Apparatus and method for providing digital audio in the fm broadcast band

-`- 2003763 , The present i~vention relates to audio broadcasting and reception, and more particularly to a method and apparatus for providing high quality digital sound signals within the FM broadcast band. ~ . .

Cable television growth has come from the development of various programming categories and by the technologies which made the delivery of these programs possible. Cable first brought distant TV signals to areas where there was little or no off- ~ air reception.

This applies to distant signals and - ;

weak signal areas where outdoor antennas are mandatory. The next category to bring major growth ., to cable was the pay service after the development of reasonable cost satellite delivery systems.

A~ter satellite delivery was accepted and less costly, super ~tations and cable networks formed another category of programming that has become customary and are often termed "extended ba~ic" services. Franchising and local politics has created a generally unprofitable but necessary category called "local origination". ~ Recently, addressable technology and aggressive marketing have caused "pay per view" proqramming to form another category of programming.

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:\ 2 2 0 03 7 63 FM (audio) broadcasting over cable has never achieved significant success for two technological reasons: signal quality is poor and the;re has ~een no way to collect revenue or control the access to the service.

New digital techniques for the reproduction of sound provide performance that is far superior to analog techniques which have been used in the past.

~n example of high ~idelity sound reproduction using digital techniques can be found in the compact disk technology which has recently en;oyed tremendous success as an alternative to phonograph records and tapes.

Digital recording and playback techniques provide reproduction of music that is extremely realistic and free from background noise and distortions which have plagued other high fidelity sound reproduction systems currently in wide scale use.

Commonly-owned, copending Canadian Patent ~pplication Serial No. 560,315, elltitled "Apparatus and Method for Providing Digital Audio on the Sound Carrier of a Standard Television Signal", filed March 2, 1988, discloses a 6y~tem wherein the FM audio portion o~ a standard television signal ln the TV band i8 replaced with digital audio.

Three digital audio ahannels are time division multiplexed on the ~ound carrier, using combined multipha~e and AM modulation. l'he audio signals are digitized using aqaptive delta modulation techniques.

Video vertical and horizontal framing, as well as the audio carrier ~ : , -: :

---` 2003~763 phase reference, audio data bit time and frame - reference, and various control data is carried using AM modulation.

The digital audio information is : .

carried using multiphase modulation.

The composite ~ ~ data stream may be serially encrypted to provide ' -security and prevent unauthorized reproduction of the video and/or audio portions of the television signal. -- U.S.

Patent No. 4,684,981, entitled "Digital ' Terminal Address Transmitting for CATV", discloses " ~ producing digital signals of up to four different `''" modes for transmission over an unused television channel in an existing cable television transmission line.

High quality audio signals may be transmitted ' and/or data channels or monaural audio signals, all ` of which may be transmitted over the single cable television transmis6ion line.

Cable television ` channels have approximately a 6 MHz bandwidth, and ' ' are transmitted in the TV band which ranges in rrequency ~rom 50 MHz ~channel 2) to 550 MHz (channel 50).

Any distribution system which transmits digital; '' ' audio data (such as a cable televislon system) must be'su¢h that the transmitted audio signal does not interrere with millions o~ radio sets already in existence which use conventional analog sound circuits.

Thus, such things as the channel width of "'' 400 kilohertz (KHz) ~or eadl channel within the FM ~' broadcast band should not be changed, sub~ect to ` 30 narrow tolerances. ''''' . . - .

.:, ~ 4 ~ 2 ~3763 The present invention provides a method and apparatus for incorporating digitized audio data within a plurality of channels in the standard FM broadcast band in a manner such that the signal will be recoverable for reproduction of the 5transmitted audio program on FM radlo receivers, with additional digital data receiving circuitry.

In accordance with the present invention, a method and apparatus are provided for transmitting, receiving, and reproducing digital audio signals as discrete carriers 10frequency allocated as standard FM broadcast signals.

An audio signal is digitized using, for example, adaptive delta modulation technl~ues.

Several channels of audio information, such as left and right stereo channels or four voice mono channels can be digitized and incorporated onto the digital 15carrier in the FM broadcast band.

The digitized audio signal i8 modulated using multiphase or multilevel amplitude or frequency modulation of the carrier in the FM broadcast band.

