United States Patent £191
`Bitzer et al.
`
`[II]
`
`[451
`
`3,743,767
`July 3, 1973
`
`[54] TRANSMITTER AND RECEIVER FOR THE
`TRANSMISSION OF DIGITAL DATA OVER
`STANDARD TELEVISION CHANNELS
`Inventors: Donald L. Bitzer, Urbana; Michael
`Johnson, Paxton; Jack Stifle,
`Champaign, all of Ill.
`
`[75]
`
`3,046,331
`3,649,750
`
`7/1962 Gebel ................................... 178/6.8
`3/1972 Gibson ......................... 178/DIG. 23
`
`Primary Examiner-Richard Murray
`Attorney-Charles J. Merriam et al.
`Attorney-Charles J. Merriam, Nate F. Scarpelli
`et al.
`
`[73] Assignee: University oflllinois Foundation,
`Urbana, Ill.
`Oct. 4, 1971
`[22] Filed:
`[ 21 J Appl. No.: 186,020
`
`[52] U.S. Cl. ............ 178/5.6, 178/6.8, 178/DIG. 23,
`178/DIG. 13
`[ 5 I J Int. Cl .............................................. H04n 7/00
`[58] Field of Search ............... 178/5.6, 6.8, DIG. 13,
`178/DIG. 23
`
`[56]
`
`3,493,674
`
`References Cited
`UNITED STATES PATENTS
`2/1970 Houghton .................... 178/DIG.23
`
`[57]
`ABSTRACT
`Apparatus and a method for the economical distribu(cid:173)
`tion of digital data to a number of data terminals using
`standard commercial television channels including a
`digital transmitter for transmitting digital data over a
`video cable and a receiver for receiving the transmitted
`digital data and selectively distributing the· recovered
`data to the desired data terminals. A method for send(cid:173)
`ing digital data bit by bit over television channels in a
`field by field manner to a plurality of terminals in a
`manner such as to greatly simplify the detection of any
`errors and offering an advantage of decreasing the
`probability of errors as more data terminals are added
`to the system.
`
`8 Claims, 11 Drawing Figures
`
`20
`
`CLOCK
`CONTROL
`
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`SIGNAL
`COMPOSER
`
`j-30
`
`25 .__..
`
`26
`
`24
`
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`COUNTER
`
`34)
`
`DIGITAL
`DATA
`
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`
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`CONTRfJL
`
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`
`(38
`
`44
`
`46-
`
`POS.
`~VIDEO
`
`TO
`MODULATORS
`
`APPLE EX. 1009
`Page 1
`
`

`
`PATENTED Jut. 3 197!
`
`3. 743,767
`
`SHEEt 1 OF 6
`
`12
`
`DIGITAL
`X'TER
`
`FIG. f.
`
`TO OTHER SITES
`
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`
`18
`
`18
`
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`
`FIG. 4:
`
`CLOCK
`CONTROL
`
`DIGITAL
`DATA
`
`DIVIDE by 525 1---3_ 4.....____.
`COUNTER
`
`POS.
`VIDEO
`
`TO
`MODULATORS
`
`INVENTOR
`Donald L. Bitzer
`Michael Johnson
`Jack Stifle
`BY
`!llerMII!J !J/t;~)et~ .5./P/;/o z tf'k.1e
`ATTYS.
`
`APPLE EX. 1009
`Page 2
`
`

`
`FIG. 2
`
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`
`APPLE EX. 1009
`Page 3
`
`

`
`PATENTED JOL 3 1973
`
`3. 743,767
`
`SHEET 3 If 6
`
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`
`INVENTORS
`Donald L. Bitzer
`Michael Johnson
`Jack Stifle
`BY
`1//tmttm) 1!4r:idl/, .5M;x-/tJ .r ,f/cfe
`ATTYS.
`
`APPLE EX. 1009
`Page 4
`
`

`
`RF
`INPUT
`50
`r----ll...-_.,
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`RECEIVER
`
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`
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`
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`
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`
`APPLE EX. 1009
`Page 5
`
`

`
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`
`APPLE EX. 1009
`Page 6
`
`

`
`PATENTEOJut 3197&
`
`3. 743.767
`
`SHEET 6 Of 6
`
`TV UNES
`
`I 12
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`
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`
`FROM TIME COUNTER 66
`
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`
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`
`FROM LINE
`COUNTER 64
`DATA
`INTERVAL
`
`FIG. 8
`
`69
`
`HORIZ.
