United States Patent [191
`Mustafa et al.
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,789,895
`Dec. 6, 1988
`
`. [54] SYSTEM FOR SYNCHRONIZING DIGITAL
`BIT STREAM FOR TELECOMMUNICATION
`SYSTEM
`
`[75]
`
`Inventors: Mehmet Mustafa, Waltham; Ernest
`P. Tweedy, Lexington; James C.
`Stoddard, Wayland; Walter J.
`Beriont, Natick, all of Mass.
`
`[73] Assignee: GTE Government Systems
`Corporation, Waltham, Mass.
`
`[21] Appl. No.: 44,387
`
`[22] Filed:
`
`Apr. 30, 1987
`
`[51]
`Int. Cl,4 ............................................... H04N 7/04
`[52] u.s. Cl ..................................... 358/147; 358/146;
`358/148
`[58] Field of Search ............... 358/148, 141, 142, 146,
`358/147, 143, 145, 86, 83; 375/95, 111, 119
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`3,743,767 7/1973 Bitzer et al ........................... 358/93
`3,746,780 7/1973 Stetten et al .......................... 358/85
`3,891,792 6/1975 Kimura ............................... 358/142
`4,104,681 8/1978 Saylor et al ......................... 358/141
`4,129,748 12/1978 Saylor ................................. 358/141
`4,500,751 · 2/1985 Darland et al ..
`4,578,535 3/1986 Simmons .
`4,600,943 7/1986 Tanabe ................................ 358/147
`4,613,827 9/1986 Takamori et al ..
`4,616,263 10/1986 Eichelberger .
`4,712,131 12/1987 Tanabe ................................ 358/147
`
`OTHER PUBLICATIONS
`NHK (Japan Broadcasting Corporation) "Present Sta(cid:173)
`tus of Still-Picture Television"; May 1978.
`Berg "Dialoog TV; Kabelexperiment Zuid-Limburg
`J2-Elektrotechniek/Elektronica"-No.
`4-1986,
`pp.
`35-39.
`Kimura, A., "Telescan Simultaneous Information Dis(cid:173)
`play System" Telecommunication Journal, vol. 42, pp.
`33-45, 1975.
`Primary Examiner-James J. Groody
`Assistant Examiner-David E. Harvey
`Attorney, Agent, or Firm-J. Stephen Yeo
`
`ABSTRACT
`[57]
`A telecommunication system for synchronizing a 'digital
`bit stream sent from a central facility to a terminal on
`lines of television frames. Each active line starts with a
`horizontal sync pulse and a color burst. A first data
`clock at the central facility provides first data clock
`pulses synchronized with said color burst. A circuit at
`the central facility provides a flag bit one clock pulse
`wide delayed by a first constant number of clock pulses
`from the start of the line's horizontal sync pulse. A
`second data clock at the terminal provides second data
`clock pulses synchronized with said color burst. A cir(cid:173)
`cuit at said terminal provides a time window delayed by
`a second constant number of clock pulses from the start
`of the line's horizontal sync pulse so as to bracket said
`flag bit. A plurality of second data clock pulses occur
`during said time window. Circuits at the terminal deter(cid:173)
`mine which of said plurality of second data clock pulses
`coincide in time with said flag bit, thereby synchroniz(cid:173)
`ing said data stream with said second data clock.
`
`2 Claims, 5 Drawing Sheets
`
`I POWERSiiiiPLiES - ~
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`
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`19
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`18
`
`SUBSYSTEM
`
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`
`17
`
`19
`
`CONCENTRATOR
`
`SUBSYSTEM
`
`19
`
`11
`
`19
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`
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`
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`
`SIGNALLING LINES FROM
`
`F'i.g: 2.
`
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`

`
`U.S. Patent Dec. 6, 1988
`
`Sheet 3 of 5
`
`4,789,895
`
`LINES FOR FIELD SYNC
`ADDRESS, MODE CODE, ERROR CHECK
`
`LINES 1-15; 263-278
`
`ACTIVE LINES 16-262; 279-525
`
`VIDEO
`OR
`AUDIO DATA
`
`Fi'i.q. .3.
`
`FLAG BIT
`
`TIME
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`
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`
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`
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`
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`
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`
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`
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`
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`
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`
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`
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`
`1
`
`4,789,895
`
`SYSTEM FOR SYNCHRONIZING DIGITAL BIT
`STREAM FOR TELECOMMUNICATION SYSTEM
`
`2
`the described system to have audio information such as
`music and voice accompany the video.
