United States Patent [19]
`Robbins
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,821,097
`Apr. 11, 1989
`
`[54] APPARATUS AND METHOD FOR
`PROVIDING DIGITAL AUDIO ON THE
`SOUND CARRIER OF A STANDARD
`TELEVISION SIGNAL
`
`Inventor: Clyde Robbins, Maple Glen, Pa.
`[75]
`[73] Assignee: General Instrument Corporation,
`New York, N.Y.
`[21] Appl. No.: 22,380
`
`Mar. 5, 1987
`[22] Filed:
`Int. Cl.4 ............................................... H04N 7/04
`[51]
`[52] U.S. Cl •...................................... 358/143; 380/10;
`380/15
`[58] Field of Search ................................... 380/10, 15;
`358/143-145, 86
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`Re. 31,735 11/1984 Davidson ............................ 358/124
`4,215,366 7/1980 Davidson ............................ 358/124
`4,266,243 5/1981 Shutterly ............................... 380/20
`4,295,223 10/1981 Shutterly ............................. 358/145
`4,353,088 10/1982 van Toonder et al. ............... 380/15
`4,402,010 8/1983 Vogelman ........................... 358/133
`4,405,944 9/1983 Eilers et al .......................... 358/144
`4,513,315 4/1985 Dekker et al ......................... 358/86
`4,513,327 4/1985 Takahashi et al. .................. 358/310
`4,555,730 11/1985 Briggs ................................. 358/144
`4,605,950 8/1986 Goldberg et al. .................... 358/11
`4,608,456 8/1986 Paik et al. ............................. 380/28
`4,644,580 2/1987 Akagane ............................. 358/144
`
`4,656,629 4/1987 Kondoh et al. ..................... 358/143
`4,679,085 7/1987 Johnson et al. ..................... 358/143
`4,682,360 7/1987 Frederiksen .......................... 380/10
`4,684,981 8/1987 Toyoshima et al ................... 358/86
`4,691,234 4/1987 Albean ................................ 358/144
`4,710,814 12/1987 Gassmann et al, .................. 358/143
`4,745,476 5/1988 Hirashima ........................... 358/143
`
`OTHER PUBLICATIONS
`Understanding Data Communications, published by
`Texas Instruments Leaming Center, 1984, pp. 4-4 to
`4-12.
`Primary Examiner-Salvatore Cangialosi
`Attorney, Agent, or Firm-Kramer, Brufsky & Cifelli
`ABSTRACT
`[57]
`A television transmission system replaces the standard
`FM audio portion of a television signal with digital
`audio. Three digital audio channels are time division
`multiplexed on the sound carrier, using combined multi(cid:173)
`phase and AM modulation. The audio signals are digi(cid:173)
`tized using adaptive delta modulation techniques. Video
`vertical and horizontal framing, as well as the audio
`carrier 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 multi-phase 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.
`
`47 Claims, 9 Drawing Sheets
`
`COLOR SUBCARRIER
`3.579545 MHz
`
`PICTURE
`CARRIER
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`
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`
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`
`APPLE EXHIBIT 1075
`APPLE v. PMC
`IPR2016-01520
`Page 1
`
`

`

`U.S. Patent
`
`Apr.11, 1989
`
`Sheet 1 of 9
`
`4,821,097
`
`COLOR SUBCARRIER
`3.579545 MHz
`
`PICTURE
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`FIG. I
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`
`APPLE EXHIBIT 1075
`APPLE v. PMC
`IPR2016-01520
`Page 2
`
`

`

`/ 2 5
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`APPLE EXHIBIT 1075
`APPLE v. PMC
`IPR2016-01520
`Page 3
`
`

`

`HEADEND
`CONTROLLER
`
`CONTROL DATA ,ENCRYPTION
`KEYS, SEEDS, ETC.
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`APPLE EXHIBIT 1075
`APPLE v. PMC
`IPR2016-01520
`Page 4
`
`

`

`DATA LSB
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`APPLE EXHIBIT 1075
`APPLE v. PMC
`IPR2016-01520
`Page 5
`
`

`

`AGC
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`APPLE EXHIBIT 1075
`APPLE v. PMC
`IPR2016-01520
`Page 6
`
`

`

`206
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`APPLE EXHIBIT 1075
`APPLE v. PMC
`IPR2016-01520
`Page 7
`
`

`

`U.S. Patent Apr.11, 1989
`
`Sheet 7 of 9
`
`4,821,097
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`APPLE EXHIBIT 1075
`APPLE v. PMC
`IPR2016-01520
`Page 8
`
`

