United States Patent (19)
`Ely
`
`USOO5982418A
`Patent Number:
`11
`(45) Date of Patent:
`
`5,982,418
`Nov. 9, 1999
`
`54) DISTRIBUTED WIDEO DATA STORAGE IN
`VIDEO SURVELLANCE SYSTEM
`
`75 Inventor: Reuel Andrew Ely, Plantation, Fla.
`73) ASSignee: Sensormatic Electronics Corporation,
`Boca Raton, Fla.
`
`21 Appl. No.: 08/635,692
`22 Filed:
`Apr. 22, 1996
`(51) Int. Cl." ....................................................... H04N 7/18
`52 U.S. Cl. ............................................. 348/153; 348/154
`58 Field of Search ..................................... 348/150, 153,
`348/159, 143, 154 H04N 71s
`s
`s
`s
`References Cited
`
`56)
`
`U.S. PATENT DOCUMENTS
`4.736,218 4/1988 Kutman ..................................... 3548
`4,833.534 5/1989 Paff et al. .....
`... 348/151
`4,943,854 7/1990 Shiota et al. ............................ 348/159
`5,109,278 4/1992 Erickson et al. ........................ 358/108
`5,111,291 5/1992 Erickson et al. ........................ 358/108
`
`5,229,850 7/1993 Toyoshima .............................. 348/153
`5,359,363 10/1994 Kuban et al.
`... 348/36
`5,521,634 5/1996 McGary .........
`... 348/153
`5,602933 2/1997 Blackwell et al. ...................
`348/143
`5,659,787 8/1997 Schieltz .............................. 395/200.56
`5,724,475 3/1998 Kristen .................................... 386/109
`Primary Examiner Bryan Tung
`Assistant Examiner Nhon T. Diep
`Attorney, Agent, or Firm-Robin, Blecker & Daley
`57
`ABSTRACT
`A video Surveillance System includes a central control
`Station and a plurality of Video cameras each mounted inside
`a dome housing unit. 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 com
`pressed form over a data network that is also used for
`command, alarm and Status messaging.
`
`22 Claims, 6 Drawing Sheets
`
`12O
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`2O
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`COMMAND, STATUS, WIDEO DATA
`
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`AO
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`
`
`
`
`CONTROL
`
`ANALOG
`WDEO
`
`
`
`
`
`CENTRAL
`STATION
`
`

`

`U.S. Patent
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`Nov.9, 1999
`
`Sheet 1 of 6
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`5,982,418
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`Motorola v. Stellar
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`Motorola Exhibit 1020
`Page 002
`
`
`

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`U.S. Patent
`
`5,982.418
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`5,982.418
`5,982,418
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`£EOI
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`NOILVLS.AVSLNS9WONSOL
`
`U.S. Patent
`U.S. Patent
`
`Nov.9, 1999
`
`Sheet 3 of 6
`
`
`
`
`
`
`
`
`
`
`
`Tit
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`Motorola v. Stellar
`
`Motorola Exhibit 1020
`Page 004
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`

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`U.S. Patent
`
`Nov. 9, 1999
`
`Sheet 4 of 6
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`5,982.418
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`TRANSMT L VE
`COMPRESSED
`WDEO DATA
`
`
`
`
`
`STORE LIVE
`COMPRESSED
`VIDEO DATA
`
`
`
`HANDLE CAMERA
`CONTROL COMMANDS
`
`
`
`
`
`
`
`
`
`
`
`ALARM
`COMMAND
`p
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`
`
`RETREVE
`COMMAND
`p
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`TRANSMT
`YES STORED
`COMPRESSED
`VIDEO DATA
`
`PROTECT PRE
`AND POST-ALARM
`STORED COMPRESSED
`VIDEO DATA
`
`
`
`
`
`CLEAR
`COMMAND
`2
`
`YES UNPROTECT
`STORED WIDEO
`DATA TO BE
`CLEARED
`
`A/G 4
`CAMERA UNIT OPERATION
`
`