According to one aspect of the invention, a method of broadcasting audio signals comprises the steps of digitizing 20a channel of audio source material to produce a compressed digltal data stream, modulating a carrler by sald data stream using multilevel modulatlon to produce an RF channel slgnal havlng a bandwidth substantially e~ual to or less than the bandwidth of a standard FM radlo channel and a fre~uency 25within an FM radlo broadcast band and transmitting the RF ;

channel signal in a standard channel allocation of the FM radio broadcast band.

According to another aspect of the invention, an -apparatus for receiving digital and analog audio signals ~'`-~''~ ''.

- 4a _ 20~3763 comprises tuner means for tuning to signals in the FM radio broadcast band, digital demodulator means for demodulating a multilevel modulated compressed dlgitized audio signal :

transmitted in a channel allocation substantially equal to or : ~:

less than the bandwidth of a standard FM radio channel in the -:

FM radio broadcast band and output from said tuner, decoder , . . . .

means for processing the demodulated compressed digitized audio signal to provide an audio output signal; and analog demodulator means for demodulating a nondigitized audio signal transmitted in a different standard channel allocation in the FM radio band and output from said tuner to provide an audlo output signal.

According to a further aspect of the invention, there is provided a radio receiver for providing reproduction of audio signals transmitted as digital data in the FM radio broadcast band comprising a tuner for receiving compressed digital audio signals in channel allocations substantially equal to or less than the bandwidth of a standard FM radio channel in the FM band, digital demodulator means coupled to an output of said tuner for demodulating a multilevel modulated compressed digital audio signal received in the FM band and decoder means, coupled to receive demodulated compressed digital audio data from said digital demodulator means, for converting the data to an analog audio signal.

According to still another aspect of the invention, there i8 provided an apparatus for broadcasting audio signals comprising means for digitizing a plurality of channels of audio source material to produce a plurality of compressed .. ~:

or1 digital data streams, means, coupled to said digitizing means, ~ 2 0 0 3 7 6 3 for modulating a plurality of RF carriers with said data streams using multilevel modulation to produce a plurality of digital RF channel signals, each having a bandwidth substantially equal to or less than the bandwidth of a standard FM radio channel and a frequency within an FM radio broadcast band and means, coupled to said modulating means, for transmitting the digital RF channel signals in said FM radio broadcast band.

The 400 KHz spacing of digital carriers in the FM band allows 50 channels of stereo digital quality audio, addressable and encrypted. In a local service area, the FCC spaces FM stations 800 KHz apart, meaning that there are a maximum of 25 local FM broadcasters in the most dense market. ~ ' . . , ~' ` ''''' :' ;:

2003763 .

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A bandwidth efficient system would use Dolby ADM and would allow data carriers at 400 KHz --spacing.

This spacing is the same as normal -~ broadcast FM.

This would allow up to 50 channels in the FM band.

The digital channels could be intermixed (staggered) with regular FM channels.

There i8 also the possibility for broadcast (wireless) application.

Another alternative would be to transmit 44 KHz sampled 16 bit linear PCM (Compact Disc Format) spaced at 1.2 NHz between channels, allowing 16 channels in the FM band.

The Dolby system could be built at low cost.

From a marketing standpoint, it is recommended that discrete carriers be u~ed rather than full video channels with time division multiplexing (TDM).

This results in lower cost, more acceptable use of spectrum to the cable operator and more robust per~orman¢e.

ao When the present invention i8 used in ` con~unction with a aable television system, three primary components are used.

These ar~ the addressable controller (also referred to as "headend controller"), the headend encoder, and the subscriber converter (also known as the "sub~criber ; ;~ terminal").

Both the addressable controller and ;~ encoder are present at the headend from which the a~ble tel~vision signals are sent by the cable system operator.

ThQ addressable controller controls all subscriber t~rminais in the cable television system, oontrols the encoders/decoders 2~)0~763 associated with the system, configure~ scrambling modes, service codes, and encryption keys, and orchestrates the dissemination of all decryption keys. The encoder of the present invention is a haadend device consisting of a number of subcomponents including an audio digitizer, audio scrambler, tag insertion logic, addressable controller interface logic, and modulator circuitry.

The subscriber converter is a device located at . ~ each subscriber's residence and contains an RF converter module, demodulator, addressable control interface logic, subscriber interface logic, audio l`~ decryptor and digital to analog ~"D/A") converter.