`

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`APPLE EXHIBIT 1075
`APPLE v. PMC
`IPR2016-01520
`Page 9
`
`

`

`U.S. Patent
`
`Apr.11, 1989
`
`Sheet 9 of 9
`
`4,821,097
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`APPLE EXHIBIT 1075
`APPLE v. PMC
`IPR2016-01520
`Page 10
`
`

`

`1
`
`4,821,097
`
`2
`digitized audio signal is modulated using multi-phase
`modulation on the sound carrier portion of a television
`signal. The modulated sound carrier is then converted
`to an intermediate frequency, and summed with the AM
`5 modulated video portion of the television signal to
`which the digitized audio signal corresponds to produce
`a composite IF output signal.
`The sound carrier is amplitude modulated with a pilot
`signal for use as a phase and timing reference in the
`multi-phase modulated signal. The pilot signal can com(cid:173)
`prise, for example, a pulse added to the sound carrier
`once for each horizontal line contained in the television
`signal. The pilot signal can be added to a portion of the
`sound carrier corresponding to the end of active video
`on each horizontal line.
`In order to broadcast the composite IF output signal,
`it can be converted to an RF output signal and transmit(cid:173)
`ted on a television signal channel. The RF output signal
`can be transmitted through the air, via satellite, over a
`cable television system, or any combination thereof. In
`the instance where the television signal contains pre-
`mium programming such as that which would be of(cid:173)
`fered on a pay-per-view basis via satellite or a cable
`television system, the video portion of the signal can be
`scrambled by suppressing the horizontal synchroniza(cid:173)
`tion pulses thereof. Such scrambling will not interfere
`with the proper transmission and subsequent reception
`and reproduction of the digital audio signal. The digital
`audio data can also be encrypted to prevent unautho(cid:173)
`rized reproduction of the audio portion of a television
`program.
`In accordance with the present invention, the adapt(cid:173)
`ive delta modulation used to digitize the audio signal
`uses an audio sampling rate that is an integer factor of
`the sound carrier center frequency. Further, the sound
`carrier center frequency is phase locked to a horizontal
`scan rate used in reproducing the video portion of the
`television signal. In a preferred embodiment, the hori-
`zontal scan rate is approximately 15.734 KHz and the
`sound carrier center frequency is 4.5 MHz.
`In order to provide stereo left, stereo right, and sec-
`ond audio program ("SAP") audio channels, these three
`channels can be time division multiplexed on the audio
`carrier of the television signal. The digitizing of the
`three audio channels can be accomplished by sampling
`the left and right stereo audio channels at an adaptive
`delta modulation rate of 13 times the horizontal scan
`rate, and sampling the SAP channel at an adaptive delta
`modulation rate of 11 times the horizontal scan rate.
`The composite data stream may be serially encrypted
`for security purposes.
`The present invention also provides a method and
`apparatus for receiving the television signal with the
`digital audio data incorporated therein and reproducing
`the sound which the digital signals represent. A multi-
`phase demodulator demodulates a received television
`signal to retrieve the audio channel data. The data is
`presented to an adaptive delta modulation decoder
`which produces conventional audio output signals for
`input to an audio amplifier or, alternately, a television
`modulator which inputs the audio signals (not in analog
`from) to a television for sound reproduction in a con(cid:173)
`ventional manner.
`
`APPARATUS AND METHOD FOR PROVIDING
`DIGITAL AUDIO ON THE SOUND CARRIER OF A
`STANDARD TELEVISION SIGNAL
`
`BACKGROUND OF THE INVENTION
`The present invention relates to television broadcast(cid:173)
`ing and reception, and more particularly to a method
`and apparatus for providing high quality digital sound
`signals within the audio portion of a standard television lO
`signal.
`New digital techniques for the reproduction of sound
`provide performance that is far superior to analog tech(cid:173)
`niques which have been used in the past. An example of
`high fidelity sound reproduction using digital tech- 15
`niques can be found in the compact disc technology
`which has recently enjoyed tremendous success as an
`alternative to photograph records and tapes. Digital
`recording and playback techniques provide reproduc(cid:173)
`tion of music that is extremely realistic and absent from 20
`background noise and distortions which have plagued
`other high fidelity sound reproduction systems cur(cid:173)
`rently in wide scale use.
`Recent advances in television technology have en(cid:173)
`abled the transmission of stereophonic sound together 25
`with a conventional television picture transmission.
`Known systems have used analog techniques in connec(cid:173)
`tion with the stereophonic sound reproduction. See, for
`example, commonly assigned and co-pending U.S. pa(cid:173)
`tent application Ser. No. 06/816, 133 filed Jan. 3, 1986 30
`and entitled "Apparatus and Method for Stereo Televi(cid:173)
`sion Sound".
`A difficulty with providing digital audio in television
`broadcasting has been the incorporation of the digital
`signals within the standard . television signal without 35
`interfering with the video portions of the television
`signal or appreciably affecting the quality or type of
`picture reproduced by conventional video circuits. In
`addition, any television distribution system which trans(cid:173)
`mits digital audio data (such as a cable television sys- 40
`tern) must be such that the transmitted television signal
`can be received and reproduced on the millions of tele(cid:173)
`vision sets already in existence which use conventional
`analog sound circuits. Thus, such things as the channel
`width of six megahertz (MHz) for each channel within 45
`the television signal spectrum, the aspect ratio of four to
`three, the video bandwidth of 4.25 MHz, the horizontal
`and vertical scanning rates of 15.734 kilohertz (KHz)
`and 60 hertz (Hz), respectively, and the number of scan(cid:173)
`ning lines per frame at 525 cannot be changed, subject 50
`to narrow tolerances.
`The present invention provides a method and appara(cid:173)
`tus for incorporating digitized audio data within the
`second carrier of a standard television signal in a man(cid:173)
`ner such that the signal will be recoverable for repro- 55
`duction of the transmitted program on black and white
`and color television sets already in existence.
`
`SUMMARY OF THE INVENTION
`In accordance with the present invention, a method 60
`and apparatus are provided for transmitting, receiving,
`and reproducing digital audio signals in the sound car(cid:173)
`rier of a standard television signal. An audio signal is
`digitized using, for example, adaptive delta modulation
`techniques. Several channels of audio information, such 65
`as left and right stereo channels and a second audio
`program ("SAP") channel can all be digitized and in(cid:173)
`corporated onto the television signal sound carrier. The
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a diagram showing the complete video
`spectrum of a standard color television transmission;
`
`APPLE EXHIBIT 1075
`APPLE v. PMC
`IPR2016-01520
`Page 11
`
`

