PCT
`WORLD INTELLECTUAL PROPERTY ORGANIZATION
`International Bureau
`INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`WO 97/40624
`
`(11) International Publication Number:
`
`(51) International Patent Classification 6 :
`H04N 7118, 9/47
`
`Al
`
`(43) International Publication Date:
`
`30 October 1997 (30.10.97)
`
`(21) International Application Number:
`
`PCT/US97/06844
`
`(22) International Filing Date:
`
`17 April 1997 (17 .04.97)
`
`(30) Priority Data:
`08/635,692
`
`22 April 1996 (22.04.96)
`
`us
`
`(71) Applicant: SENSORMATIC ELECTRONICS CORPORA(cid:173)
`TION [US/US]; 951 Yamato Road, Boca Raton, FL 33431-
`0700 (US).
`
`(72) Inventor: ELY, Reuel, Andrew; 560 N.W. 66th Avenue,
`Plantation, FL 33317 (US).
`
`(74) Agent: TORRENTE, John, J.; Robin, Blecker, Daley &
`Driscoll, 330 Madison Avenue, New York, NY 10017 (US).
`
`(81) Designated States: AL, AM, AT, AU, AZ, BA, BB, BG, BR,
`BY, CA, CH, CN, CU, CZ, DE, DK, EE, ES, Fl, GB, GE,
`HU, IL, IS, JP, KE, KG, KP, KR, KZ, LC, LK, LR, LS,
`LT, LU, LV, MD, MG, MK, MN, MW, MX, NO, NZ, PL,
`PT, RO, RU, SD, SE, SG, SI, SK, TJ, TM, TR, TT, UA,
`UG, UZ, VN, ARIPO patent (GH, KE, LS, MW, SD, SZ,
`UG), Eurasian patent (AM, AZ, BY, KG, KZ, MD, RU, TJ,
`TM), European patent (AT, BE, CH, DE, DK, ES, FI, FR,
`GB, GR, IE, IT, LU, MC, NL, PT, SE), OAPI patent (BF,
`BJ, CF, CG, CI, CM, GA, GN, ML, MR, NE, SN, TD, TG).
`
`Published
`With international search report.
`Before the expiration of the time limit for amending the
`claims and to be republished in the event of the receipt of
`amendments.
`
`(54) Title: DISTRIBlITED VIDEO DATA STORAGE IN VIDEO SURVEILLANCE SYSTEM
`
`100----..
`
`120
`
`120
`
`• • • SENSOR
`
`COMMAND, STATUS, VIDEO DATA
`112
`118
`
`i18
`
`114
`
`114
`
`106
`CENTRAL
`STATION
`
`104
`
`102
`
`116
`
`116
`
`(57) Abstract
`
`A video surveillance system includes a central control station (102) and a plurality of video cameras each mounted inside a dome
`housing unit (114). A video data buffer memory, storing compressed video data generated by the camera, is mounted with each camera in
`the respective dome unit. Data buffered at the dome units may be selectively protected from over-writing in response to alarm signals and
`then retrieved for display or tape-recording by the central control station. Both live and buffered video signals are transmitted in compressed
`form over the data network (112) that is also used for command, alarm and status messaging.
`
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`Codes used to identify States party to the PCT on the front pages of pamphlets publishing international applications under the PCT.