`PULSE
`FROM
`SYNC.
`DET. 62
`
`FIG. 9
`
`HORIZ.
`SIGNAL
`
`TO LINE
`COUNTER
`64
`
`CWCK
`
`INVENTORS
`Donald L. Bitzer
`BY
`Michael Johnson
`Jack Stifle
`life~; 1Jl&,.f14{_; _jjQjJiit' ~ !(/t4{;
`ATTYS.
`
`APPLE EX. 1009
`Page 7
`
`

`
`1
`TRANSMITTER AND RECEIVER FOR THE
`TRANSMISSION OF DIGITAL DATA OVER
`STANDARD TELEVISION CHANNELS
`
`3,743,767
`
`2
`
`15
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a schematic block diagram illustrating a
`transmitter and receiver system for distributing digital
`5 data via standard television channels from a data center
`to a number of data terminals;
`FIG. 2 is a representation of the FCC standard syn(cid:173)
`chronization signals required for commercial televi(cid:173)
`sion;
`FIG. 2A is an expanded view of a portion of the hori(cid:173)
`zontal blanking and synchronization interval shown in
`FIG. 2;
`FIG. 2B is an expanded representation of a portion
`of the vertical synchronization and blanking interval
`shown in FIG. 2;
`FIG. 3 is a representation of the composite signal in
`accordance with the present invention including digital
`data and containing the . required synchronizing and
`blanking signals for commercial television channels;
`FIG. 4 is a block diagram schematically illustrating a
`transmitter in accordance with the present invention
`for transmitting digital data over standard television
`channels;
`FIG. 5 is a representation of the pulses present at the
`output of the pulser unit shown in FIG. 4;
`FIG. 6 is a schematic block diagram illustrating a re(cid:173)
`ceiver in accordance with the present invention for re(cid:173)
`covering the digital data from standard television chan-
`nels and generating data addresses for sending the re(cid:173)
`spective digital data to the required data terminal;
`FIG. 7 is a timing diagram controlling the receiver
`synchronization with respect to the incoming compos(cid:173)
`ite television signal containing the digital data;
`FIG. 8 is an illustration of the format for transmitting
`digital data within a television field in accordance with
`another aspect of the present invention; and
`FIG. 9 illustrates the phase lock and oscillator com(cid:173)
`ponents of the receiver-distributor 16.
`
`This invention relates to digital data communica(cid:173)
`tions, and more particularly to apparatus and a method
`for transmitting and receiving digital data such as sup(cid:173)
`plied by a computer using standard commercial televi(cid:173)
`sion channels.
`It has become extremely desirable to be able to send 10
`and distribute digital data from a single source, such as
`a computer to a number of receivers or users at com(cid:173)
`puter terminals. As an example, in the computer(cid:173)
`assisted instruction system developed at the University
`of Illinois (commonly known as the PLATO system) up
`to 4,000 remote computer terminals, each requiring a
`nominal 1200 bits per second bps channel are to be
`connected to a centrally located computer. Reference
`may be made to Donald L. Bitzer U.S. Pat. No. 20
`3,405,457, assigned to the same assignee herein de(cid:173)
`scribing one embodiment of the PLATO system. In
`such a system, voice grade telephone lines could serve
`as the 1200 bps communication channel. However, in
`systems involving more than 1000 terminals, it be- 25
`comes especially important to obtain economical distri(cid:173)
`bution of the digital data to the terminals. The intra(cid:173)
`state tarriffs for leasing such voice grade lines range
`from about 50 per mile per month per line for a ser(cid:173)
`vice involving 240 channels to about $4.50 per mile per 30
`month for a single line.
`On the other hand, the tariffs for an intra-city educa(cid:173)
`tional television ( ETV) channel range from approxi(cid:173)
`mately $30 per mile per month downward with the
`number of channels leased. Such a channel would dis- 35
`tribute digital data to computer terminals in class
`rooms in a manner not unlike the distribution of com(cid:173)
`mercial television programs, via CATV systems to pri(cid:173)
`vate homes. In such a system one ETV channel could 40
`provide 1200 bps service to more than l 000 terminals
`resulting in a per terminal charge for the channel of less
`than 5.5
`per mile per month.