`

`4,821,097
`
`3
`FIG. 2 is a graphical representation showing the
`interleaving of the luminance and digital audio signals
`with color signal in the frequency spectrum of a televi(cid:173)
`sion signal;
`FIG. 3 is a block diagram of a cable television hea- 5
`dend which modulates and adds digital audio signals to
`a television signal in accordance with the present inven(cid:173)
`tion;
`FIG. 4 is a block diagram of a multi-phase modulator
`for use in modulating the digital audio information in 10
`accordance with the present invention;
`FIG. 5 is a block diagram of a cable television con(cid:173)
`verter for receiving and reproducing television signals
`with digital audio data in accordance with the present
`invention;
`FIG. 6 is a block diagram of a multi-phase demodula(cid:173)
`tor for use in the converter shown in FIG. 5;
`FIG. 7 is a polar diagram illustrating the multi-phase
`modulation technique used in accordance with the pres-
`ent invention;
`FIG. 8 depicts various waveforms including the mul(cid:173)
`ti-phase modulated input, detected peak AM, detected
`most significant bit, and detected least significant bit
`information from a television signal having digital audio
`data in accordance with the present invention;
`FIG. 9 is a logic diagram of a data encryptor/decryp(cid:173)
`tor used to encode or decode digital data transmitted
`with a television signal;
`FIG. 10 is a timing diagram of horizontal synchroni-
`zation and data pulses; and
`FIG. 11 is a timing diagram of AM modulated data
`within a portion of a video information frame.
`
`20
`
`4
`by placing the chrominance information at odd multi(cid:173)
`ples of one-half the line frequency. This was possible
`because the scanning rates for the chrominance signal
`and luminance signal are the same (approximately
`15.734 KHz). Interleaving of the chrominance signals is
`shown in FIG. 2 at the 453rd harmonic 36 of half the
`line frequency 27, as well as at the 455th harmonic 38,
`457th harmonic 40, and 459th harmonic 42. Those
`skilled in the art will recognize that luminance and
`chrominance signal energy will continue along the fre(cid:173)
`quency spectrum at harmonics above those shown in
`FIG. 2.
`The intent of the present invention is to provide digi(cid:173)
`tal audio in the sound carrier of a standard television
`15 signal in such a manner that the audio signals will not
`interfere with the video portions of the television signal,
`thereby providing compatibility with the video process(cid:173)
`ing stages of standard television receivers. In order to
`do this, the digital audio signals are synchronized with
`the horizontal line frequency so that the harmonics of
`the modulated audio signals will appear in registration
`with the luminance portions of the television signal
`frequency spectrum. Thus, the digital audio signals will
`be interleaved with the color signals and will not inter(cid:173)
`fere with the color reproduction in a video program.
`In order to accomplish the objective of the present
`invention, the stereo left, stereo right, and SAP audio
`channels are time division multiplexed on the 4.5 MHz
`audio carrier of a standard television signal using com(cid:173)
`bined multi-phase modulation and Am modulation. The
`4.5 MHz intercarrier is phase locked to the video hori-
`zontal scan rate of 15.734 KHz. In the preferred em(cid:173)
`bodiment, the audio signals are digitized using adaptive
`delta modulation ("ADM") techniques and particularly,
`the ADM system proposed by Dolby Laboratories in a
`paper entitled "Recent Developments in Digital Audio
`Techniques", K. J. Gundry, D. P. Robinson, and C. C.