`
`FOR THE PURPOSES OF INFORMATION ONLY
`
`AL
`AM
`AT
`AU
`AZ
`BA
`BB
`BE
`BF
`BG
`BJ
`BR
`BY
`CA
`CF
`CG
`CH
`Cl
`CM
`eN
`cu
`CZ
`DE
`DK
`EE
`
`Albania
`Annenia
`Austria
`Australia
`Azerbaijan
`Bosnia and Herzegovina
`Barbados
`Belgium
`Burkina Faso
`Bulgaria
`Benin
`Brazil
`Belarus
`Canada
`Central African Republic
`Congo
`Swilzerland
`COte d' lvoire
`Cameroon
`China
`Cuba
`Czech Republic
`Germany
`Denmark
`Estonia
`
`ES
`Fl
`FR
`GA
`GB
`GE
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`GR
`HU
`IE
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`JP
`KE
`KG
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`
`KR
`KZ
`LC
`LI
`LK
`LR
`
`Spain
`Finland
`France
`Gabon
`United Kingdom
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`Hungary
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`Israel
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`Italy
`Japan
`Kenya
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`Democratic People's
`Republic of Korea
`Republic of Korea
`Kazakstan
`Saint Lucia
`Liechtenstein
`Sri Lanka
`Liberia
`
`LS
`LT
`LU
`LV
`MC
`MD
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`MK
`
`ML
`MN
`MR
`MW
`MX
`NE
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`NO
`NZ
`PL
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`RO
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`SD
`SE
`SG
`
`Lesotho
`Lithuania
`Luxembourg
`Latvia
`Monaco
`Republic of Moldova
`Madagascar
`The fonncr Yugoslav
`Republic of Macedonia
`Mali
`Mongolia
`Mauritania
`Malawi
`Mexico
`Niger
`Netherlands
`Norway
`New Zealand
`Poland
`Portugal
`Romania
`Russian Federation
`Sudan
`Sweden
`Singapore
`
`SI
`SK
`SN
`sz
`TD
`TG
`TJ
`TM
`TR
`TT
`UA
`UG
`us
`uz
`VN
`YU
`zw
`
`Slovenia
`Slovakia
`Senegal
`Swaziland
`Chad
`Togo
`Tajikistan
`Turkmenistan
`Turkey
`Trinidad and Tobago
`Ukraine
`Uganda
`United States of America
`Uzbekistan
`Viet Nam
`Yugoslavia
`Zimbabwe
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`DISTRIBUTED VIDEO DATA STORAGE IN
`VIDEO SURVEILLANCE SYSTEM
`
`FIELD OF THE INVENTION
`
`5
`
`10
`
`to video surveillance
`is related
`invention
`This
`systems, and more particularly to surveillance systems of
`the type which include sensors for advising the system of
`alarm conditions.
`
`20
`
`25
`
`BACKGROUND OF THE INVENTION
`surveillance
`A conventional closed-circuit video
`system,
`indicated generally by reference numeral 10, is
`illustrated in block d~agram form in Fig. 1.
`15 The surveillance system 10 includes a host computer 12,
`which controls the operation of the system, a video switch
`14 which selectively interconnects
`input video signals
`(represented by dashed lines 16) with outputs of the switch
`(represented by dashed lines 18). Also included in the
`system 10 are video cameras 20, monitors 22, control
`consoles 24, and sensor devices 26. A local area network 28
`is provided for interchange of command, status and alarm
`data between the system host 12 and the cameras 20, control
`consoles 24 and sensors 26.
`A user interface 30 is also provided to permit a system
`operator to control ope?="ations of the system. The interface
`30 may incorporate a monitor and a control console which are
`not shown separately from the user
`interface 30. Also
`included in the system 10 are one or more video cassette
`recorders 32 which are available to record video signals
`that are generated by the video cameras 20 and selectively
`routed to the VCRs 32 by way of the video switch 14. Video
`signals previously recorded by the VCRs 32 can be played
`back by the VCRs 32 and selectively routed through the
`switch 14 for viewing on one or more of the monitors 22. As
`is well known to those who are skilled in the art, system
`host 12 also controls the video switch 14 so that,
`in
`response to control signals entered via the control consoles
`
`30
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`35
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`24, video signals generated by a selected one of the video
`cameras 20 is routed for viewing at a selected one of the
`monitors 22. Sensors 26 generate alarm signals in response
`to anomalous events such as unauthorized opening of doors,
`5 windows or display cases. The system host 12 operates to
`inform the human operator of the alarm conditions, and
`predetermined camera movements and routing of corresponding
`video
`signals
`to pre-selected monitors
`22 may
`be
`automatically actuated
`in
`response
`to alarm signals.
`10 Movement of the cameras can also be controlled through
`commands issued by the system host, in response to pre(cid:173)
`programmed surveillance routines, or in response to operator
`input through the control consoles 24 or the user interface
`30.