`
`SUMMARY OF THE INVENTION
`In accordance with the principles of the present in(cid:173)
`vention there is provided apparatus and a method for
`the economical distribution of digital data to a number
`of data terminals using standard commercial television
`channels. The data is transmitted in a synchronous 50
`time-division multiplex mode which is compatible with
`standard television practice, therefore providing low
`cost input and distributions equipment. In addition, the
`particular format of the digital data within a television
`field as described hereinafter, greatly simplifies the de- 55
`tection of any errors and the distinct advantage of de(cid:173)
`creasing the possibility of error as more data terminals
`are added to the system.
`Thus, the present invention provides the following
`advantages:
`1. Compatible transmission and reception with televi-
`sion standards;
`2. May be used in standard CATV systems;
`3. Ease of error detection;
`4. Decreasing error possibilities with addition of
`more terminals; and
`S. Low cost inp!lt and data distribution.
`
`60
`
`65
`
`DETAILED DESCRIPTION OF A PREFERRED
`EMBODIMENT
`Referring now to FIGS. 1-8, there is illustrated the
`apparatus and operation thereof of one embodiment of
`the invention. In FIG. 1, the overall system 10 of the in-
`45 vention is shown, including a digital transmitter 12 for
`transmitting digital data over a video cable 14 in a
`mode compatible with standard commercial television
`practice, and a receiver 16 for receiving the transmit-
`ted digital data and selectively distributing the recov(cid:173)
`ered data to the desired data terminals 18.
`The data terminals 18 comprise, for instance, student
`terminals each having a display device such as a cath(cid:173)
`ode ray tube or a plasma panel as disclosed in the D.
`L. Bitzer et al. U.S. Pat. No. 3,559,190 assigned to the
`same assignee here. It is to be understood that the ter(cid:173)
`minals 18 also include keysets, each communicating
`through a keyset multiplexor and voice grade phone
`lines to a large scale, general purpose computer and the
`digital transmitter 12. This additional apparatus is men(cid:173)
`tioned here only for setting the environment within
`which the present invention is concerned, and thus has
`not been illustrated in FIG. 1, in order to avoid encum(cid:173)
`bering the drawings.
`Therefore, in an overall view of the drawings illus(cid:173)
`trating one embodiment of the invention:
`FIG. 4 illustrates the components of the digital trans(cid:173)
`mitter 12;
`
`APPLE EX. 1009
`Page 8
`
`

`
`3,743,767
`
`4
`bins of each line (0.02H + 0.08H + 0.06H) are used for
`horizontal synchronization and blanking purposes
`while each of the remaining 84 bins contains a bit of
`digital information.
`
`3
`FIG. 6 shows the components of the receiver-
`distributor 16;
`FIGS. 2, 3, S and 7 show the various control, timing
`and data signals for operating the apparatus;
`FIG. 8 shows a television field, wherein in another as- 5
`pect of this invention the format of the digital data has
`been entered in a novel manner for reducing data er(cid:173)
`rors as the number of terminals 18 is increased; and
`FIG. 9 illustrates the phase lock and oscillator com(cid:173)
`ponents of the receiver-distributor 16.
`
`DIGITAL TRANSMITTER
`The digital transmitter 12 generates the standard
`television synchronization and blanking signals of FIG.
`2 and combines these signals with the digital data into
`10 a composite signal compatible with FCC standards, one
`such line signal being shown in FIG. 3. The composite
`signal is then delivered to the common carrier supply(cid:173)
`ing the television channel for RF modulation and trans(cid:173)
`mission over standard cable television (CATV) equip-
`IS ment.
`A block diagram of the compatible television-digital
`transmitter apparatus 12 is shown in FIG. 4.
`A clock control circuit 20 contains a 1.575 MHz
`crystal controlled oscillator (clock) which drives a di-
`20 vide by SO counter. The outputs of the counter and the
`clock signal are sent to a pulser circuit 22 where they
`are used to generate the four pulses - V, H, E and B
`shown in FIG. 5.
`In a constructed embodiment of the invention, the
`25 divide by SO counter and pulser comprised conven(cid:173)
`tional logic circuits, many in the form of integrated cir(cid:173)
`cuits readily available in the industry. As an example,
`the following may be utilized, it being understood that
`other equivalent specific components and logic circuits
`may be readily employed by those skilled in the art in
`accordance with the teachings herein to practice the
`invention and yet fall within the scope of the invention
`(commercially available integrated circuit numbers are
`indicated where applicable):
`1. Decade counters (two required-one wired as divi(cid:173)
`de/5) -No. 7490.