`Todd, Dolby Laboratories, San Francisco and London,
`presented at the 1984 NCTA Show, which paper is
`incorporated herein by reference. An integrated circuit
`chip for providing Dolby ADM decoding is available
`from Signetics Corporation under Model No. NE5240.
`Also pertinent and incorporated herein by reference is a
`paper entitled "Digital Audio for Cable Television",
`Clyde Robins, NCTA Technical Papers, Mar. 15-18,
`1986.
`The adaptive delta modulation audio sampling rate
`used in digitizing the audio data in the preferred em(cid:173)
`bodiment of the present invention is an integer factor of
`50 the intercarrier frequency. Three digital components
`are established for each digitized channel in the Dolby
`ADM technique. These are amplitude information,
`frequency companding data, and amplitude compand-
`ing data. In the best mode embodiment, the digitized
`audio is transmitted via multi-phase modulation on the
`aural carrier. Forty-four bits of data are transmitted in
`each horizontal line. The first four bits are used as pre(cid:173)
`amble for bit synchronization. The preamble is transmit(cid:173)
`ted with a unique phase and amplitude, identifying it as
`a synchronization symbol.
`As noted above, three channels of digital audio are
`preferably transmitted; namely, stereo left, stereo right,
`and second audio program channels. The left and right
`stereo channels each require 13 bits of digitized (ADM)
`data for each horizontal line of video in the television
`signal, and one bit per channel per line for frequency
`and amplitude companding data. The SAP channel
`requires 11 bits of digitized audio data per horizontal
`
`25
`
`30
`
`35
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`FIG. 1 illustrates the complete video spectrum of a
`standard color television transmission. The luminance
`signal 14 is shown in graph 10, and the color or "chro(cid:173)
`minance" signal 20 is shown in graph 12. A color sub(cid:173)
`carrier 16 is transmitted at 3.579545 MHz with side- 40
`bands that extend 0.6 MHz above and 1.5 MHz below
`this frequency. The sound carrier 18 is centered at 4.5
`MHz.
`The use of the specific frequency of 3.579545 MHz
`for the chrominance subcarrier results in an interleaving 45
`of the luminance and chrominance signals as is well
`known in the art. This interleaving of signals makes it
`possible to transmit both the luminance and chromi(cid:173)
`nance signals within the same channel width used for
`the transmission of a monochrome television signal.
`In standard monochrome television signal transmis(cid:173)
`sion, the transmitted signal is comprised of recurring
`wave forms having frequencies that are harmonics of
`the horizontal line scanning frequency. Thus, the fre(cid:173)
`quency spectrum contains a concentration of energy at 55
`each harmonic, i.e., at whole multiples of the horizontal
`line frequency. FIG. 2 illustrates the frequencies at
`which such concentrations of energy occur. A first
`concentration appears at the line frequency 29. Subse(cid:173)
`quent concentrations of energy are centered around the 60
`second harmonic 31, the third harmonic 33, and so forth
`as shown at the 226th harmonic 35, 227th harmonic 37,
`228th harmonic 39, and 229th harmonic 41. As shown in
`FIG. 2, nearly half of the video spectrum is unused by
`the luminance signal which has harmonics at whole 65
`multiples of the line frequency. Therefore, in providing
`color television transmission, it was possible to inter(cid:173)
`leave the chrominance signal with the luminance signal
`
`APPLE EXHIBIT 1075
`APPLE v. PMC
`IPR2016-01520
`Page 12
`
`

