United States Patent [19]
`Mustafa et al.
`
`[111
`
`[45]
`
`Patent Number:
`
`Date of Patent:
`
`4,789,895
`
`Dec. 6, 1988
`
`[54] SYSTEM FOR SYNCHRONIZING DIGITAL
`BIT STREAM FOR TELECOMMUNTCATION
`SYSTEM
`
`‘
`
`[75]
`
`Inventors: Mel1metMustafa,Wa1tham; 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
`
`Int. Cl.‘ ............................................... H04N 7/04
`[51]
`[52] U.S. Cl. .................................. .. 358/147; 358/146;
`358/148
`358/148, 141, 142, 146,
`[58] Field of Search .........
`358/147, 143, 145, 86, 83; 375/95, 111, 119
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`.........
`
`
`
`..... 358/93
`7/1973 Bitzer et al.
`3,743,767
`. 358/85
`3,746,780 7/1973 Stetten et al.
`.. 358/142
`3,891,792
`6/1975 Kimura ........
`358/141
`4,104,681
`8/1978 Saylor et a1.
`4,129,743 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 a1.
`.
`4,616,263 10/1986 Eichelberger.
`4,712,131 12/1987 Tanabe ................................ 358/147
`
`OTHER PUBLICATIONS
`
`NHK (Japan Broadcasting Corporation) “Present Sta-
`tus of Still—Picture Television”; May 1978.
`Berg “Dialoog TV; Kabelexperiment Zuid—Lirnburg
`I2-Elektrotechniek/E1ektronica”—No.
`4-1986,
`pp.
`35-39.
`Kimura, A., “Telescan Simultaneous Information Dis-
`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
`[57]
`ABSTRACT
`
`A telecommunication system for synchronizing a'digita1
`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-
`cuit at said terminal provides a time window delayed by
`a second constant number of clock pulses from the start
`of the 1ine’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-
`mine which of said plurality of second data clock pulses
`coincide in time with said flag bit, thereby synchroniz-
`ing said data stream with said second data clock.
`
`2 Claims, 5 Drawing Sheets
`
`|
`
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`US. Patent
`
`Dec. 6, 1988
`
`Sheet 3 of 5
`
`4,789,895
`
`LINES I-15; 263-278
`
`ACTIVE LINES 16-262; 279-525
`
`LINES FDR FIELD SYNC
`ADDRESS, MODE CODE, ERROR CHECK
`
`
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`1
`
`4,789,895
`
`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
`sending still pictures with accompanying program
`sound. Frames for a number of programs are time multi-
`plexed. Each video frame has a code identifying the
`program to which it belongs. The sound signals of all
`the programs are digitally encoded and time division-
`ally multiplexed on lines within an audio frame. Both
`the video and audio frames are in the standard NTSC
`format. The video frames are sent serially with each
`video frame followed by two audio frames. Each video
`frame carries analog video information corresponding
`to one program, while each audio frame carries digi-
`tized audio information corresponding to all programs.
`A receiver seizes and records video frames of a se-
`lected program for repetitive display. The analog video
`may be digitized and stored in a solid state memory
`while apparently the audio is converted to analog and
`played as received. The NHK system is appropriate for
`broadcasting a plurality of predetermined programs,
`but it is not optimal for interactive broadband services
`where many users can initiate and interact with pro-
`grams or services at different times.
`
`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-
`tal sync pulse and a color burst. A first data clock at the
`central facility provides first data clock pulses synchro-
`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-
`chronized with said color burst. A circuit at said termi-
`nal provides a time window delayed by a second con-
`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.
`
`l0
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`SYSTEM FOR SYNCHRONIZING DIGITAL BIT
`STREAM FOR TELECOMMUNICATION SYSTEM
`
`RELATED APPLICATIONS
`
`The following applications filed simultaneously with
`this application pertain to different features of the tele-
`communication systems.
`Ser. No. 044,393, filed Apr. 30, 1987 Telecommunica-
`tion System With Video and Audio Frames is con-
`cerned with addressed still frame video and time-com-
`pressed audio frames.