Control data communicated over the FM path time ;

division multiplexed with the digital audio data between the headend controller and the encoder typically includes a signature used to protect sensitive information communicated over the path, tag and audio encryption keys, key usage identiSiers, sampling mode data, audio service code, and priae and morality rating data. Data whioh pert~ins to the digital audio se~vice and is sent to the subscriber terminal over the FM path includes a ~ignature used to protect ~ensitive ~;

inSormation communicated over the path, tag and audio decryption keys, and authorization inSormation.

~he following commonly-used abbreviations may ~:

be used throughout thi~ application: kilohertz , : ' ' 2003763 ,:

(KHz), megahertz (MHz), freguency modulation (FM), television (TV), adaptive delta modulation ~ADM), amplitude modulation ~AM), cable television (CA~V), pulse code modulation (PCM), time division multiplex : .

(TDM), pulse modulation (PM), pulse amplitude modulation (PAM), pulse width modulation (PWM), frequency division multiplexing (FDM), Quadrature Phase Shift Keying (QPSK), radio frequency (RF), audio frequency (AF), direct current (DC), and Federal Communications Commission (FCC). ;

Other objects, features and advantages of the ~:

invention will become apparent in light of the :~:

following description thereof. .~.

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2003763 Figure 1 is a schematic block diagram of the digital audio system of this invention.

~ igure 2 is a schematic block diagram of a digital FN broadcast and cable interconnection system, according to the invention. -~ Figures 3A and 3B, combined, are a schematic ~block diagram of a digital FM receiver, according to the invention.

Figure 4 i8 a schematic oP a multiphase modulator suitable for use in the digital audio system of this invention.

Figure 5 is a polar diagram showing phase relationships for the multiphase modulator of Figure 4.

Figure 6 is a schematic of a Costas loop QPS~ detector suitable for use in the digital receiver of this invention.

Figure 7 is a echematic block diagram of a digital FM broadcast and cable interconnection system, a¢cording to the invention.

Figure 8 is a schematic block diagram of a digitnl FM re¢eiver, according to the invention.

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2~037~3 Digital information such as digitized audio, addressing data, and auxiliary data may be combined together to form a composite digital data stream.

ThiC digital signal may then be modulated on to a carrier for transmission. The modulation may cause amplitude, phase, or freguency variation of the carrier. In order to maintain channel spacing similar to previously established analog transmission standards, multilevel (AM), multiphase (PM, i.e., QPSK) or multi frequency (FM) must be - used. QPSK or eight frequency FM are approaches, ;

when combined with an e~ficlent digital audio sampling system such as Dolby ADM that can aliow coexistent digital and analog modulated carriers in the FM broadcast band at the normal frequency ;

allocations. QPSK is the preferred transmission modulation means as its signal to interference ratio required for operation without data errors is less than with eight level FM.

Figure l shows the ma~or elements of the digltal audio system of this invention, portions of which will be described in greater detail hereinaftex. As described herein, the digital audlo system is applied to a aable television network, but lt wlll be evldent that the teahniques descrlbed are applicable to wir~less broadca~t of dlgital audio.

At the headend, or cable transmission center 10, a plurality of tele~islon chalmel transmissions 12 are provided for distribution over a cable transmission llne 14, according to known technlgues.

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' ' ' . .'`.'., ~ '., : ' ' ~.' Z003~63 Further, according to known technique~, a pluralityof subscribers 16 (one shown) are connected, each by a cable drop 18, to a distribution network 20. Each subscriber is provided with either a cable-ready television, capable of tuning in exce6s of loo TV channels, or is provided with a converter 22 which converts a selected one of the hundred or so TV channels to a pre~elected channel, such as channel 3, which may be received by an ordinary, non-cableready television set 24. It is further known to provide "~pecial" channels within the band of television channels that may only be viewed by a subscriber on a ~pecial basis. To this end, digital address signals 26 are provided on the ca~le 14, and the converter 22 either permits or restricts viewing the special TV channels in response to the digital address signals, again according to known te¢hniques.

A¢cording to the invention, audio-only source material is also provided over the cable 14 in the ~ollowing manner. A channel 30 o~ audio 80urce material 32 is provided to a digitizer 34 which converts the source material into digital format.

Such conversion of the audio souraQ material to dlgitized audio may be per~ormed according to a variety of known techniques. The digitized aud~o source material i~ provided to an FM band exciter 36 which provides the digitized audio 60urae material as a radio frequency ~RF) signal to the cable 14.