`
`15
`
`20
`
`It is generally a desired goal in video surveillance
`that video signals representative of conditions
`systems
`during and after the onset of an alarm event be
`before,
`recorded for current and future reference, subsequent legal
`proceedings, and so forth, and it is for this purpose that
`the VCRs 32 are provided. However, surveillance systems
`frequently include a large number of video cameras (e.g.,
`dozens or hundreds of cameras) , and the cost of video
`cassette recorders is too high to permit recording of the
`signals generated by more than a few of the cameras at any
`25 given
`time.
`In conventional
`systems, video signals
`indicative of conditions prior to an alarm event cannot be
`recorded, unless the output of a video camera has been
`routed for recording by a VCR prior to the time an alarm
`event occurs.
`To maximize the recording capacity of the VCRs, it is
`known to provide space-division-multiplexed (e.g., quad
`multiplexed) signals for recording on VCRs 32, and it is
`also known to. operate the VCRs in a time lapse mode, with a
`reduced frame rate and switching among a number of cameras.
`35 When
`an alarm occurs,
`the
`time
`lapse mode
`can be
`automatically stopped in favor of a standard frame rate
`recording operation for the critical camera or cameras.
`In
`addition, critical cameras can be connected at all times for
`
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`There are, however,
`a dedicated VCR.
`recording by
`disadvantages with all of these techniques. As noted above,
`it is quite expensive to provide a large number of VCRs, and
`if space-division-multiplexing is employed,
`the spatial
`resolution of the image signal is reduced, which may impair
`recognition of critical elements of the recorded images. On
`the other hand, time l'apse recording suffers from reduced
`temporal resolution, so that again critical events may be
`missed. And, as noted before, if a camera having a view of
`10 an alarm event is not being recorded prior to the alarm
`event, the critical video information may simply fail to be
`captured.
`An "intrusion monitoring system" disclosed in U.S.
`5,109,278
`and
`5,111,291
`Patent Nos.
`(assigned
`to
`15 Commonwealth Edison) attempts to address some of these
`problems by providing an analog-to-digital converter and
`frame buffers at
`the output of a video multiplexer.
`Al though the system disclosed in the Com Ed patents provides
`some capability for capturing and storing signals indicative
`20 of conditions at or before the time that an alarm event
`occurs, the system does not seem to be adapted to operate
`with a large number of cameras, and the temporal resolution
`is poor, especially when the system includes more than a
`minimal number of cameras.
`
`25
`
`3 O
`
`OBJECTS AND SUMMARY OF THE INVENTION
`It is accordingly an object of the invention to provide
`a video surveillance system having an improved capability
`for storage of video signals.
`It is another object of the invention to provide a
`video surveillance system in which buff er storage
`is
`provided for video signals generated by all of the cameras
`in the syste~, and it is a further object to provide such
`buffer storage for all cameras in the system even when the
`35 number of cameras is large.
`invention to
`the
`It is still a further object of
`in which a human
`provide a video surveillance system
`operator can conveniently control buffering and permanent
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`20
`
`recording of video signals generated by all of the video
`cameras in the system.
`It is yet another object of the invention to provide a
`video surveillance system in which storage for video signals
`5 generated by each camera is carried out at low cost.
`It is yet a further object of the invention to provide
`a video surveillance system which requires less cabling than
`conventional systems.
`there is
`According to an aspect of the invention,
`10 provided a video surveillance system, including a central
`station, a plurality of video cameras located remotely from
`the central station for generating respective video signals,
`a plurality of storage devices, each located remotely from
`the central station and associated with a respective one of
`the video
`cameras
`for
`storing digital video data
`representative of
`the video signal generated by
`the
`respective video camera,
`and connection circuitry for
`forming a signal path between the central station and each
`of the storage devices:
`the
`this aspect of
`Further
`in accordance with
`invention, the connection circuitry may include a plurality
`of control devices each operatively connected
`to
`a
`respective one of the storage devices for controlling the
`respective storage device to operate as a ring buffer in
`25 which digital video data representative of video signals
`currently generated by
`the respective video camera is
`written over oldest digital video data stored in the
`respective storage device. The surveillance system provided
`according to this aspect of
`the
`invention may further
`include a sensor device for generating an alarm signal
`indicative of an alarm condition,
`and circuitry for
`transmitting the alarm signal from the sensor device to the
`central station.