`Ser. No. 044,388 filed Apr. 30, 1987 Telecommunica-
`tion System With Burst and Continuous Audio Signals
`discusses attenuating a continuous background channel
`when burst audio is played.
`Ser. No. 044,394 filed Apr. 30, 1987 Telecommunica-
`tion System With Selectable Audio Channels describes
`a system having two or more continuous audio signals
`sent on a first transmission medium and selected in re-
`sponse to a signal sent on a second transmission me-
`dium.
`
`Ser. No. 044,395 filed Apr. 30, 1987 Telecommunica-
`tion System With Frame Selected Continuous Audio
`Signals describes a system having framed audio and two
`or more continuous audio channels which are selected
`in response to a control signal.
`
`INCORPORATION BY REFERENCE
`
`U.S. Pat. No. 3,746,780 is hereby incorporated by
`reference.
`.
`
`BACKGROUND OF THE INVENTION
`
`This invention pertains to telecommunications and,
`more particularly, is concerned with interactive tele-
`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,
`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-
`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
`dual media systems are the subjects of U.S. Pat. Nos.
`3,746,780 and 4,616,263, and described in I2——E1ek-
`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
`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
`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
`conventional television receiver.
`Examples of video information include menus, forms,
`data, text, and still pictures. It is desirable but lacking in
`
`50
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`55
`
`FIG. 1_is a schematic diagram of a telecommunica-
`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-
`nal;
`FIG. 4 illustrates waveforms on a bit synchronization
`system; and
`FIG. 5 sheets 1 and 2 are block diagrams of a Termi-
`nal used in the system of FIG. 1.
`
`DESCRIPTION OF INVENTION
`
`65
`
`A telecommunication system 10 embodying the in-
`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
`
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`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-
`lowing duplex transmission of control signals not re-
`quiring a large bandwidth. The local switched tele-
`phone system is a satisfactory second transmission me-
`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-
`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-
`tures, or other images sent as a series of addressed video
`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-
`uous and specific, both of which may be played simulta-
`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
`because it is specific to the displayed still frame. Typical
`specific audio is voiced narrative or instructions. Spe-
`cific audio is intended for only certain selected termi-
`nals at a time and is sent in time-compressed bursts by
`addressed frames having the same address as the accom-
`panying video frame. Only one channel of time-com-
`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-
`tems 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-
`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
`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.
`Schemes for sending auxiliary information during one
`or more lines of VBI have been developed such as de-
`scribed in U.S. Pat. No. 3,493,674 and in North Ameri-
`can Broadcast Teletext, Specification-Engineering and
`Development Department, CBS Television, Newark,
`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
`that a correspondingly addressed frame is to be ac-
`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
`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-
`tems: a Concentrator Subsystem 15, a Server Subsystem
`16, a Video Subsystem 17, and Control and Mainte-
`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. However,
`in smaller configura-
`tions, one or more of these subsystems may be imple-
`mented on a single computer while still maintaining the
`software interfaces that allows simple expansion to mul-
`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-
`ings of the overall system. The Control and Mainte-
`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-
`mission medium 14 which may be a dial-up connection
`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-
`ler of a session using input from the Terminal 12, via the
`Concentrator Subsystem 15, to access the appropriate
`databases and to send instructions back to the Concen-
`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 the 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 dc 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-
`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-
`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-
`ated with a particular video frame) are previously digi-
`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
`
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`about 1 Gbyte will accommodate about 1000 minutes of
`audio at 124 kb/s.
`
`4,789,895
`
`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
`frame, the audio is compressed. One or more seconds of
`real time audio are sent on each 1/30 second frame,
`depending upon the type of compression.
`Central processor 23 identifies those audio frames to
`accompany a selected video frame and unloads them
`from the correct addresses in a Winchester disc 26 into
`a buffer memory 28. After the transfer has been com-
`pleted, the Central processor 23 calls for a correspond- '
`ing number of black frames via the switch to accommo-
`date the audio data. These black frames include color
`bursts for clock recovery in the terminal. Each active
`line is at black level. In the NTSC system, black is rep-
`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
`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
`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,
`for example,’ 10.5 microseconds (60 data clock pulses)
`after the leading 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.