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`U.S. Patent
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`Nov. 9, 1999
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`Sheet 5 of 6
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`5,982.418
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`HANDLE USER
`CAMERA CONTROL
`NSTRUCTIONS
`
`HANDLE USER
`DISPLAY CONTROL
`INSTRUCTIONS
`
`HANDLE USER
`VCR CONTROL
`INSTRUCTIONS
`
`
`
`USER
`INSTRUCTION TO
`RECORD ALARM
`VIDEO DATA
`
`ALARM
`CONDITION
`p
`
`USER
`NSTRUCTION TO
`DISPLAY ALARM
`VIDEO DATA
`
`
`
`
`
`SEND ALARM
`COMMAND TO
`CORRESPONDING
`CAMERA
`LOCATIONS
`
`A/G 5A
`SYSTEM OPERATION
`
`

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`U.S. Patent
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`Nov. 9, 1999
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`Sheet 6 of 6
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`5,982.418
`(B) 494
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`SEND RET REVE
`COMMAND TO
`SELECTED CAMERA
`LOCATION
`
`184
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`DISPLAY
`RETREVED ALARM
`VIDEO DATA
`ON SELECTED
`MONTOR
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`186
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`196
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`188
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`USER
`NSTRUCTION
`TO RECORD RETREVED
`ALARM WIDEO
`DATA 2
`
`NO
`
`
`
`
`
`SEND RET REVE
`COMMAND TO
`SELECTED CAMERA
`LOCATION
`
`SEND COMMAND
`TO CLEAR
`RETRIEVED ALARM
`VIDEO DATA
`FROM CAMERA
`BUFFER
`
`OUTPUT RETRIEVED
`ALARM WDEO
`DATA TO WCR;
`CONTROL VCR
`TO RECORD
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`198
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`
`
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`
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`
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`SEND COMMAND
`TO CLEAR
`RETRIEVED ALARM
`VIDEO DATA
`FROM CAMERA
`BUFFER
`
`OUTPUT RETRIEVED
`ALARM WDEO
`DATA TO WCR;
`CONTROL V CR
`TO RECORD
`
`A/G 5A9
`SYSTEM OPERATION
`
`

`

`1
`DISTRIBUTED WIDEO DATA STORAGE IN
`VIDEO SURVELLANCE SYSTEM
`
`FIELD OF THE INVENTION
`This invention is related to Video Surveillance Systems,
`and more particularly to Surveillance Systems of the type
`which include Sensors for advising the System of alarm
`conditions.
`
`BACKGROUND OF THE INVENTION
`A conventional closed-circuit Video Surveillance System,
`indicated generally by reference numeral 10, is illustrated in
`block diagram form in FIG. 1.
`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 operations 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 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, windows or display cases.
`The System host 12 operates to inform the human operator
`of the alarm conditions, and predetermined camera move
`ments and routing of corresponding video signals to pre
`Selected monitorS 22 may be automatically actuated in
`response to alarm Signals. Movement of the cameras can
`also be controlled through commands issued by the System
`host, in response to pre-programmed Surveillance routines,
`or in response to operator input through the control consoles
`24 or the user interface 30.
`It is generally a desired goal in Video Surveillance Systems
`that Video Signals representative of conditions before, during
`and after the onset of an alarm event be 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 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
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`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. 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
`recording by a dedicated VCR. There are, however, disad
`Vantages 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 resolu
`tion of the image Signal is reduced, which may impair
`recognition of critical elements of the recorded images. On
`the other hand, time lapse 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
`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. Pat.
`Nos. 5,109,278 and 5,111,291 (assigned to 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. Although the System
`disclosed in the Com Ed patents provides Some capability
`for capturing and Storing Signals indicative 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
`CCS.
`
`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 buffer Storage is provided for
`Video signals generated by all of the cameras in the System,
`and it is a further object to provide such buffer storage for
`all cameras in the System even when the number of cameras
`is large.
`It is still a further object of the invention to provide a
`Video Surveillance System in which a human operator can
`conveniently control buffering and permanent 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
`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 con
`ventional Systems.
`According to an aspect of the invention, there is 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.
`Further in accordance with this aspect of the invention,
`the connection circuitry may include a plurality of control
`devices each operatively connected to a respective one of the
`
`