`2. BCD to Decimal Decode (two) - No. 7442.
`3. Combinational Logic - Including two - No. 7420.
`4. Flip-flop circuits (one for each V, H, E, B pulse
`and Clear Shift).
`The V pulse is used to drive a divide by 52S counter
`24, comprising standard flip-flop and gate circuits, the
`outputs of which, including odd and even field designa(cid:173)
`tions, are interpreted as the horizontal line count within
`a frame. The line count is shown at the top of FIG. 2
`as previously indicated.
`the signal composer circuit 26 assembles the compos-
`ite synchronization and blanking signals using the four
`pulses supplied by the pulser circuit 22 as building
`blocks. The outputs of the divider by 525 counter 24
`on line 25, including signals representing the horizontal
`line count and odd and even field signal designations,
`are used by the signal composer 26 to supervise assem(cid:173)
`bly of the composite signals.
`In particular, one output line 28 couples the compos(cid:173)
`ite horizontal and vertical sync signal while another
`output line 30 couples the composite horizontal and
`vertical blanking and the horizontal blanking to the
`60 data control circuit 32. Output line 34 of the counter
`24 supplies the correct data interval for formation of
`the composite sync, blanking and data signal.
`A constructed signal composer 26 comprised a com(cid:173)
`binational logic circuit including integrated circuit Nos.
`7400, 7410 and 7420 for receiving the horizontal line
`count designations from line 25, and standard gate and
`flip-flop circuits for combining the output of the combi(cid:173)
`national logic circuit with the V, H, E and B pulses and
`
`65
`
`COMPOSITE TELEVISION SIGNAL
`The composite television signal (black and white) is
`assembled from three signals. These are:
`1. A composite synchronizing signal which is made
`up of two parts, a horizontal sync signal and a vertical
`sync signal;
`2. A composite blanking signal which is also com(cid:173)
`posed of two parts, a horizontal blanking signal and a
`vertical blanking signal; and
`3. A video signal which contains the picture informa(cid:173)
`tion. This signal is normally used by television receivers
`to intensity modulate the horizontal scanning Jines. In
`the system described here the video signal contains the
`digital data.
`
`SYNCHRONIZATION SIGNAL
`Details of the Federal Communications Commission
`(FCC) standard synchronization signal are shown in
`FIG. 2. All times are given relative to H, the time inter- 30
`val between horizontal sync pulses (H = 1/15750 sec=
`63.2 microsec.). The vertical blanking time shown is
`the minimum allowed by FCC standards. The dashed
`line circled horizontal blanking and sync interval in the
`upper portion of FIG. 2 is more clearly shown in the ex- 35
`panded view of FIG. 2A. Similarly, the dashed line cir(cid:173)
`cled vertical blanking and sync interval in FIG. 2 is
`more clearly shown in the expanded view of FIG. 2B.
`The equalization pulses and the serrating pulses in
`the vertical sync interval are of little value in the system 40
`described in this disclosure and therefore the reasons
`for their existence wiii not be discussed here. They
`must, however, still be generated by the digital trans(cid:173)
`mitter 12, in addition to the illustrated FCC sync sig(cid:173)
`nals, in order that standard commercial equipment may 45
`be used for transmission and reception.
`In the United States television system, there are 30
`frames transmitted per second, each frame containing
`525 lines. In FIG. 2 the lines zero to 23 are shown, con(cid:173)
`tinuing to lines 514-525 as shown at the left portion of 50
`FIG. 2. To reduce flicker in the picture, each frame is
`transmitted as two fields of 262-lh lines each at a rate
`of 60 fields per second, with the lines of one field inter(cid:173)
`laced between the lines of the other. The vertical
`blanking interval requires up to 21 lines (see FIG. 2)
`leaving a maximum of 241-lh lines to be used for video.
`
`55
`
`VIDEO (DATA) SIGNAL
`Because digital information is binary in nature, only
`two voltage levels are required to represent the infor(cid:173)
`mation. In the system described here the white level is
`chosen as logical "one" and the black level as logical
`"zero". An example of a line carrying digital data is
`shown in FIG. 3.