`The central station may
`respond
`to
`receiving the alarm signal by transmitting a storage signal
`to at least one of the control devices, such control device
`or devices responding to the storage signal by inhibiting
`over-writing of
`the oldest digital video data
`in the
`respective storage device or devices controlled by such
`
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`
`control device or devices. Alternatively, the sensor device
`may be coupled to the connection circuitry for transmitting
`the alarm signal directly to the control device or devices,
`such device or devices responding to the alarm signal by
`inhibiting over-writing of the oldest digital video data in
`the respective storage device or devices controlled by such
`control device or devices.
`Still further in accordance with this aspect of the
`invention, the central station may selectively transmit a
`retrieve signal to a selected one of the control devices,
`which responds to the retrieve signal by transmitting to the
`central station digital video data read out from the storage
`device controlled by the selected control device.
`The system provided in accordance with this aspect of
`invention may
`further
`include
`a display device
`the
`associated with the central station for displaying images
`corresponding to the digital video data read out from the
`storage device and transmitted to the central station, as
`well as a video tape ·recorder controlled by the central
`20 station for recording signals corresponding to the digital
`video data read out from the storage device and transmitted
`to the centrai station.
`The connection circuitry may include a common data
`transmission
`channel
`by which
`command
`signals
`are
`transmitted from the central station to the video cameras
`and digital video data read out from the storage devices is
`transmitted to the central station. The digital video data
`stored in the storage devices and transmitted by the data
`transmission
`channel may be
`compressed video data.
`r'
`30 Preferably each of the storage devices is a solid state
`memory and each is disposed in a respective housing together
`with the respective video camera.
`According to another aspect of the invention, there is
`provided a surveillance video camera unit,
`including a
`35 housing which includes a light transmissive portion, a video
`camera disposed within the housing for forming an image from
`light which passes through the light transmissive portion of
`the housing and for converting the
`image into a video
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`signal, and a storage device, disposed within the housing,
`for storing digital data representative of the video signal.
`
`5
`
`10
`
`Further in accordance with the latter aspect of the
`invention, the video signal generated by the video camera
`may be an analog video signal, in which case the camera unit
`includes conversion circuitry disposed within the housing
`for converting the analog video signal into a digital video
`signal, and processing circuitry disposed within the housing
`for applying·a data compression algorithm to the digital
`video signal to form compressed video data, the compressed
`video data being stored in
`the storage device.
`The
`conversion circuitry and the processing circuitry may be
`integrated with the video camera so that the video camera
`15 outputs the compressed video data to the storage device.
`Still further in accordance with this aspect of the
`invention, the camera unit may include a control circuit
`disposed within the housing for controlling the storage
`device to operate as a ring buffer in which digital data
`representative of a video signal currently formed by the
`video camera is written over oldest digital data stored in
`the storage device.
`In response to receiving an event
`signal,
`the control circuit inhibits over-writing of the
`oldest digital data.
`There may further be provided in the camera unit a
`plurality of motors disposed in the housing for changing
`pan, tilt and zoom conditions of the video camera, and the
`control circuit generates control signals for controlling
`the plurality of motors, the camera unit further including
`30 circuitry for transmitting the control signals from the
`control circuit to the .plurality of motors. Preferably the
`storage device is a solid state memory.
`According to still another aspect of the invention,
`there
`is provided a method of storing video signals,
`including the steps of generating digital video signals
`using a plurality of video cameras, storing the digital
`video signals generated by each camera in a respective
`storage device provided adjacent
`to
`the camera,
`and
`
`20
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`15
`
`selectively transmitting to a central station the digital
`video signals stored in each storage device. Further in
`accordance with this aspect of the invention, the method may
`include operating the storage devices as ring buffers in
`5 which currently generated digital video signals are written
`over oldest digital video signals stored in the storage
`devices, and inhibiting the over-writing of the oldest
`digital video signals in response to receipt of an event
`signal.