`In a publication dated May 1978, NHK (the Japan
`Broadcasting Corporation) a system is described for
`5 sending still pictures with accompanying program
`RELATED APPLICATIONS
`sound. Frames for a number of programs are time multi-
`The following applications filed simultaneously with
`plexed. Each video frame has a code identifying the
`this application pertain to different features of the tele-
`program to which it belongs. The sound signals of all
`communication systems.
`Ser. No. 044,393, ftled Apr. 30, 1987 Telecommunica-
`the programs are digitally encoded and time division-
`tion System With Video and Audio Frames is con- 10 ally multiplexed on lines within an audio frame. Both
`the video and audio frames are in the standard NTSC
`cerned with addressed still frame video and time-com-
`pressed audio frames.
`format. The video frames are sent serially with each
`Ser. No. 044,388 filed Apr. 30, 1987 Telecommunica-
`video frame followed by two audio frames. Each video
`tion System With Burst and Continuous Audio Signals
`frame carries analog video information corresponding
`discusses attenuating a continuous background channel 15 to one program, while each audio frame carries digi-
`when burst audio is played.
`tized audio information corresponding to all programs.
`Ser. No. 044,394 flled Apr. 30, 1987 Telecommunica-
`A receiver seizes and records video frames of a se-
`tion System With Selectable Audio Channels describes
`lected program for repetitive display. The analog video
`a system having two or more continuous audio signals
`may be digitized and stored in a solid state memory
`sent on a first. transmission medium and sele~t~d in re- 20 while apparently the audio is converted to analog and
`s~onse to a s1gnal sent on a second transm1ss1on me-
`played as received. The NHK system is appropriate for
`broadcasting a plurality of predetermined programs,
`dmm.
`.
`. Ser. No. 044,~95 flled Apr. 30, 1987 Te~ecommumc~-
`but it is not optimal for interactive broadband services
`tion System W1th Frame Selected Contmuous Audto
`where many users can initiate and interact with pro-
`Signals descri?es a sys~e~ having frame~ audio and two 25 grams or services at different times.
`or more contmuous audto channels whtch are selected
`in response to a control signal.
`
`INCORPORATION BY REFERENCE
`U.S. Pat. No. 3,746,780 is hereby incorporated by 30
`reference.
`
`BACKGROUND OF THE INVENTION
`This invention pertains to telecommunications and,
`more particularly, is concerned with interactive tele- 35
`communication systems.
`A number of telecommunication systems have been
`suggested and deployed which allow a terminal user to
`request particular video information from a remote
`repository. Services possible with such systems include, 40
`but are not limited to, information searches, retrievals,
`financial transactions, reservations, and shopping.
`With some systems, both the user requests and the
`video information are sent on a single duplex medium
`such as telephone lines. Examples of such single me- 45
`dium systems are described in U.S. Pat. Nos. 4,500,751
`and 4,578,535.
`In other systems, requests are sent over telephone
`lines and the video information is sent over a broader
`bandwidth medium, such as cable. Examples of such 50
`dual media systems are the subjects of U.S. Pat. Nos.
`3,746,780 and 4,616,263, and described in I2-Elek(cid:173)
`trotechniek/Elektronica No. 4-1986, pp. 35-39.
`With the system of the aforecited U.S. Pat. No.
`3,746,780, a user wanting a service telephones a code 55
`number to a central facility. At the central facility, the
`selected video information is recovered from video
`discs or other storage means and sent as still television
`frames to the user over cable or other media. As many
`terminals are usually coupled to the same medium, each 60
`still frame contains a location number or address which
`is read by address detectors located at the terminals.
`Only those frames with the same address as the terminal
`are accepted. An accepted frame may be digitally
`stored by the terminal and repetitively displayed on a 65
`conventional television receiver.