One channel 30 is illustrated, but several channels ., .

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Z00;~763 may be provided. Each channel may contain stereo program material.

The RF output o~ each channel 30 ~ ~occupies up to 400 kilohertz (KHz), preferably in the standard FM broadcast band which ranges typically from 88-108 megahertz (MHz). There are fifty 400 KHz channels available in the standard FM band. Thus, although up to ~ifty distinct audio channels 30 could be provided, it is preferable to provide only up to 25 channels of digitized audio material in alternate (every other) channels in the standard FM band.

The digitized audio signals from the audio channels are provided from the subscriber cable drop 18 to a digital FM band receiver 38, which is described in greater detail hereinafter with respect to Figures 3A and 3B. As with the "special" TV channels, access to some or all of the digitized audio channels may be restricted by ths use of address signals 26 which are imbedded in the multiplexed digital channel.

It is also possible to provide nondigitized audio source material over the cable 14. A channel 40 oP audio source material 42 is provided to a conditioning circuit 44 to ad~Ust the signal level of the source material. The conditioned audio source material is provided to an FM exciter 46 which provides the conditioned audio 30urce material aB a r~dio frequency signal to the cable 14. One ahannel 40 is illustrated, but several channel~ may be provided. As with the digitized channels 30, the . ~ ~, : : , . .

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~003763 .

12 RF output o~ each nondigitized channel 40 occupies 400 KHz in the 20 MHz FM band, and can be provided as a nonspecial (non-address restricted) channel to the subscriber.

Advantageously, the nondigitized channels 40 can be interspersed between the digitized channels - 30.

Alternatively, but less effectively, the ~digitized channels 30 can be assigned to a portion, 6uch as the upper 10 MHz of the standard FM band while the nondigitized channels 40 reside in the lower 10 MHz of the standard FM band.

A combiner 48 combines the signal outputs of the TV channels 12, the address information 26, the digitized audio channels 30 and the nondigitized audio channels 40 onto the cable 14.

Figure 2 shows an FM broadcast station 50 and cable interconnection system. A studio 52 provides audio source material (3imilar to elements 32, 42 of Figure 1) as unprocessed audio signals in stereo along "lQft" and "right" ~ignal paths.

In one instance, the audio signal~ are provided to an FM stereo encoder and loudness proces~or 54 ~similar to element 44 o~ Figure 1), and from there are provided onto an FM exciter 56 (similar to element 46 of Figure 1).

The output of the exciter 56 i5 amplified by a high power ampliPier 58 and broadcast over the airwaves by an antenna 60 as stereo multiplex (MPX) FM in the FM broadca~t band.

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2()03763 In another instance, the aud~o signals are provided to a Dolby adaptive delta modulation (ADM) encoder 62, and from there are provided as a data stream to a digital processor and combiner 64. The digital processor and combiner 64 operates under the control of a text and control computer 66.

The output of the digital processor and combiner 64 is provided in one instance to an FM exciter 68, similar to the FM exciter 56.

The . 10 output of the exciter 68 is amplified by a high power amplifier 70, similar to the amplifier 58, and broadcast over the airwaves by an antenna 72, . similar or unitary with the antenna 60, as digital ..

FM in the FM broadcast band for reception by a digital FM receiver, such as i8 disclosed in Figures ~ 3A, 3B.

In another instance, the output of the digital ~.

proce~sor and combiner 64 i8 provided as 8-level data to an FM modulator 74 which inserts the 20 digitized audio signals onto a oable telQvision transmission line 76 via a directional splitter 78 to a cable headend 80 (corresponding to element 10 of Figure 1).

Preferably, the digital audio signals are transmitted over the cable 76 at 5-30 MHz, which : :

: 25 i~ reserved for upstream (to the.headend) communication over the cable 76, and which is outside of the spectrums o~ both the audio channels (30 and 40 o~ Figure 1) and television channels (12 o~ Figure 1) that are provided by the headend 80 to ~- 30 .subscribers 8~ .'..: ~'.

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2003~63 14 .:

In Figure 2, the exciter 68 could as well be a QPSX modulator, and the modulator 74 could as well be a QPSK modulator. ~:

In the arrangement shown in Figure 2, the headend 80 is provided with a digital demodulator and remodulator 84 for receiving and demodulating the digitized audio signals from the off-site : - .

modulator 74 in the 5-30 MHz band, and for remodulating and transmitting digitized audio . signals on the transmission line 76 in the FM band (88-108 MHz) to subscribers 82.