`
`5
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`
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`
`6
`to make available many more distinctly different back-
`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-
`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-
`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-
`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—R.T-11 telephone jack
`Auxiliary phone—RJ-11 telephone jack.
`Through these connectors, the terminal 12 is coupled
`to a CATV channel 13, telephone loop 14 and a televi-
`sion receiver or monitor 35.
`Set-top module 33 contains hardware and software
`for data communications to and from the Central Facil-
`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-
`top module also contains a power supply, and a control-
`lable modem 36 as the interface to the telephone loop
`14.
`
`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
`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. 1/30 second) into the video input of a CATV
`modulator 26. The video frames are also passed on to
`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-
`tion frames are delivered. Each channel modulator is
`selected for transmission through a single coaxial distri-
`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 charmel. Video and
`audio frames of a program are time multiplexed with
`others on a CATV video channel and sent to remote
`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-
`nels 30 in the NTSC composite signal. A SAP (second
`audio program) channel is used. Three possible chan-
`nels are the monaural (left plus right), the stereo differ-
`ence channel, and the SAP channels. It is also possible
`
`45
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`CATV demodulator 38 receives a selected NTSC
`channel signal over the CATV network. This compos-
`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-
`trol signal from the Central Facility 11 sent on the verti-
`cal blanking interval, or alternatively through the tele-
`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-
`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-
`ated by audio control 43 when audio is being played
`from the alternate audio-frame source (i.e., audio specif-
`ically to accompany the display). This is so the back-
`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
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`4,789,895
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`5
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`30
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`35
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`7
`then connected into the audio input of a channel of
`modulator 45 which couples it to television set 35 for
`playing.
`Each video and encoded audio frame received from
`the Central Facility 11 had been tagged with a three-
`byte terminal address and a two-bit mode code and a
`background sound control code in line 12 of the Verti-
`cal Blanking Interval (VBI). A VBI correlation circuits
`46, 47, 48 uses the address to decide whether the current
`frame belongs to the terminal, and the command code
`to determine the handling of the frame. The mode code
`can have one or four meanings:
`Video frame;
`Video frame; stop preceding audio
`Audio playout; initial frame
`Audio playout; continuation frame.
`The vertical and horizontal drives are used to identify
`all frame lines by number. Line 12 is read out and the
`address and mode (video or audio) bits are connected
`over to the VBI processor 46, a mode/tag register 48
`and error detector 47. The address is correlated with
`the user address which is resident in an EPROM 49, and
`then the mode bits are checked to determine the type of
`frame, so that appropriate action can be taken. Each
`frame transmitted from the Central Facility is tagged
`with numbers that repeat in 64 blocks. These can be
`interrogated by the Central Facility to check the deliv-
`ery of any specific still frame recently transmitted.
`If the address in line 12 correlates with the terminal
`ID, then depending upon the mode (video or audio), the
`frame is inserted into the appropriate store. In the case
`of audio,
`they are stored in bursts (line-by-line) of
`5.7272 Mb/s and clocked into audio RAM 50, line-by-
`line through the appropriate set of frames.
`After the audio data segment has been received, it is
`played out from the RAM 50 at the appropriate speed
`(e.g., 128 kb/s) through D/A converter 51 which in-
`cludes a 7.5 KHz low pass filter. The analog output
`from the D/A converter is passed to the sound summer 40
`44 which connects into the audio input of the channel 3
`modulator 45. It is summed with the selected continu-
`ous audio channel which is attenuated until the se-
`quence is completed.
`'
`The frames from demodulator 38 are connected, via a 45
`do restorer 52, to a color subcarrier (3.58 MHz) band-
`pass filter 53. Its output is passed on to a color-subcar-
`rier regenerator chip 54. For this to operate, it is neces-
`sary for it to be provided with a burst flag gate. To
`achieve this, the synchronization pulses are stripped off
`the incoming video frames by sync stripper 55 and con-
`nected into a synchronization pulse generator 56. The
`outputs from generator 56 include horizontal drive,
`vertical drive, and color subcarrier burst flag. All clock-
`ing frequencies for synchronization are derived from
`the color subcarrier bursts always present on the incom-
`ing still frames. For sync generator 56 to operate, it
`needs an input of four times the color subcarrier fre-
`quency (l4.32 MHz). This is provided by a phase-
`locked loop 57. The output of the regenerated color
`subcarrier frequency is connected into the phase-locked
`loop which locks in a 8 time color subcarrier oscillator
`from which the 4 times color subcarrier is derived.