`

`5
`line and one bit per line for frequency and amplitude
`companding data. This bit distribution is summarized in
`Table 1.
`
`4,821,097
`
`10
`
`6
`other scrambling systems. When using sync suppression
`scrambling, the timing recovery signals sent as AM on
`the sound carrier may be offset in time from the video,
`increasing the security of the video signal, while making
`5 the sound unrecoverable by unauthorized boxes.
`The AM modulated data in the signal transmitted
`from the cable television headend to subscriber convert(cid:173)
`ers contains 16 bits of data for 6/10 db sync suppression
`scrambling selection, program specific data such as
`price, morality rating, and like data as well known in the
`art. An example of a prior art headend controller which
`transmits such data is the model AH-4 controller manu(cid:173)
`factured and sold by the Jerrold Division of General
`Instrument Corporation.
`In accordance with the preferred embodiment of the
`present invention, 13 new bits of data are appended to
`the AM data tag to carry seeding and key data for the
`decryptor, service code data for the digital audio con(cid:173)
`verter, and time shift descrambling data. This data is
`necessary to enable an authorized digital audio con(cid:173)
`verter to receive, decrypt, and reproduce sound from
`the digital audio signals transmitted with the television
`signal. A service code can also be provided in the data
`that will be read by non-digital audio converters to
`deauthorize them so that they cannot receive any televi(cid:173)
`sion program channels carrying digital sound.
`When the present invention is used in conjunction
`with a cable television system, three primary compo(cid:173)
`nents are used. These are the addressable controller
`(also referred to as "headend controller"), the headend
`encoder, and the subscriber converter (also known as
`the "subscriber terminal"). Both the addressable con(cid:173)
`troller and encoder are present at the headend from
`which the cable television signals are sent by the cable
`system operator. The addressable controller controls all
`subscriber terminals in the cable television system, con(cid:173)
`trols the encoders/ decoders associated with the system,
`configures scrambling modes, service codes, and en-
`cryption keys, and orchestrates the dissemination of all
`decryption keys. The encoder of the present invention
`is a headend device consisting of a number of subcom(cid:173)
`ponents including an audio digitizer, video scrambler,
`tag insertion logic, addressable controller interface
`logic, and modulator circuitry. These components are
`described below in connection with the description of
`FIG.1.
`The subscriber converter is a device located at each
`subscriber's residence and contains an RF converter
`module, demodulator, addressable controller interface
`logic, subscriber interface logic, audio decryptor and
`digital to analog ("D/ A") converter, together with a
`video descrambler and modulator. Each of these ele(cid:173)
`ments will be described below in connection with the
`description of FIG. 5.
`In addition to the AM data path, certain data is trans(cid:173)
`mitted over an FM data path from the cable television
`headend controller to the subscriber converters. This
`data is typically modulated using frequency-shift keying
`("FSK") techniques well known in the art. The model
`AH-4 addressable controller referred to above is an
`example of a prior art headend controller which con(cid:173)
`trols data which is both AM modulated for certain
`tagging data and FSK modulated for encryption data,
`authorization codes, and the like.
`Table 2 lists various terms and their definitions which
`are used herein in connection with the description of the
`transmission of data from the headend controller to the
`
`TABLE 1
`AUDIO DATA FRAME
`Bits
`Data
`
`0-12
`13
`14-26
`27
`28-38
`39
`
`left audio
`left audio companding
`right audio
`right audio companding
`SAP audio
`SAP audio companding
`
`Number of Bits
`
`13
`1
`13
`1
`11
`1
`40 bits total
`
`The horizontal scan rate ("fII') in a conventional 15
`NTSC color television signal is 15.734264 KHz. The
`audio intercarrier is 286 times the horizontal scan rate,
`or 4.5000 MHz. Factors of the audio intercarrier are 2,
`11, and 13 (2X 11 X 13=286). Thus, as noted above, the
`adaptive delta modulation audio sampling rates work 20
`out to be integer factors of the intercarrier frequency.
`The modulation rate of the multi-phase modulated
`audio data is 22 X fH, which provides 22 "symbol times"
`pre horizontal line. Thus, there will be 13 intercarrier
`cycles per symbol time (13X22=286). The multi-phase 25
`modulation provides two bits of data per symbol, which
`results in the transmission of 44 audio data bits in each
`horizontal line. This translates to a bit rate of 692.3
`kilobits per second ("KBPS"). Using non-return to zero
`("NRZ") coding makes the maximum symbol transition 30
`rate one-half of the symbol rate or 11 fH. The Nyquist
`required bandwidth is ± 173.07 KHz.
`In accordance with the bit distribution for the audio
`data shown in Table 1, the stereo audio delta modula(cid:173)
`tion sampling rate for each of the left and right channels 35
`in the preferred embodiment is 13 fH. The second audio
`program delta modulation sampling rate is 11 fH. Fi(cid:173)
`nally, the adaptive companding sampling rate is one(cid:173)
`half fH. In the Dolby ADM system, both amplitude and
`frequency companding data are used. In the present 40
`implementation of Dolby ADM, the companding bits
`provided in each horizontal line are alternated between
`amplitude data and frequency data. Each line contains
`either amplitude or frequency companding data, and
`the contents of sequential lines alternate from one line to 45
`the next. Thus, the adaptive companding sampling rate
`for each type of companding data can be half the hori(cid:173)
`zontal scan rate.
`The audio data is carried by the aural intercarrier of
`a television signal using multi-phase modulation. The 50
`same carrier uses AM modulation for video vertical and
`horizontal framing, as well as the audio carrier phase
`reference, and audio data bit time and frame reference.
`When the present invention is used in connection with a
`cable television system, program identifications and 55
`decryption seeds are also carried on the aural intercar(cid:173)
`rier using AM modulation. In the cable television envi(cid:173)
`ronment, it is advantageous to encrypt the audio data,
`and the composite data stream may be serially en(cid:173)
`crypted so that only authorized subscribers will be able 60
`to recover and reproduce the digital audio program(cid:173)
`ming.
`In a cable television system, digital audio may be
`transmitted in accordance with the present invention
`along with a video signal that is scrambled using, e.g., 65
`conventional 6/10 db dynamic sync suppression tech(cid:173)
`niques and/or video inversion scrambling systems.
`Those skilled in the art are well aware of these and
`
`APPLE EXHIBIT 1075
`APPLE v. PMC
`IPR2016-01520
`Page 13
`
`