The techniques of i providing "special" audio channels, as well as the .. : .

spacings of digitlzed and nondigitized audio ~:~ channels within the FM band, discussed with respect to Fiqure 1, are equally applicable in the system of ;~ Figure 2.

Figure 2 contemplates that several stations 50 ~ :

will provide digital audio channels, generally one .

channel per station, to the cable system operator (CS0) 80.

As noted therein, this i~ readily ::

accomplished over existing cable transmission lines .~ 76 in a band (~uch a~ 5-30 MHz) reserved rOr . .

up~tream communication to the CS0.

More detAils of the system are shown with re~peot to Figure 7.

Figures 3A and 3B show a digital FM band :.: :

receiver.

Generally, Figure 3A shows a tuner section 100, and Fi~ure 3B ~hows a deaoder section 101.

The input 102 to the receiver i~ either a : -:

;~ cable transmission line (14 of Figure 1; 76 o~ :. 30 Figure 2), or a suitable antenna and preampli~ier ~ .. .

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~,,; ,, ' . ~ 20~3763 (not shown).

It is contemplated in this example that the receiver will tune from 72-120 MHz, in an "expanded" FM band, to provide a large number of audio channels while avoiding the used TV channels, and gaps therebetween, such as the 4 MHz gap between TV channels 4 and 5.

The signals rom the input 102 are provided to a double tuned tracking filter (DTTF) 104, from there to an amplifier 106, on to a single tuned tracking filter (STTF), and to a mixer 110, according to known techniques.

The mixer 110 receives a second input from an oscillator 112, so that the ou~put of the mixer 110 is at an intermediate frequency (IF) of 10.7 MHz for a selected channel.

The channel selection process is under control oE a tuning synthesizer 114, integrating amplifier 116, ST~F 118 and amplifier 120, interconnected as shown, and impressing an appropriate signal on a line 124 to the DrTF 104, S~TF 108 and oscillator 112 to ef~ect channel selection, according to known technigues.

The selected audio channel i8 provided at the intermediate ~requency ~IF~ to a ~ilter network comprising a bandpass filter 126, amplirier 128 and bandpass ~ilter 130, as shown, according to known technigues, and is ready for detection.

In one embodiment of the invention, a detector 132 is provided which comprises an FM detector 134, such as a Sanyo (trade mark) LA1150, which provides an 8 level data output to an analog-to-digital (A/D) converter ' - , . .

2003763 :

136, such as a 4-bit CMOS device. The detector 132 is suitable ~or digital audio received in 8- frequency modulated FM format. The output of the A/D device 136 is provided as a data strean~ over a bu~ to a demultiplexer and decryption logic circuit 138 which separates the data stream ~nto control bits and channel indication (tag bits) and encrypted digital audio data bits (demultiplexing functions) and decrypts the digitized audio data into a suitable form for a Dolby decoder 140.

The audio data is decrypted into three serial streams per audio channel consisting of basic delta modulation parameters for "left" and "right" channels and companding data streams for "left" and "right" channels.

The demultiplexed control and channel data separated out ~rom the data stream by the element 138 are provided to a mi¢roprocessor (uP) 142 which controls the overall operation of the receiver.

Channel selection i8 provided by an in~rared ~IR) receiver and/or a keyboard 144, which in~ormation is pas~ed on by the microproces~or 142 to the tuning synthesizer 114. ~ unique address, or serial number ;` Por the receiver i8 stored in a nonvolatile memory (NVM) 145, for addressing by the CS0, as di6cussed with respect to Figure 1.

The output o~ the Dolby decoder 140 is provided i;

as "le~t" and "right" audio channels to a stereo ampli~ier 146, and to stereo outputs 148 ~or use ; 30 with standard audio components.

It is anticipated : . .

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~'''~ `; 2003763 17 that a relay will be provided at the output 148 toswitch between other ~ource material (not shown) and the digital audio output of the receiver,; to cover instances where a user's amplifier component has limited inputs available.

In an alternate embodiment, the detector 132 is a quadrature phase shift key (QPSK) detector. This, of course, presupposes that the digital modulation o~ audio data signals occurring, for instance at element 34 of Figure 1 and element 64 of Figure 2, occurs in the QPSK mode.