`The method may also include the steps of tape-
`recording signals corresponding to the digital video signals
`transmitted to the central station, and displaying images
`corresponding to the digital video signals transmitted to
`the central station.
`According to yet another aspect of the invention, there
`is provided a video surveillance system
`including
`a
`plurality of video cameras for generating the video signals,
`digital storage circuitry for storing the video signals in
`the form of digital video data, control circuitry for
`generating camera control
`commands, data
`transmission
`20 circuitry for forming a data communication channel by which
`the camera control commands are transmitted from the control
`circuitry to the plurality of video cameras, and circuitry
`for retrieving the stored digital data from the digital
`storage circuitry and for transmitting the retrieved digital
`25 video data
`to
`the control circuitry via
`the data
`communication channel
`formed by
`the data
`transmission
`circuitry.
`The video signals generated by the video cameras may be
`analog video signals, in which case the digital storage
`30 circuitry includes circuitry for converting the analog video
`signals
`into digital . video signals and circuitry for
`compressing the digital video signals to form compressed
`digital video data. Alternatively,
`the video signals
`generated by· the video cameras may be in the form of
`35 compressed digital video data.
`The digital storage circuitry may include a plurality
`of digital storage devices, each located adjacent to a
`respective one of the video cameras, and the retrieving
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`s
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`circuitry may include a plurality of control devices, each
`associated with the respective one of the digital storage
`devices. Preferably, each of the storage devices is a solid
`state memory.
`By providing a video surveillance system that includes
`camera units having digital storage for signals generated in
`the respective camera unit, video signals corresponding to
`conditions before, during and after an alarm event can be
`conveniently and reliably stored, and the locally stored
`10 video can be read out and transmitted to an operator(cid:173)
`attended station at a convenient
`time for viewing and
`permanent recording on video tape. The data communication
`network used for transmission of command and status data
`between the system central station and the video cameras may
`lS be provided with sufficient capacity to handle transmission
`of compressed video data buffered or currently generated at
`the video camera units, in which case separate video signal
`transmission cabling can be dispensed with.
`and
`features
`The
`foregoing
`and other objects,
`20 advantages of the invention will be further understood from
`the following detailed description of preferred embodiments
`and practices of
`the
`invention and
`from
`the drawings,
`wherein like reference numerals identify like components and
`parts throughout.
`
`2S
`
`30
`
`3S
`
`DESCRIPTION OF THE DRAWINGS
`Fig. 1 is a block diagram of a conventional video
`surveillance system.
`Fig. 2 is a block diagram of a video surveillance
`system provided in accordance with the present invention.
`Fig. 3 is a schematic illustration of a video camera
`unit used in the system of Fig. 2.
`Fig. 4 is a flow chart of processes carried out in a
`control circuit that is part of the camera unit of Fig. 3.
`Figs. SA and SB together form a flow chart of processes
`carried out in a host computer that is part of the system of
`Fig. 2.
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`DESCRIPTION OF PREFERRED EMBODIMENTS AND PRACTICES
`An embodiment of the invention will now be described,
`initially with reference to Fig. 2. Reference numeral 100
`generally indicates a video surveillance system provided in
`s accordance with the invention. The system 100 includes a
`central station 102 at which a host computer 104
`is
`provided. Associated with the host computer 104 are a video
`data encoding/decoding device 106 and hardware and software
`required to provide a user interface, represented by block
`10 108.
`The host computer 104 controls a video cassette
`recorder 110. A local area network (LAN) 112 is connected
`to the host computer 104. Data communicated between the
`host 104 and other components of the system 100 is carried
`over the LAN 112. Other system components connected to the
`15 host computer 104 via the LAN 112 include camera units 114,
`monitors 116
`(each connected to the LAN 112
`through a
`respective video data decoding device 118) and sensor
`devices 120.
`The sensor devices 120 are preferably of
`conventional types that detect, for example, unauthorized
`20 opening of doors, windows and display cases and the like and
`generate alarm signals indicative of these occurrences. The
`sensors 120 may also include conventional motion and/or heat
`sensing devices. The monitors 116 and the VCR 110 are also
`preferably conventional commercially-available items.
