gray code
`reduces the overall probability of error.
`4QAM detector 114 also includes circuitry for recov-
`ering the original clock frequency for use in later syn-
`chronizing other circuitry (not shown) to the digital
`data. 4QAM detector 114 recovers this frequency from
`the transmitted waveform irrespective of the phase
`shift, if any, and supplies the resulting clock signal on
`the clock line. Scrambling and descrambling of the bit
`stream guarantees that the 4QAM signal carries suffi-
`cient phase changes to enable clock recovery by detec-
`tor 114.
`One alternative embodiment, shown in block diagram
`form in FIG. 6, employs two subcarries, one at 61 kHz
`and the other at 97 kHz. 61 kHz subcarrier modulator
`50, also shown inside dotted lines in FIG. 1, has already
`been described. 97 kHz subcarrier modulator 52 con-
`tains the same functional blocks as modulator 50 but
`operates with a 97 kHz subcarrier and is designed to
`generate the lower sideband instead of the upper side-
`band as in modulator 50, the notch and bandpass filters
`being designed accordingly. The outputs of modulators
`50 and 52 are combined in summing amplifier 141 along
`with a stereo signal generated, as already described, by
`stereo multiplexer 43. The output signal from summing
`amplifier 141 is the FM modulating signal for FM mod-
`ulator 44, which generates an FM signal as described
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`5
`with reference to FIG. 1. The FM signal is amplified in
`RF amplifier 46 and transmitted from antenna 48. The
`receiver for this embodiment is similar to the receiver
`already described with reference to FIG. 5, the differ-
`ence being the addition of a second subcarrier detector
`similar to subcarrier detector 150, shown inside dotted
`lines in FIG. 5.
`Alternatively, higher order modulation techniques,
`such as 8-phase PSK or 16-ary QAM, may be used in
`place of 4QAM in the phase oriented modulator, modu-
`lator 14, and in the phase oriented detector, detector
`114, without departing from the scope of the invention.
`In the case of 16-ary QAM, the modulated signal has
`one of three different amplitudes in addition to having
`one of at least four phase angles. Accordingly, modula-
`tor 14 is adapted to provide variable amplitude as well
`as phase, and detector 114 is correspondingly adapted
`to detect changes in amplitude as well as phase. The
`phase and amplitude relationship for 16-ary QAM is
`further described in Feher, Digital Communications:
`Microwave Applications, Prentice-Hall, Inc., Engle-
`wood Cliffs, N.J. 07632, 1981, pp. 122-3.
`It will thus be understood that an improved system
`for modulating an SCA subcarrier with both digital and
`audio information has been described. This system pro-
`vides improved signal-to-noise ratio while transmitting
`digital data received from a data source at a desired
`baud rate of 9600 baud.
`While the invention has been illustrated and de-
`scribed in connection with the preferred embodiment,
`the same is to be considered as illustrative and not re-
`strictive in character,
`it being understood that all
`changes and modifications that come within the spirit of
`the invention are desired to be protected.
`What is claimed is:
`
`1. A receiver which comprises:
`(a) an FM detecting means,
`tuned to a radio fre-
`quency signal of a standard FM broadcast station,
`for detecting frequency modulation of a received
`carrier in said radio frequency signal;
`(b) an amplitude sensitive detecting means connected
`to the output of said FM detecting means, and
`tuned to a subcarrier frequency band above 60
`kHz, for detecting amplitude modulation of a sub-
`carrier, said amplitude sensitive detecting means
`including means for reconstructing from said de-
`tected amplitude modulation a composite signal
`having audio and digital components in adjacent
`portions of its baseband spectrum;
`(c) a phase oriented detecting means connected to the
`output of said amplitude sensitive detecting means
`for detecting modulation at any one of at least four
`discrete phase angles and at least one amplitude
`and for producing digital bit groups therefrom;
`((1) means for producing digital data from the digital
`. bit groups from said phase oriented detector;
`(e) a low pass filter connected to the output of said
`amplitude sensitive detecting means; and
`(f) means for producing an audio output signal from
`the output of said low pass filter.
`2. The receiver of claim 1 in which said amplitude
`sensitive detecting means is a single-sideband detector.
`3. The system of claim 1 in which said phase oriented
`detecting means differentially decodes said modulation
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`6
`detected at any one of at least four discrete phase angles
`and at
`least one amplitude and produces digital bit
`groups from said differentially decoded modulation.
`4. The receiver of claim 1 in which said phase ori-
`ented detecting means is a quadrature detector for de-
`tecting an in-phase and a quadrature signal.
`5. The receiver of claim 4 in which said phase ori-
`ented detecting means is a 4QAM detector.
`6. The receiver of claim 5 in which said phase ori-
`ented detecting means differentially decodes said modu-
`lation detected at any one of at least four discrete phase
`angles and at least one amplitude and produces digital
`bit groups from said differentially decoded modulation.
`7. The receiver of claim 6 in which said amplitude
`sensitive detecting means is a single-sideband detector.
`8. The receiver of claim 7 in which said single-side-
`band detector reinserts an about 61 kHz carrier and
`detects the upper sideband and in which said 4QAM
`detector tunes to about 9.6 kHz.
`
`9. A digital transmission system using standard FM
`broadcast stations which comprises:
`(a) a source of digital data;
`(b) means for sequentially producing bit groups of
`digital signals from said source;
`(c) a phase oriented modulating means for modulat-
`ing a first subcarrier with said bit groups to pro-
`duce a modulated signal at any one of at least four
`discrete phase angles and at least one amplitude,
`said phase oriented modulating means being con-
`nected to the output of said means for producing;
`(d) means for combining said modulated signal with
`an audio signal to form a composite signal having
`audio and digital components in adjacent portions
`of its baseband spectrum;
`' (e) an amplitude modulating means connected to the
`output of said phase oriented modulating means
`and responsive to said composite signal from said
`combining means for amplitude modulating a sec-
`ond subcarrier at a frequency above 60 kHz; and
`(i) an FM modulating means for frequency modulat-
`ing a radio frequency signal of a standard FM
`broadcast station in response to the output of said
`amplitude modulating means.
`10. The system of claim 9 in which said amplitude
`modulating means is a single-sideband modulator.
`11. The system of claim 9 in which said phase ori-
`ented modulating means is a differential modulator for
`differential encoding of said first subcarrier.
`12. The system of claim 9 in which said phase ori-
`ented modulating means is a quadrature modulator for
`modulating an in-phase and a quadrature signal.
`13. The system of claim 12 in which said phase ori-
`ented modulating means is a 4QAM modulator.
`14. The system of claim 13 in which said phase ori-
`ented modulating means is a differential modulator for
`differential encoding of said first subcarrier.
`15. The system of claim 14 in which said amplitude
`modulating means is a single-sideband modulator.
`16. The system of claim 15 in which said second
`subcarrier is about 61 kHz and said single-sideband
`modulator produces the upper sideband and in which
`said first subcarrier is about 9.6 kHz.
`t
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