It is apparent that reception of multilevel AM or FM modulated digital signals can suffer from multipath reception problems (reflections) when transmitted over the airwaves ~see 72, Figure 2) especially with respect to stereo transmissions. QPSK displays greater immunity to this problem.

QPSK techniques are well known, and are disclosed for instance in the aforementioned commonly-owned, copending Canadian Patent Application No. 560,315, filed March 2, 1988.

Figure 4 shows a multiphase modulator 200 suitable for use as the FM band exciter 36 of Figure 1.

Serial data input is provided to a serial/parallel converter 202, filtered ~y two di~ital rilters 204 and 205 and provided to two digital-to-analog converters 206 and 207, as shown.

The output of each digital-to-analog converter 206 and 207 i6 provided to a balanced mixer 208 and 209, : ., '' 2003763 18 respectively. The output of a carrier oscillator 210, operating in the FM band, is split by a 6plitter 211 and provided, in one instance, to one of the mixers 209, and in another instance is phase shifted 90 degrees by a phase shifter 212 prior to being provided to the other mixer 208. The outputs of the two mixers are combined at a combiner 213, the output o~ which is digitized audio RF output in the FM band. Multiphase modulation technique is described in greater detail in the aforementioned Ca~adiar~ Pate~t Application No. 560,315 filed March 2, 1988.

The multiphase modulator 200 is used to ;~ modulate the digital audio data.

As shown in the polar diagram 220 in Figure 5, the audio data is modulated such that each two bit symbol appears 90 degrees apart on the axes of the polar diagram.

The rightmost bit in each o~ the two bit symbols is shifted out of the transmitting shift register ~irst, and into the receiving shi~t register ~irst.

There are ~our data points 222, 224, 226, 228 shown in polar diagram 220 on the circle 221 which represent the normal amplitude o~ the carrier signal.

Figure 6 shows A known costas loop carrier recovery system 250, whi¢h is suitable ~or decoding a QPSK signal according to known tecbniques.

Such an arrangement could be advantageously employed as the dete¢tor 132 of Figure 3B.

.. ,~ :, : -, :" 20~13'763 19 Figure 7 ~hows a digital broadca~t 8yBtem 3 00 .

A playli6t computer 302 ~ for instance at a ~tation - controls the selection of audio source material from a disc player 3 04. The output of the disc player is digitized by a Dolby digitizer 306, and passed on as one of 16 inputs (channels) 308 to a formatter/encryptor/tag inserter/EDC inserter ~"inserter~) 310.

The inserter 310 combines the digital audio output of the digitizer 306 with other source material, which may or may not be digital - audio.

The inserter 310 formats and encrypt~ the source material on each channel 308 ~ tags it to identify a program access level, and provides error detection and correction t~DC) functions.

The output of the inserter 310 i6 multiplexed by a multiplexer 312, modulated by a modulator 314 and transmitted over a single video satellite uplink 316.

, At the receiving end, a satellite dish 320 receives the ~ultiplexed output of the in~erter 310, demodulatQs it at a de~odulator 322, demultiplexes it at a demultiplexer 324 and provides it as a data strQam to an EDC correct/control data insertion device 326.

Each of the 16 demultiplexed data streams is error corrected by the device 326 and ` provided to a QPSK broadcast modulator 328, ~uch as been hereinbefore described. A computerized billing system 330 exercises control over a radio controller ~ 332, which is comparable to the address module 26 of t,~ 30 Figure l ~or permitting/restrictinq access to --~ . . .

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20~3763 program material by subscribers.

The address ~ information from the radio controller 332 iS inserted by the device 326 into the data streams.

As shown in Figure 7, 16 individual outputs 334 of the device 326, each corresponding to a channel of source material, is provided to a QPSK modulator (one, 328 shown), and combined by a combiner 336 onto a transmission line 338 for distribution to subscribers (one shown) having an appropriate terminal 340.

Another beneficial feature of this system 300 :

is that locally (versus remotely, by satellite) originated audio source material 342, such as for simulcast with a television program, may be combined by a combiner 344 onto the transmission line. This would be achieved by digitizing the source material 342 with a digitizer 346, for each of a plurality of channels 348, providing the digitized source material to an inserter similar to the in~erter 310 (but not requiring the EDC insertion function), and QPSK modulating the combined output thereof with a modulator 350 for broadca~t on the tran~mis~lon line 338.