`It
`should be understood that although only one VCR 110 is shown
`in Fig. 2, two or more VCRs may be provided as part of the
`system 100.
`represented
`is
`interface 108
`the user
`Although
`separately from the monitors 116, it should be understood
`that the user interface preferably includes one or more of
`such monitors, as well as other
`input/output devices
`commonly provided as peripherals to personal computers.
`In
`addition, the user interface 108 may include one or more
`camera control devices such as the "TOUCHTRACKER" control
`3 5 consoles marketed by the assignee of this application.
`Additional camera control consoles, although not shown, may
`be provided in the system at or remote from the central
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`station 102, and connected to the host 104 through the LAN
`112.
`
`5
`
`10
`
`The camera units 114 are preferably of a design
`provided in accordance with an aspect of the invention, as
`illustrated in Fig. 3.
`Each camera unit may be mounted at a ceiling 122 or at
`another appropriate location, and includes a housing 124
`constituted, at least in part, by a base portion 126 and a
`dome portion 128. In accordance with conventional practice,
`the dome portion 128 is formed of a
`light-transmissive
`material that allows light to enter the interior of the
`housing 124, although the material of which dome 128 is made
`be "one way" so that the interior of
`the unit 114 is
`screened from view.
`Mounted within the housing 124 of the camera unit 114,
`and preferably mounted on the base portion 126, are a video
`camera 130, motors and other structure 132 for changing pan,
`tilt, zoom and/or other conditions of the camera 130, a
`control circuit 134, and a memory board 136. The camera 130
`20 may be of conventional design, and includes an optical
`system 138 that receives and forms into an optical image
`light 140 which passes
`through the dome 128
`into
`the
`interior of the camera unit 114. The image formed by the
`optical system 138 is converted into an electrical signal at
`25 CCD
`142
`and
`the
`resulting
`signal
`is
`subjected
`to
`conventional analog signal processing at video signal
`processing circuit 144. A suitable camera for use in the
`camera unit 114 is commercially available from the assignee
`the present application, Sensormatic Electronics
`of
`Other suitable
`30 Corporation, as part number 2003-0037-00.
`cameras are also commercially available.
`The camera mounting structure, pan, tilt, and zoom
`motors, and so forth indicated by reference numeral 132 are
`also preferably of conventional design, such as are provided
`35 on commercially available camera dome units. The control
`circuit 134 may be a conventional microcontroller, such as
`is currently used in camera dome units to receive command
`signals from, and transmit status data to, a system central
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`control station, and also for controlling the camera motors
`and detecting the status (such as position) of the camera
`motors,
`in a conventional manner.
`However,
`the control
`circuit 134 shown in Fig. 3 is also programmed, according to
`5 practices of the present invention, to carry out memory
`control, as well as data storage and retrieval functions
`that will be described below.
`The memory board 136 is provided in accordance with the
`invention to receive an analog video signal output from the
`camera 130 and to digitize, compress and store the video
`signal. Components of the memory board 136 include a video
`digitizer circuit 146, a video data coding/decoding circuit
`148 and a memory device 150. The video digitizer 146 may be
`suitably realized by the "WAVEWATCHER TV II" video digitizer
`15 card
`available
`from AITech
`International,
`Fremont,
`California. A suitable video codec is the model VCI-10,
`available from VistaCom, OY, Tietajantie 14, 02130, Espoo,
`Finland.
`The model VCI-10 compresses video data
`in
`accordance with the H.261 standard, and also provides the
`function of
`reconstituting video data
`that has been
`compressed in accordance with that standard.
`The memory
`device 150 may be constituted by one or more of RAM, EEPROM,
`flash memory, and so forth. As will be seen, the memory
`device 150 functions as a buffer for
`the video signal
`25 generated by the camera 130, and is controlled by the
`control circuit 134.
`It is expected that, within a short time, there will
`become
`commercially
`available
`video
`cameras which
`incorporate the video signal digitizing and compression
`functions carried out at blocks 146 and 148 in Fig. 3. For
`example, commercial introduction of a video camera which
`outputs compressed digital video data in accordance with the
`above-mentioned H. 261 standard is anticipated.