`

`3
`Storage devices for controlling the respective Storage device
`to operate as a ring buffer in 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 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 the
`invention may further include a display device 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 Station for recording
`Signals corresponding to the digital video data read out from
`the Storage device and transmitted to the central Station.
`The connection circuitry may include a common data
`transmission channel by which command Signals are trans
`mitted 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. Pref
`erably 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
`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 signal, and a Storage device, disposed within the
`housing, for Storing digital data representative of the Video
`Signal.
`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 conver
`Sion circuitry and the processing circuitry may be integrated
`with the Video camera So that the Video camera outputs the
`compressed Video data to the Storage device.
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`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 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 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 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 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 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
`circuitry includes circuitry for converting the analog video
`Signals into digital Video signals and circuitry for compress
`ing 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 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 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
`
`

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`5,982.418
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`S
`Video can be read out and transmitted to an operator
`attended Station at a convenient time for viewing and
`permanent recording on Video tape. The data communica
`tion network used for transmission of command and Status
`data between the System central Station and the Video
`cameras may 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.
`The foregoing and other objects, features and advantages
`of the invention will be further understood from the follow
`ing detailed description of preferred embodiments and prac
`tices of the invention and from the drawings, wherein like
`reference numerals identify like components and parts
`throughout.
`
`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. 5A and 5B together form a flow chart of processes
`carried out in a host computer that is part of the System of
`FIG. 2.
`
`6
`camera control consoles, although not shown, may be pro
`vided in the system at or remote from the central station 102,
`and connected to the host 104 through the LAN 112.
`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
`VeW.
`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 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
`CCD 142 and the resulting Signal is Subjected to conven
`tional 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 of the present
`application, Sensormatic ElectronicS Corporation, as part
`number 2003-0037-00. Other Suitable 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
`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
`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 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 card available from AITech International,
`Fremont, Calif. 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 accor
`dance with the H.261 standard, and also provides the func
`tion 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 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
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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
`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
`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 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
`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
`55
`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.
`Although the user interface 108 is represented 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 consoles
`marketed by the assignee of this application. Additional
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`the video Signal digitizing and compression functions car
`ried 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 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 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 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
`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
`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
`video codec 148 is also stored in the memory 150. Prefer
`ably 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
`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 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 cor
`responding to a time interval beginning at a predetermined
`time prior to receipt of the alarm command, and continuing
`for a predetermined time after receipt of the alarm com
`mand.
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`The control circuit 134 is also conditioned (as 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
`other data that has previously been stored in the buffer
`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 compressed Video data that was read out from the
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`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 does not interrupt ongoing Storage of
`“live' video data.
`As indicated by decision block 166, the control circuit 134
`is also conditioned to receive “clear commands, in response
`to which the control circuit removes a previously protected
`block of stored video data from protection, thereby permit
`ting “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. 5A and 5B, which
`illustrate processes carried out by the host computer 104.
`As indicated at block 170 in FIG. 5A, one ongoing
`process performed by the host computer 104 entails con
`trolling camera movements, etc., in response to instructions
`input by users of the System. The instructions may be
`received either through the user interface 108, or via sepa
`rate 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 transmis
`Sion of the command data Via LAN 112, and camera
`movements (pan, tilt, Zoom, etc.) in response to the com
`mands are all preferably carried out in a conventional
`manner, and therefore will not be described further. The
`process 170 can also be considered as encompassing receipt
`and processing of Status data transmitted on the LAN 112 by
`the camera units 114, in accordance with conventional
`practice. Although the discussion up to this point has Sug
`gested that all user camera control instructions are processed
`in the host 104 and then relayed to the cameras 114 via the
`LAN 112, it is also contemplated that camera control con
`Soles (not shown) connected to the LAN 112 may generate
`commands that are addressed directly to, and processed only
`by, camera units 114.
`Another ongoing function carried out by the host com
`puter 104 is indicated at block 172 in FIG. 5A, and relates
`to handling user instructions for displaying “live' video
`Signals generated by the cameras 114. In essence, the user
`instruction, as in conventional Systems, Specifies a particular
`camera unit, as well as a particular monitor on which the
`Signals generated by the camera unit are to be d