`Each horizontal scanning line in the television field
`which carries data is divided into I 00 time bins of
`0.01 H seconds each, as shown in FIG. 3. The first 16
`
`APPLE EX. 1009
`Page 9
`
`

`
`3,743,767
`
`6
`minal, and may not be needed where the terminal can
`directly utilize digital data. Thus, although the data
`modem 73 is not a part of the present invention, refer(cid:173)
`ence may be made to a copending application entitled
`"Data Modem", U.S. Ser. No. 160,429, assigned to the
`same assignee here, which describes a data modem.
`
`5
`the odd and even field designations to provide the com(cid:173)
`posite sync, composite blanking, vertical and horizon(cid:173)
`tal blanking, and Select H signals.
`The composite sync, blanking and data interval sig(cid:173)
`nals along with a 1.575 MHz (period=O.OlH) clock sig- 5
`nal on output line 36 are sent to the data control circuit
`32 which performs two functions:
`1. The generation of the timing and control signals
`necessary for the transfer of digital data into the trans(cid:173)
`mitter; and
`2. The assembly of the data, the sync, and the blank(cid:173)
`ing signals into a composite signal.
`The digital data to be transmitted is assumed to be in
`the form of 84 bit words. At the start of each line a data
`word is loaded into the shift register 38 by a data trans(cid:173)
`fer signal on output line 40 through gate 42. At the con(cid:173)
`clusion of the horizontal blanking interval the data is
`shifted into the data control circuit 32 by a shift ( 1.575
`MHz) signal supplied on output line 44. Standard flip(cid:173)
`flop and gate circuits under control of the aforemen(cid:173)
`tioned signals, and as shown in FIG. 4, provide this data
`transfer operation. The composite signal comprising
`composite sync, composite blanking and data coupled
`through a respective current source and switch is then
`sent on output line 46 to standard television RF modu(cid:173)
`lators for transmission over standard television chan-
`nels.
`
`35
`
`RECEIVER SYNCHRONIZATION
`Sucessful recovery of the incoming digital data in
`I 0 television format depends upon the synchronization of
`the addressing circuits with the incoming signal. The
`details of receiver synchronization are shown in FIG. 7.
`The horizontal sync signal is viewed by the address(cid:173)
`ing circuits through a 6 microseconds window. This
`IS window is initiated by the clock time 90 (T 90 ) from the
`1.575 MHz clock and terminated by the trailing edge
`of the incoming horizontal sync pulse. Between clock
`time 99 and 00 a sample H pulse is generated by the
`time counter 66 ·and is used to sample the horizontal
`20 sync signal as seen through the window. During this
`sample H interval the frequency of the 1.575 MHz os(cid:173)
`cillator 68 is adjusted such that the oscillator 68 re(cid:173)
`mains phase locked to the trailing edge of the horizon(cid:173)
`tal sync pulse. At the conclusion of the horizontal sync
`25 pulse a clear time counter pulse is generated (see FIG.
`7) which sets the time counter 66 to zero thus placing
`the counter "in step" with the train of sync pulses.
`Reference may be made to FIG. 9 wherein there is
`illustrated the phase comparator 69, low pass filter 71
`30 and the interconnection of various gate and flip-flop
`circuits for adjusting the frequency of oscillator 68 as
`described above.
`The line counter 64 is not actually incremented by
`the horizontal sync pulse but instead by an increment
`line counter pulse at clock time 3 from the time
`counter 66. Noise which may be present in the horizon-
`tal sync signal is thus prevented from entering false
`counts into the line counter. It is apparent, of course,
`that noise present on the sync signal during the window
`interval can still cause errors by generating erroneous
`clear pulses. The window, however, is open only for 6
`microseconds per line, and the clearing of the time
`counter 66 (generation of the clear pulse from gate 74)
`is permitted only for the first 12 horizontal sync pulses
`in a field plus approximately 3 lines following the verti(cid:173)
`cal sync pulse. The addressing circuits are thus exposed
`to the horizontal sync signal for approximately
`3/262.5 + 6/63.5 12/262.5 x 100= 2 percent
`50 of the time.
`Except for the vertical sync interval, the phase(cid:173)
`locking operation occurs for every line in a field.