`Examples of video information include menus, forms,
`data, text, and still pictures. It is desirable but lacking in
`
`SUMMARY OF THE INVENTION
`Briefly, according to one aspect of the invention, a
`system is provided for synchronizing a digital bit stream
`sent from a central facility to a terminal on lines of
`television frames. Each active line starts with a horizon(cid:173)
`tal sync pulse and a color burst. A first data clock at the
`central facility provides first data clock pulses synchro(cid:173)
`nized with said color burst. A circuit at the central
`facility provides a flag bit one clock pulse wide delayed
`by a first constant number of clock pulses from the start
`of the line's horizontal sync pulse. A second data clock
`at the terminal provides second data clock pulses syn(cid:173)
`chronized with said color burst. A circuit at said termi(cid:173)
`nal provides a time window delayed by a second con(cid:173)
`stant number of clock pulses from the start of the line's
`horizontal sync pulse so as to bracket said flag bit. A
`plurality of second data clock pulses occur during said
`time window. Circuits at the terminal determine which
`of said plurality of second data clock pulses coincide in
`time with said flag bit, thereby synchronizing said data
`stream with said second data clock.
`
`BRIEF DESCRIPTION OF THE ORA WINGS
`FIG. 1 is a schematic diagram of a telecommunica(cid:173)
`tion system embodying the system;
`FIG. 2 shows in more detail a video server subsystem
`of the Central Facility seen in FIG. 1;
`FIG. 3 represents video and audio frames used to
`carry information from the Central Facility to Termi(cid:173)
`nal;
`FIG. 4 illustrates waveforms on a bit synchronization
`system; and
`FIG. 5 sheets 1 and 2 are block diagrams of a Termi(cid:173)
`nal used in the system of FIG. 1.
`
`DESCRIPTION OF INVENTION
`A telecommunication system 10 embodying the in(cid:173)
`vention is shown in FIG. 1. A Central Facility 11 is
`coupled to a plurality of terminals 12 through a first
`transmission medium 13 having a bandwidth sufficient
`to carry standard television frames. First transmission
`
`7
`
`

`
`4,789,895
`
`3
`medium 13 may be cable, such as a CATV network.
`Optical fiber, air, and other wideband media are also
`suitable. The Central Facility 11 and terminals 12 are
`also coupled by a second transmission medium 14 al(cid:173)
`lowing duplex transmission of control signals not re(cid:173)
`quiring a large bandwidth. The local switched tele(cid:173)
`phone system is a satisfactory second transmission me(cid:173)
`dium, allowing low bit rate signals to be sent between
`the facility and a specific terminal. Alternatively, the
`low bit rate signals can be transmitted over the afore- 10
`mentioned broadband medium 13.
`Central Facility 11 provides both video and audio
`information in response to subscribers' requests. The
`video information represents still frames of text, pic(cid:173)
`tures, or other images sent as a series of addressed video 15
`frames. Each still frame may be displayed for several
`seconds on a television set or monitor connected to the
`terminal, during which time it is desirable to provide
`audio to accompany the video.
`The system preferably has two types of audio: contin- 20
`uous and specific, both of which may be played simulta(cid:173)
`neously. Continuous audio is sent on a real time basis on,
`for example, audio channels. An example of continuous
`audio is background music. Specific audio is so called 25
`because it is specific to the displayed still frame. Typical
`specific audio is voiced narrative or instructions. Spe(cid:173)
`cific audio is intended for only certain selected termi(cid:173)
`nals at a time and is sent in time-compressed bursts by
`addressed frames having the same address as the accom- 30
`panying video frame. Only one channel of time-com(cid:173)
`pressed audio is carried in an audio frame.
`Both audio frames and video frames are in the same
`television format, such as the NTSC system. The same
`principle is directly applicable to other television sys- 35
`terns like PAL or SECAM.
`In the NTSC system, each frame is 1/30 second long
`and consists of two fields with 262.5 horizontal lines
`each. The lines of two fields are interlaced for a total of
`525 lines per frame. Approximately 21 lines occur dur- 40
`ing a period called the vertical blanking interval (VBI)
`which is at least 1.33 mS long. These lines do not appear
`on the television screen, leaving about 483 lines of video
`in a frame.
`The NTSC system is described briefly in "Federal 45
`Communication Commission, Public Notices" of Dec.
`17, 1953 and June 6, 1954.
`When the NTSC standard was written, a minimum
`VBI of 1.33 mS was necessary to allow the scan to
`return to the top of the picture tube between fields. 50
`Schemes for sending auxiliary information during one
`or more lines of VBI have been developed such as de(cid:173)
`scribed in U.S. Pat. No. 3,493,674 and in North Ameri(cid:173)
`can Broadcast Teletext, Specification-Engineering and
`Development Department, CBS Television, Newark, 55
`N.J., June 22, 1981.