`The dc restored frames from DC restorer 52 are con-
`nected to a buffer which generates TTL logic levels
`from the bits on the raster. These are connected into the
`clock generator and data extraction circuit 59. This data
`extraction makes efficient use of the raster frames.
`
`8
`The leading edges of the horizontal synchronization
`pulses slope too much to accurately synchronize the bit
`train. Accordingly, a bit synchronization circuit is used.
`It will be recalled that the first bit on each data line is a
`“I,” placed 10.5 microseconds (60 data clock pulses
`after the leading edge of the horizontal sync pulse. This
`is called a flag bit. The 8 times color subcarrier source
`is connected to clock generator and data extraction 59
`which divides by 5 to derive 174.6 nanosecond data
`clock pulses. The leading edges of the horizontal syn-
`chronization pulses are used to start a count using the
`data clock 59. Turning now to FIG. 4, after a count of
`58 data clock pulses (l0.l27 microseconds) a gate or
`time window is opened for the 8 times color subcarrier
`clock (28.64 MHz) for a period of 5 data clock pulses
`(873 ns). These 5 data clock pulses are “and”ed with the
`data bit stream so that when the leading “1” (e.g., flag
`bit) is present, its position with respect to the five data
`clock pulses is identified and is used to correctly syn-
`chronize the data clock to the rest of the data on the
`line.
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`Returning now to FIG. 5, a flash A/D converter
`60A, part of video coder 60, clocked at about 14.32
`million 8-bit samples per second, continually provides
`5 data into input register 61, but it is not transferred into
`memory bank 62 (3.82 Mb) until a strobe is received. If
`the mode register 48 recognizes the incoming frame as
`being addressed to the user terminal and being for video
`display, a strobe is sent to the video frame ‘store. The
`memory bank 62 is then loaded from register 61 for the
`period of exactly one frame (approx. 1/30 second.) It is
`then played out as still frame through D/A converter
`60B repetitively using the 4 times color subcarrier clock
`(14.32 MHz). Since there is a color subcarrier phase
`jump between every other frame, this is corrected in the
`frame jump correction block 63.
`Before being connected into the video input of the
`modulator 45, the video signal is passed through a char-
`acter generator 64. This is used to insert characters as
`they are entered from the user keypad 34. They are
`typically inserted close to the bottom of the screen
`display.
`The system provides interactive data retrieval and
`transaction. The infra-red keypad 34 provided with the
`system is equipped with an adequate number of keys
`(e.g., 53) to interact with the system, including alpha-
`numeric and various special function keys to provide
`simplified, dedicated operation of on/off, page forward,
`page back, pause, purchase and other functions.
`The keypad 46 transmits PCM-encoded IR com-
`mands to the set-top module via an infra-red receiver
`65. The keypad is the mechanism for the user to control
`the set-top module and to communicate with the Cen-
`tral Facility 11 via the telephone loop 14. The keys are
`used to control the terminal, Central Facility functions,
`as well as to input transaction specific data by the users.
`IR receiver 65 includes a decoder to convert the
`special encoded IR pulses into a convenient form for
`inserting onto the 8085 CPU bus 66.
`Data communications with the Central Facility 11
`may be provided by modem 36 that transfers asynchro-
`nous ASCII data at a rate of 300 or 1200 bps via a tele-
`phone switching office. Modem 36 has autodial capabil-
`ity for automatic placement of calls to the Central Facil-
`ity. LED indicator 74 is a data carrier detector.
`The terminal is controlled by microprocessor 67 cou-
`pled to system bus 66. A 8085 CPU may be used.
`MUART 36 is connected to system bus 66.
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`Decoder interface 68 is used as a buffer between the
`bus 66 and the circuits associated with the line 12 data.
`Address decoder 69 is adapted to the 3085 bus 6