`In the constructed embodiment of this invention, the
`line counter comprised standard flip-flop and gate cir(cid:173)
`cuits, as well as a nine stage binary counter formed of
`integrated circuits Nos. 7473 and 7493. The decoded
`circuit included integrated circuits Nos. 7442, 7402,
`7410 and 7400. The time counter included two decade
`counters, formed of integrated circuit No. 7490, and
`the associated decode circuit comprised two BCD to
`decimal decode, integrated circuit No. 7442, and a
`combinational logic, including integrated circuit No.
`7402.
`.
`
`RECEIVER
`Two basic functions are performed by the data re(cid:173)
`ceiver 16, (a) the recovery of the digital data, and (b)
`the generation of data addresses. This latter function is
`necessary to facilitate separation of the data when it is
`transmitted in a time-division-multiplex mode and must
`be delivered to multiple destinations.
`A block diagram of the data receiver 16 is shown in
`FIG. 6. An RF receiver 50 similar to the front end of
`a standard commercial television receiver is used to re(cid:173)
`cover the composite video signal from the input RF
`carrier. The composite signal on line 52 is delivered to 40
`the TV interface circuit 54 where it is clamped and
`then separated into sync and video (data) components.
`The video signal on line 56 is placed on the data bus 58
`while the composite sync signal on line 60 is sent to the
`sync detector circuit 62.
`The sync detector circuit 62 including two inte(cid:173)
`grator/comparators separates the vertical and horizon(cid:173)
`tal sync signals and supplies these signals to the data ad(cid:173)
`dressing circuits.
`Data addresses are specified by:
`a. a line address designating the television field line
`on which a data bit occurs; and
`b. a time address specifying the time bin along that
`field line which contains the data bit.
`The line address is specified by a line counter 64
`which effectively counts horizontal sync pulses. The
`time address is obtained from the time counter 66
`which is driven by a 1.575 MHz oscillator 68. This os(cid:173)
`cillator is phase locked by a phase lock circuit 70 to the 60
`horizontal sync pulse and provides an accurate source
`for strobing the data bins along a field line. The lower
`left hand part of FIG. 6 shows how a particular bit of
`data may be recovered from the data stream through
`line and time address gates 72 for transmission to a re- 65
`spective data terminal through a data modem 73. The
`data modem converts the respective digital data on
`data bus 58 into a form suitable for the particular ter-
`
`45
`
`55
`
`FORMAT OF TELEVISION FIELD
`Referring now to FIG. 8 there is illustrated the begin(cid:173)
`ning and ending portions of the 240 line television field
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`30
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`35
`
`7
`and the format of presenting digital data in the field in
`accordance with this invention. As shown in FIG. 8,
`each terminal bit i of a data word within the first n
`lines of the field, followed by bit i + 1 within the next
`12 lines, etc. As illustrated, data terminal T 0 receives
`the first bit 0 (T0°); the second terminal T 1 then re(cid:173)
`ceives its first bit 0 (T 1°), etc. The data transmission
`continues in the first line of the television field·until ter(cid:173)
`minal 83 has been presented with its bit 0 (T83°). The
`bit 0 for all terminals is thus sequentially transmitted
`within the first l21ines in the television field. As shown
`in FIG. 8, during the next 12 lines the next respective
`bit (bit I) for each terminal is sequentially transmitted
`as previously described for the first 12 lines. This for(cid:173)
`mat continues with bit 2 for all terminals transmitted
`during the next 12 lines, unit! bit 1!.9 is transmitted to
`all of the terminals to complete the TV field.
`It must be noted that a distinct advantage results
`from utilizing the format shown in FIG. 8. Specifically,
`if spurious noise occurs during the transmission time 20
`within the first 12 lines, for instance, only bit 0 for all
`of the terminals may be lost. Thus, in this system the
`noise burst would have to be at least 12 lines in dura(cid:173)
`tion (each line is 63.5 microseconds) to eliminate one
`bit from the terminals. In· fact, one unique feature of 25
`this aspect of the invention is that the error probability
`decreases with an increase in terminals, since if we dou(cid:173)
`bled the number of terminals it would take a noise burst
`of 24 lines duration to eliminate one bit from each ter-
`minal.
`The format shown in FIG. 8 is developed from the
`fact that the data rate for the system is given by 60NLN8
`bits per sec.; where NL is the number of teleivision lines
`per field carrying data; and N 8 is the number of bits per
`television line.