`In the present invention, addresses, mode codes and
`error detection/correction are sent on one or more lines
`(e.g., line 12) during the VBI of both video frames and
`audio frames. The address alerts an addressed terminal 60
`that a correspondingly addressed frame is to be ac(cid:173)
`cepted. The mode code identifies the frame as either
`video or audio, and its sequence with other frames so
`that the terminal can process it accordingly.
`The audio frame information, which may be digital or 65
`analog, is sent in time-compressed bursts, allowing one
`or more seconds of audio to· be carried by each 1/30
`second frame. The terminal stores the time-compressed
`
`4
`audio frame and plays it at normal speed through a
`television receiver.
`.The Central Facility 11 consists of four basic subsys(cid:173)
`tems: a Concentrator Subsystem 15, a Server Subsystem
`16, a Video Subsystem 17, and Control and Mainte(cid:173)
`nance Subsystem 18.
`Each of the subsystems may be implemented on a
`commercially available general purpose computer, with
`the interconnections being provided by a Local Area
`Network (LAN) 19. The software of each of these
`subsystems may be designed as a self-contained entity,
`with the inter-subsystem interfaces conforming to a
`standard inter-processor protocol. This allows for a
`complete Central Facility system configuration where
`each subsystem consists of a separate processor or
`group of processors. Howev~r, in smaller configura(cid:173)
`tions, one or more of these subsystems may be imple(cid:173)
`mented on a single computer while still maintaining the
`software interfaces that allows simple expansion to mul(cid:173)
`ti-computer configurations.
`The Control and Maintenance Subsystem 18 provides
`the administration for the Central Facility 11 and is also
`responsible for the gathering of statistics on the work(cid:173)
`ings of the overall system. The Control and Mainte(cid:173)
`nance Subsystem 18 is not necessary to practice the
`present invention and will not be discussed further.
`The Concentrator Subsystem 15 is the interface to the
`terminals for all control and communication purposes.
`It is accessed by a remote terminal on the second trans(cid:173)
`mission medium 14 which may be a dial-up Cl'>nnection
`through the public telephone network, or an RS232C
`direct terminal access interface for high usage control
`terminal activities.
`The Server Subsystem 16 acts as the overall control(cid:173)
`ler of a session using input from the Terminal12, via the
`Concentrator Subsystem 15, to access the appropriate
`databases and to send instructions back to the Concen(cid:173)
`trator Subsystem 15 as to which video and audio frames
`to send to the user.
`The Video Subsystem 17 which is seen separately in
`FIG. 2 stores and sends to t·he terminals 12 via the
`CATV link 13 video and encoded audio information
`frames. Video still frames are stored in optical Video
`Disk Units 20 (VDU) in standard NTSC composite
`baseband format. About 54000 still frames can be stored
`on each disc.
`Since the seek time of the video disc units is longer
`than desired, many more than the minimum number of
`video disc units to accommodate all the still frames is
`needed. The video disc units 20 provide ac coupled
`signals, so a de restorer 21 is needed to precede a non-
`blocking video matrix switch 22.
`.
`If the user calls for a video still frame, a central pro(cid:173)
`cessor 23 identifies its location on an available video
`disc unit. After the unit has reached the requested still
`frame, it advises switch 22 which then switches one
`frame into a pulse insertion device 24. The pulses in(cid:173)
`serted come directly from the station sync master. The
`frame is then connected into a channel processor 25
`which inserts addressing, mode code and error codes on
`a VBI line. The addressed frame is connected into the
`video input of a CATV modulator 26.
`Audio frames (of which more than one may be associ(cid:173)
`ated with a particular video frame) are previously digi(cid:173)
`tally stored on Winchestertype magnetic discs 26 in a
`8-bit PCM encoded format at a sampling rate of 16 KHz
`for example. A disc drive unit which can accommodate
`
`8
`
`

`
`4,789,895
`
`25
`
`5
`about 1 Gbyte will accommodate about 1000 minutes of
`audio at 124 kb/s.
`It is also possible to store time-compressed audio in
`analog format to be transmitted as an analog signal.
`In order to store as much audio as possible on each 5
`frame, the audio is compressed. One or more seconds of
`real time audio are sent on each l/30 second frame,
`depending upon the type of compression.