Although not shown, video channel~ could also `~ be combined for broadcast over the line 338.

Figure 8 shows a digital FM receiver 400, similar in many respects to that shown and described with respect to Figures 3A and 38. AB will become evident, however, a notable di~ference iG that the receiver 400 oP Figure 8 is suitable for receiving both QPSK digitized and nondigitiz-d FM signal~ ''," ;"' '' '' ' ' '.. ~ ~ ~ - 2003763 :` 21 Signals received on an ante~na 402 are provided to a tunable RF bandpass filter 404, to a variable gain amplifier 406, and to a tunable RF bandpass filter 408.

The output of the RF bandpass filter 408 is provided to a mixer 410, which receives a second input from a variable frequency oscillator 412, so that the output of the mixer 410 is at an intermediate frequency (IF) for a selected channel.

The channel selection process is under control of a tuning synthesizer 414 which receives the output of the variable frequency oscillator 412, and provides a signal based on the output of the oscillator 412 to effect channel eelection by the RF bandpass filters 404 and 408.

The output of the mixer 410 is provided to an intermediate frequency (IF) bandpass filter 416, tuned to 10.7 MHz, to an amplifier 418, and to a second IF bandpass filter 420 tuned to 10.7 NHz.

The IF bandpass filters 416 and ~20 are preferably wide type ceramic filters.

The output of the second IF filter is the #ignala received on the antenna 402 aorresponding to a selected ahannel in the FM band. A dotted line 422 encloses the elements 404, 406, 408, 410, 412, 416, 418 and 420, su¢h as would be found in a standard FN tuner.

lrhe output of the second IF bandpass filter 420 is provided to both a QPSK demodulator 430 and to an FN detector 432. ~n one signal path, the FM detector detQ¢ts the audio component of the incoming signal and provides such as an AF ~ignal to a stereo demultiplexer 434 (~or stereo broadca~ts), the ;.: ' "' ' ` ' ' ,, : ~`. 2~)0376.3 , 2z output of which i8 provided to a digital or analogBWitCh 436 as left and right audio channel signal~.

The FM detector 432 also provides a ~ignal to the ~ variable gain amplifier 406 to automatically control the gain thereof in accordance with known automatic gain control (AGC) techniques. In another signal path, the QPSK demodulator provides a bit stream to a logic array 438, when there i8 a digital signal received in the selected ~channel~ An indication of the existence of such a :

digital signal, indicative of a digitized audio broadcast being received, is provided as a logic signal to the digital analog switch 436.

The output of the logic array 438 iB provided to a Dolby ADM decoder 440, the output of which is provided to the digital analog switch 436 as left and right audio channel signals (for stereo broadcasts).

The analog/digital switch 436 selects between the output~ of the Dolby AD~ decoder 440 and the stereo demultiplexer 434, under control of the logic array 438, and provides le~t and right audio signals from one or the other to audio amplifiers 442 and 444.

The advantages of the invention are multifaceted.

As-mentioned hereinbefore, the digitized audio channels can be interspersed between nondigitized audio channels, each occupying 400 KHz in the FM band.

The Federal Communication~ ~` Commission (FCC) requires at least 800 KHz between `~ 30 standard FM channels in a market, which tran61ates ........... ..... ............................................................... .

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2003763 into only 25 stations in the 88-108 MHz band.

However, since digitized audio channels may be interspersed between standard (nondigitized~ channels, up to 50 channels (stations) could be provided in the standard FM, 20 MHz wide band. This is highly pertinent to both airwave and cable transmission. Due to the availability of, in essence, twice the number of stations, there is plenty o~ room created for original local stations, basic premium stations and "pay-per-listen" stations, all, in marked contrast to the aforementioned U.S. patent no. 4,684,981, without usurping a TV video channel.

Another advantage is that most of the degradation in a standard FM signal occurs within the cable network (transmission line) itself. This problem iB overcome by the use of digitized audio channels as one-to-one replacements for the standard audio channels. The ultimate result is that listener~ will be able to receive audio broadcasts that are more in line with digital disc ~CD) recordings which are becoming 80 popular. Further, the possibility o~ providing high quality audio via cable may add a new impetus to the radio induEtry.

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