`

`4,821,097
`
`TAG DECRYPTION KEY
`
`7
`encoder and subscriber converters in accordance with
`the preferred embodiment of the present invention.
`TABLE 2
`Set of 8 nibbles
`AUDIO DECRYPTION KEYS
`transmitted over the FM
`(FSK) data path to the
`subscriber terminal
`(converter) and encoder.
`The nibbles are used to
`seed the cryptors in the
`headend encoder and
`subscriber terminal
`decoder. At any one time
`four of the nibbles are in
`use. The unused nibbles
`may be changed out while
`idle. Data identifying
`the four nibbles to be
`used, and the order of
`usage is transmitted from
`the headend controller to
`the encoder, and
`transmitted from the
`encoder to the subscriber
`terminal as AM modulated
`tag data.
`Specifies which of the
`audio decryption keys to
`use in each of the 4
`nibble slots in the
`subscriber terminal
`descrambler circuit. Any
`of the 8 keys can be
`assigned to any of the 4
`slots. This assignment
`data is sent in bits 16-23
`of the AM tag, and is
`specific to each
`television program.
`4 bit nibble sent from the
`headend controller to the
`subscriber terminal over
`the FM data path. This
`key is used to decrypt the
`encrypted service code and
`encrypted seed that is
`sent in the AM tag. Two
`tag decryption keys exist
`in the system, but only
`one is in use at any time.
`A key may be changed out
`when it is not in use.
`The key that is to be used
`to decrypt the tag is
`specified in the tag.
`Service code sent in bits
`0-7 of the AM tag, and
`used to determine
`authorization for the
`video portion of the
`program. A subscriber
`terminal not authorized
`for this service code will
`disallow viewing of the
`video portion of the
`program. A non-digital
`audio subscriber terminal
`will also use it to allow
`or disallow viewing (non(cid:173)
`digital audio converters
`should always be
`deauthorized for digital
`audio services).
`Service code of the audio
`portion of the program,
`sent in an encrypted form,
`decrypted by tag
`decryption key. The audio
`service code is one of the
`fields of data sent in
`bits 16-23 of the AM tag.
`This service code is used
`by the digital audio
`
`KEY USAGE SPEC
`
`VIDEO SERVICE CODE
`
`AUDIO SERVICE CODE
`
`8
`TABLE 2-continued
`converter to determine
`authorization for stereo
`audio. If the upper tag
`bits are not present, the
`standard service code
`(bits 0-7) are used for
`authorization validation,
`and audio is transmitted
`in the standard TV format.
`Data specifying the
`m