`Central processor 23 identifies those audio frames to
`accompany a selected video frame and unloads them 10
`from the correct addresses in a Winchester disc 26 into
`a buffer memory 28. After the transfer has been com(cid:173)
`pleted, the Central processor 23 calls for a correspond(cid:173)
`ing number of black frames via the switch to accommo(cid:173)
`date the audio data. These black frames include color l5
`bursts for clock recovery in the terminal. Each active
`line is at black level. In the NTSC system, black is rep(cid:173)
`resented by a low signal level. The data is inserted by
`the channel processor 25. The data bits are converted to
`pulses which are first conditioned by a Nyquist filter to 20
`reduce inter-symbol interference, as well as to prevent
`interference on the sound channel and the spreading of
`energy on adjacent channels before they are summed
`onto the black frame raster.
`A transmission bit rate of 8/5 of the color subcarrier
`frequency 3.579545 MHz (5.727272 Mb/sec) may be
`used. The data clock has a period of 174.6 nanoseconds.
`While this is the same bit rate as Teletext, which is
`approximately the fastest bit rate which can be carried 30
`by most CATV systems, the preferred formatting of the
`data and the method of clocking the incoming data are
`much more efficient and significantly different.
`Turning briefly to FIG. 4, on each line on which
`there is data, a flag bit in the form of a "i" bit is placed, 35
`for example,10.5 microseconds (60 data clock pulses)
`after the leadmg edge of the horizontal synchronization
`pulse. It is then followed by 288 bits (i.e., 36 bytes) of
`data. For the audio data, this results in total of 142272
`bits/frame.
`Referring to FIG. 3, in both audio and video frames
`four lines within the vertical blanking interval are
`served for addressing and mode (video or audio) and
`error codes, and future use. In audio frames, line 16 to
`262 on field one and line 279 to line 525 on field two are 45
`used for the audio data.
`·
`Referring again to FIG. 2, the black frame with the
`audio data is switched for the period of one frame
`(approx. l/30 second) into the video input of a CATV
`modulator 26. The video frames are also passed on to so
`the video input of the CATV channel modulator 26.
`There is always a continuous stream of still frames with
`black burst frames being switched in when no informa(cid:173)
`tion frames are delivered. Each channel modulator is
`selected for transmission through a single coaxial distri- 55
`bution system and a single coaxial cable system 13. With
`a set of projected traffic patterns, there might typically
`be over 100 active users sharing one channel. Video and
`audio frames of a program are time multiplexed with
`others on a CATV video channel and sent to remote 60
`terminals. Continuous audio is sent on an audio channel.
`Central Facility 11 employs the BTSC multichannel
`television sound format to place continuous audio (e.g.,
`background music) in up to three separate audio chan(cid:173)
`nels 30 in the NTSC composite signal. A SAP (second 65
`audio program) channel is used. Three possible chan(cid:173)
`nels are the monaural (left plus right), the stereo differ(cid:173)
`ence channel, and the SAP channels. It is also possible
`
`40
`
`6
`to make available many more distinctly different back(cid:173)
`ground music channels to the user.
`Both the continuous audio and the framed specific
`audio described above may occur simultaneously.
`When the video specific audio is played out in real time
`at the user terminal, the background component, if
`present, is attenuated automatically. The central proces(cid:173)
`sor 23 controls the selection of the background channel
`(or silence) at the user's terminal either through control
`signals inserted by the channel processor 25 into line 12
`of the vertical blanking interval or by control signals
`sent over. the telephone loop 14 by Concentration Sub(cid:173)
`system 11. The user may also have the capability of
`muting it. The continuous audio sources are connected
`into the audio input of the same modulator 26.
`The audio inputs of the channel modulators 26 can be
`driven either from individual SAP channel encoders 31
`or can all be driven from a single encoder using a suit(cid:173)
`able distribution unit 32.
`The video and audio frames, as well as the continuous
`audio, are distributed through the CATV network 13 to
`the terminals.
`.
`Turning now to FIG. 5, a terminal 12 consists of a
`set-top module 33 and a remote keypad 34.
`A connector panel (not shown) on the set-top module
`provides various physical connectors, including:
`Cable in 13 75 ohm coaxial F-connector
`TV out-75 ohm coaxial F-connector
`Telephone line-RJ-11 telephone jack
`Auxiliary phone-RJ-11 telephone jack.