`Such a
`camera could .be used in place of the camera 130 shown in
`35 Fig. 3, and the separately-provided video digitizer 146 and
`video codec 148 could be omitted, with the compressed
`digital video signal output from the camera being stored
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`directly in the memory device 150 under the control of the
`control circuit 134.
`Operation of the camera unit 114 will now be described,
`with reference
`to Fig. 4, which illustrates processes
`5 carried out by the control circuit 134 under the control of
`a stored program.
`{It will be understood that the program
`memory is not shown separately from the control circuit
`134.)
`One ongoing process carried out under normal conditions
`10 by the control circuit 134 is transmission to the central
`station
`(via LAN 112) of compressed video data which
`represents video signals currently being generated by the
`camera 130.
`This process,
`represented by block 152,
`includes receiving the live compressed video data from the
`15 video codec 148 and coupling the data onto the LAN 112,
`either according to predetermined time slots, or in response
`to commands from the central station, or according to some
`other protocol. In parallel with the process represented by
`block 152,
`the live compressed video data formed by the
`20 video codec 148
`is ·also stored
`in
`the memory 150.
`Preferably the memory addressing and writing of data into
`the memory is controlled by the control circuit 134 so that
`the memory 150 serves as a ring buffer, in which currently
`generated compressed video data is written over the oldest
`compressed video data that had previously been stored in the
`memory 150. However, over-writing of the oldest data is
`subject to inhibition in response to alarm commands. On
`such occasions, the live compressed video data is written
`into a portion of the memory device 150 which does not hold
`30 video data that is to be preserved.
`In a third ongoing process, represented by block 156,
`the control circuit 134 handles camera control commands
`received via the LAN 112. The handling of camera control
`commands is in accordance with conventional practice, and a
`further discussion thereof consequently will be omitted.
`Concurrently with the ongoing processes of blocks 152,
`154 and 156,
`the control circuit 134 is conditioned to
`receive occasional alarm commands, as indicated by decision
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`block 158. When an alarm command is received, a process
`(indicated by block 160) is carried out in which the control
`circuit 134 protects from over-writing compressed digital
`video data previously stored
`in
`the memory 150,
`and
`a
`5 corresponding
`to
`a
`time
`interval
`beginning
`at
`predetermined time prior to receipt of the alarm command,
`and continuing for a predetermined time after receipt of the
`alarm command.
`(as
`is also conditioned
`The control circuit 134
`indicated by decision block 162) to receive commands calling
`for reading-out and transmission to the central station of
`previously-stored compressed video data. The stored data to
`be retrieved may be data that was previously protected from
`over-writing in response to an alarm command, or may be
`15 other data that has previously been stored in the buff er
`memory 150. When a retrieve command is received, a process
`for reading out the stored data from the buffer memory and
`· transmitting the read-out data via the LAN 112 is carried
`out, as represented by block 164. The transmission of the
`20 compressed video data that was read out from the memory 150
`may be in place of, or virtually in parallel with, continued
`transmission of "live" compressed video data. Preferably,
`as suggested by Fig. 4, transmission of stored compressed
`video data in response to corresponding retrieval commands
`25 does not interrupt ongoing storage of "live" video data.
`As indicated by decision block 166, the control circuit
`134 is also conditioned to receive
`11 clear" commands,
`in
`response to which the control circuit removes a previously
`protected block of stored video data
`from protection,
`thereby permitting "live" video data to be written over the
`now-unprotected stored video data.
`Overall operation·of the system 100 (Fig. 2) will now
`be described, with reference to Figs. SA and SB, which
`illustrate processes carried out by the host computer 104.
`As indicated at block 170 in Fig. SA, one ongoing
`process performed by
`the host
`computer
`104 entails
`etc.,
`controlling
`camera movements,
`in
`response
`to
`instructions input by users of the system. The instructions
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`may be received either through the user interface 108, or
`via separate camera control consoles, which are not shown.
`The receipt of user instructions, processing of same by the
`host
`computer 104, generation of
`command data
`and
`transmission of the command data via LAN 112, and camera
`movements
`(pan,
`tilt, zoom, etc.)
`in response
`to
`the
`commands are all pr