`In the system described here, 240 television lines per
`field, each containing 84 bits are used, giving a data
`rate of 1.2096 X 106 bps. In a PLATO type system as
`previously mentioned, each data terminal 18 operates
`on a word size of 20 bits in length and requires up to 40
`60 words per second, or a data rate of 1200 bps. There(cid:173)
`fore, one television field as shown in FIG. Scan supply
`data to 1 ,008 terminals.
`In a general digital data transmitting system accord(cid:173)
`ing to the present invention, the following relationships 45
`can be given:
`BF = T R/60; L8 = 240/BF; and NT= (84) LB,
`where: TR is the terminal operating rate in bps; NTis the
`number of terminals per TV channel; BF is the number 50
`of bits transmitted per TV field per terminal; and L 8 is
`the number of TV lines required to send a bit to all ter(cid:173)
`minals.
`The aforementioned advantage in error reduction of
`this invention becomes even more pronounced if a
`transmitting device is used that has a slower transmis(cid:173)
`sion rate than the illustrated 1200 bps here, since we
`could spread the transmission over the entire 12 or
`more lines. For instance, in a teletypewriter system 60
`which can send information out at approximately 110
`bps, the transmission rate would be about 1/10 of the
`1200 bps rate of the present system. Therefore, using
`the same system as we have here, up to 10 times as
`many terminals ( 10,080) can be serviced with the same 65
`error rate as the present 1200 bps, 1080
`terminals.
`That is, with teletypewriters running about 11 0 bps (or
`about 120 bps for computation), we could use 2 bits
`
`per field or about !20 lines per bit and obtain I 0,080
`terminals.
`While the actual apparatus in terms of the circuits in(cid:173)
`volved has been herein illustrated in block diagram
`s form, such circuits are well known to those skilled in
`the art. For instance, reference may be made to "Pulse
`and Digital Circuits", Millman & Taub, McGraw-Hill
`Book Co., Inc., 1956,
`particularly pp. 505-534,
`wherein the principles and components of television
`10 transmission are described. As previously described,
`many of the illustrated components here are readily
`available as integrated circuits in the 7400 series. This
`is to be understood as only an example of the present
`invention and not as a limitation to this particular em-
`15 bodiment.
`The foregoing detailed description has been given for
`clearness and understanding only, and no unnecessary
`limitations should be understood therefrom, as modifi(cid:173)
`cations will be obvious to those skilled in the art.
`What is claimed is:
`1. A system for transmitting and receiving digital data
`selectively distributable to a plurality of data terminals,
`utilizing composite line scan horizontal and vertical
`synchronization (sync) and blanking signals for a
`teleivision field, compatible with commercial television
`practice, said system comprising:
`means for generating said composite line scan hori(cid:173)
`zontal and vertical sync and blanking signals for a
`television field;
`clock control means for generating a timing signal di(cid:173)
`viding a line of said television field into discrete
`time intervals each associated with a respective
`data terminal;
`data storage means for storing said digital data;
`data control means receiving said composite sync
`and blanking signals, said digital data and said tim(cid:173)
`ing signal for providing a line scan television field
`television
`signal compatible with commercial
`practice;
`said data control means including means for sequen(cid:173)
`tially entering said digital data bit by bit into re(cid:173)
`spective time intervals throughout said line of said
`television field in response to said timing signal,
`each bit associated for distribution to a respective
`data terminal;
`a receiver for recovering said digital data from said
`television field signal for distribution to selected
`data terminals;
`said receiver including means for separating said hor(cid:173)
`izontal and vertical sync information from said
`television field signal;
`·
`means responsive to said horizontal sync information
`for generating respective line addresses;
`phase locked oscillator means including oscillating
`means providing an oscillating signal at the same
`rate as said clock control means;
`means responsive to said oscillating signal for gener(cid:173)
`ating time addresses of each of said discrete time
`interval in each line of said television field; and
`means responsive to a respective line and time ad(cid:173)
`dress for coupling said data bits sequentially to re(cid:173)
`spective data terminals.
`2. A system as claimed in claim 1 wherein said clock
`control means includes an oscillator providing an out(cid:173)
`put frequency with a corresponding period related to
`said discrete time intervals in each line of said televi(cid:173)
`sion field.
`
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`9
`3. A system as claimed in claim 1, including means
`respon