`Through these connectors, the terminal12 is coupled
`to a CATV channel 13, telephone loop 14 and a televi(cid:173)
`sion receiver or monitor 35.
`Set-top module 33 contains hardware and software
`for data communications to and from the Central Facil(cid:173)
`ity 11 and for receiving and processing video frames
`and audio frames sent from the Central Facility and
`delivering them to the television receiver 35. The set(cid:173)
`top module also contains a power supply, and a control(cid:173)
`lable modem 36 as the interface to the telephone loop
`14.
`CATV demodulator 38 receives a selected NTSC
`channel signal over the CATV network. This compos(cid:173)
`ite signal contains video frames, digitally encoded audio
`frames, and BTSC encoded audio channels. CATV
`demodulator 38 demodulates the incoming signal to
`baseband and splits the audio channels from the video
`and audio frames.
`The audio channels extracted from the audio output
`of demodulator 38 are separated by a low-pass filter 39
`and a SAP channel decoder 40. They are connected
`into an analog switch 41, which is controlled by a con(cid:173)
`trol signal from the Central Facility 11 sent on the verti(cid:173)
`cal blanking interval, or alternatively through the tele(cid:173)
`phone loop 14 to select baseband audio or SAP audio.
`Preferably, however, a selection code stripped from a
`frame controls switch 41 to select the desired continu(cid:173)
`ous audio signal. A locally generated "mute" command
`overrides the control signal. The selected channel is
`passed through a switched attenuator 42 (e.g., 12 dB)
`which is switched in automatically by a signal gener(cid:173)
`ated by audio control 43 when audio is being played
`from the alternate audio-frame source (i.e., audio specif(cid:173)
`ically to accompany the display). This is so the back(cid:173)
`ground audio, e.g., music, does not obscure the video
`specific audio channel, e.g., voice. The output of the
`attenuator is then connected to a summer 44 which adds
`the analog signal derived from the audio frame, which is
`
`9
`
`

`
`4,789,895
`
`7
`8
`The leading edges of the horizontal synchronization
`then connected into the audio input of a channel of
`modulator 45 which couples it to television set 35 for
`pulses slope too much to accurately synchronize the bit
`train. Accordingly, a bit synchronization circuit is used.
`playing.
`Each video and encoded audio frame received from
`It will be recalled that the first bit on each data line is a
`the Central Facility 11 had been tagged with a three-
`"1," placed 10.5 microseconds (60 data clock pulses
`after the leading edge of the horizontal sync pulse. This
`byte terminal address and a two-bit mode code and a
`is called a flag bit. The 8 times color subcarrier source
`background sound control code in line 12 of the Verti-
`cal.Blanking Interval (VBI). A VBI correlation circuits
`is connected to clock generator and data extraction 59
`which divides by 5 to derive 174.6 nanosecond data
`46, 47,48 uses the address to decide whether the current
`frame belongs to the terminal, and the command code 10 clock pulses. The leading edges of the horizontal syn-
`to determine the handling of the frame. The mode code
`chronization pulses are used to start a count using the
`can have one or four meanings:
`data clock 59. Turning now to FIG. 4, after a count of
`Video frame;
`58 data clock pulses (10.127 microseconds) a gate or
`Video frame; stop preceding audio
`time window is opened for the 8 times color subcarrier
`15 clock (28.64 MHz) for a period of 5 data clock pulses
`Audio playout; initial frame
`(873 ns). These 5 data clock pulses are "and"ed with the
`Audio playout; continuation frame.
`The vertical and horizontal drives are used to identify
`data bit stream so that when the leading "1" (e.g., flag
`all frame lines by number. Line 12 is read out and the
`bit) is present, its position with respect to the five data
`clock pulses is identified and is used to correctly syn(cid:173)
`address and mode (video or audio) bits are connected 20 chronize the data clock to the rest of the data on the
`over to the VBI processor 46, a mode/tag register 48
`line.
`and error detector 47. The address is correlated with
`Returning now to FIG. 5• a flash AID converter
`the user address which is resident in an EPROM 49, and
`60A, part of video coder 60, clocked at about 14.32
`then the mode bits are checked to determine the type of million 8-bit samples per second, continually provides
`frame, so that appropriate action can be taken. Each 25 data into input register 61, but it is not transferred into
`frame transmitted from the Central Facility is tagged
`memory bank 62 (3.82 Mb) until a strobe is received. If
`with numbers that repeat in 64 blocks. These can be
`the mode register 48 recognizes the incoming frame as
`interrogated by the Central Facility to check the deliv-
`being addressed to the user terminal and being for video
`ery of any specific still frame recently transmitted.
`display, a strobe is sent to the video frame 'store. The
`If the address in line 12 correlates with the terminal 30 memory bank 62 is then loaded from register 61 for the
`period of exactly one frame (approx. 1/30 second.) It is
`ID, then depending upon the mode (video or audio), the
`frame is inserted into the appropriate store. In the case
`then played out as still frame through D/ A converter
`of audio, they are stored in bursts (line-by-line) of
`60B repetitively using the 4 times color subcarrier clock
`~.7272Mb/sand clocke~ into audio RAM 50, line-by-
`(14.32 MHz). Since there is a color subcarrier phase
`lme through th~ appropnate set of frames.
`.
`. . 35 jump between every other frame, this is corrected in the
`After the audto data segment has been rec:tved, tt ts
`frame jump correction block 63.
`played out from the RAM 50 at the appropnate. spe.ed
`Before being connected into the video input of the
`(e.g., 128 kb/s) through D/ A converter 51 whtch m-
`modulator 45, the video signal is passed through a char-
`eludes a 7.5 KHz low pass filter. The analog output
`acter generator 64. This is used to insert characters as
`from t~e D/ A con":erter is pas~ed. to the sound summer 40 they are entered from the user keypad 34. They are
`44 whtch connects mto the audto mput of the channel 3
`typically inserted close to the bottom of the screen
`modulat?r 45. It is sum~ed. with the selecte~ continu-
`display.
`ous audto channel whtch ts attenuat~d unttl the se-
`The system provides interactive data retrieval and
`quence is completed.
`transaction. The infra-red keypad 34 provided with the
`The frames from demodulator 38 are connected, via a 45 system is equipped with an adequate number of keys
`de restorer 52, to a color subcarrier (3.58 MHz) band-
`(e.g., 53) to interact with the system, including alpha-
`pass filter 53. Its output is passed on to a color-subcar-
`numeric and various special function keys to provide
`rier regenerator chip 54. For this to operate, it is neces-
`simplified, dedicated operation of on/off, page forward,
`sary for it to be provided with a burst flag gate. To
`page back, pause, purchase and other functions.
`achieve this, the synchronization pulses are stripped off so
`The keypad 46 transmits PCM-encoded IR com-
`the incoming video frames by sync stripper 55 and con-
`mands to the set-top module via an infra-red receiver
`nected into a synchronization pulse generator 56. The
`65. The keypad is the mechanism for the user to control
`outputs from generator 56 include horizontal drive,
`the set-top module and to communicate with the Cen-
`vertical drive, and color subcarrier burst flag. All clock-
`tral Facility 11 via the telephone loop 14. The keys are
`ing frequencies for synchronization are derived from 55 used to control the terminal, Central Facility functions,
`the color subcarrier bursts always present on the incom-
`as well as to input transaction specific data by the users.
`ing still frames. For sync generator 56 to operate, it
`IR receiver 65 includes a decoder to convert the
`needs an input of four times the color subcarrier fre-
`special encoded IR pulses into a convenient form for
`quency (14.32 MHz). This is provided by a phase-
`inserting onto the 8085 CPU bus 66.
`locked loop 57. The output of the regenerated color 60 Data communications with the Central Facility 11
`subcarrier frequency is connected into the phase-locked
`may be provided by modem 36 that transfers asynchro-
`loop which locks in a 8 time color subcarrier oscillator
`nous ASCII data at a rate of 300 or 1200 bps via a tete-
`from which the 4 times color subcarrier is derived.
`phone switching office. Modem 36 has autodial capabil-
`The de restored frames from DC restorer 52 are con-
`ity for automatic placement of calls to the Central Facil-
`nected to a buffer which generates TTL logic levels 65 ity. LED indicator 74 is a data carrier detector.
`from the bits on the raster. These are connected into the
`The terminal is controlled by microprocessor 67 cou-
`clock generator and data extraction circuit 59. This data
`pled to system bus 66. A 8085 CPU may be used.
`extraction makes efficien