Abstract:
A system for surveillance of a region of interest. The system includes a number of monitors, each monitor operative to acquire and to transmit periodically a description of a respective portion of the region of interest. The monitors are operative to transmit the descriptions in a first communication channel with insufficient bandwidth for all the monitors to transmit the descriptions simultaneously. The system also includes a control station for allocating respective portions of the channel to the monitors for the transmissions according to a number of rules. One or more of the rules includes feedback from the monitors that is separate from the descriptions.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to video surveillance and, more particularly, to a system that can be deployed ad hoc for surveillance of a region of interest. 
         [0002]    Closed circuit television systems for local surveillance are well-known. Such systems deploy a group of video cameras at fixed locations in a region of interest such as a building complex. The cameras are connected by coaxial cables to a monitoring station where an operator views the video streams from the cameras on video screens or on respective portions of a single video screen. If the number of cameras is relatively small, each camera has a dedicated screen or a dedicated screen portion. Otherwise, there are fewer screens or screen portion than cameras, and either the cameras are rotated among the screens or among the screen portions automatically or the operator rotates the cameras among the screens or among the screen portions, with each screen or screen portion being assigned to different cameras at different times. Usually, the video streams also are recorded. 
         [0003]    There are situations, especially in security operations, where it would be highly advantageous to set up a similar system temporarily, e.g. for the duration of a short, local, military or police operation. Examples of such security operations include: 
         [0004]    Hostage rescue. A building in which hostages are held would be placed under video surveillance for the duration of the rescue mission. In addition, video cameras would be deployed as needed by security forces entering the building as the forces advance towards the hostages. 
         [0005]    Ambush. Video cameras would be deployed to monitor possible arrival routes of a target to be intercepted. 
         [0006]    Perimeter defense. Video cameras would be deployed at the perimeter of a position held by a military unit to warn against attack. 
         [0007]    Emergency Roadblock. Video cameras would be deployed around the roadblock and around roads leading to the roadblock. 
       SUMMARY OF THE INVENTION 
       [0008]    According to the present invention there is provided a system for surveillance of a region of interest, including: (a) a plurality of monitors, each monitor operative to acquire and to transmit periodically a description of a respective portion of the region of interest, the monitors being operative to transmit the descriptions in a first communication channel having insufficient bandwidth for all the monitors to transmit the descriptions simultaneously; and (b) a control station for allocating respective portions of the channel to the monitors for the transmissions according to a plurality of rules; wherein at least one of the rules includes feedback from the monitors that is separate from the descriptions. 
         [0009]    According to the present invention there is provided a method of surveillance of a region of interest, including the steps of: (a) deploying, in the region of interest, a plurality of monitors, each monitor operative to acquire and transmit periodically a description of a respective portion of the region of interest; (b) providing a communication channel having insufficient bandwidth for all the monitors to transmit the descriptions simultaneously; and (c) allocating respective portions of the communication channel to the monitors for the transmissions according to a plurality of rules, at least one of the rules including feedback from the monitors that is separate from the descriptions. 
         [0010]    A basic system of the present invention, for surveillance of a region of interest, includes a plurality of monitors and a control station. Each monitor acquires and periodically transmits, preferably wirelessly, a “description” of the portion of the region of interest that that monitor monitors. A “description” of a portion of the region of interest is one or more signals that constitute an instantaneous description of an aspect of that portion of the region of interest. Examples of such “descriptions” include a video frame, an audio signal sample, and a set of measured values of environmental variables such as temperature, pressure and chemical species concentrations. The monitors transmit their descriptions in a first communication channel that does not have enough bandwidth for all the monitors to transmit their descriptions simultaneously. Therefore, the control station allocates respective portions of the first communication channel to the monitors, which means that the control station decides which monitors are allowed to transmit at any given time, and in which respective portions of the first communication channel. For example, the monitors may share a set of frequency sub-channels that are fewer in number than the number of monitors, in which case the control station decides which monitors are allowed to transmit, and in which respective frequency subchannels the monitors are allowed to transmit; or the monitors may rotate among a set of time slots that are fewer in number than the number of monitors, in which case the control station decides which monitors are allowed to transmit, and in which respective time slots the monitors are allowed to transmit. The portions of the first communication channel are allocated according to a plurality of rules. The rules do not provide merely for periodic cycling through all the monitors. At least one of the rules takes into account feedback from the monitors that is separate from the transmitted descriptions (although the feedback could be related to the transmitted descriptions, or could be based on the transmitted descriptions). 
         [0011]    Preferably, the feedback from the monitors is transmitted, most preferably wirelessly, by the monitors to the control station in a second communication channel that is separate from the first communication channel. Most preferably, the second communication channel has sufficient bandwidth for all the monitors to transmit their feedback substantially simultaneously. This is because the feedback typically is very sparse, in terms of data volume, compared to the descriptions. In fact, the second communication channel typically is much narrower than the first communication channel and nevertheless is wide enough to accommodate substantially simultaneous transmissions of all the feedback. 
         [0012]    Preferably, each monitor is operative to identify one of the descriptions that the monitor acquires as an unusual description. The feedback from the monitors includes notification by the monitors of the occurrences of the unusual descriptions. The processing of the descriptions in this preferred system of the present invention is distributed, in the sense that it is the monitors, and not the control unit, that decide which descriptions are unusual. The criteria that a monitor uses to identify unusual descriptions may be either absolute criteria or relative criteria, for example criteria relative to a plurality of descriptions previously acquired by the monitor. Examples of absolute criteria include the brightness of a video frame exceeding an absolute threshold and the value of a pressure measurement exceeding an absolute threshold, either of which is indicative of an explosion in the region of interest. Examples of relative criteria include the brightness of a video frame exceeding the brightness of N preceding video frames by a threshold and a value of a pressure measurement exceeding the values of N preceding pressure measurements by a threshold, either of which is indicative of an explosion in the region of interest. Most preferably, the descriptions include video frames and each monitor identifies, as an unusual description, a video frame that includes unusual motion relative to a plurality of video frames previously acquired by the monitor. 
         [0013]    Preferably, the monitors transmit the descriptions by broadcasting the descriptions, and the system also includes one or more terminals, separate from the control station, for receiving and displaying the broadcast descriptions. For example, if the descriptions include video frames, each terminal includes a video screen for displaying the video frames; if the descriptions include audio samples, each terminal includes a microphone for “displaying” the audio samples; and if the descriptions include values of temperature measurements, each terminal includes a display such as an LCD display for displaying the temperature values. Most preferably, each terminal is operative to receive descriptions from only one monitor at a time, and the control station is operative to decide, for each terminal, which monitor&#39;s descriptions are received by the terminal. 
         [0014]    Preferably, the control station includes a plurality of display devices for displaying the descriptions currently being transmitted by the monitors. Most preferably, there are as many display devices as the number of monitors that transmit simultaneously in the first communication channel, and the control station assigns each display device to a respective one of the simultaneously transmitting monitors. 
         [0015]    Preferably, each monitor is operative to record the displays that the monitor acquires while the monitor is denied permission to transmit in the first communication channel. 
         [0016]    Corresponding to the basic system of the present invention, the present invention includes a method of surveillance of a region of interest. The monitors are deployed in the region of interest. Because the communication channel that the monitors use to transmit the descriptions is not wide enough for all the monitors to transmit simultaneously, respective portions of the communication channel are allocated to the monitors according to a plurality of rules. At least one of the rules takes into account feedback from the monitors that is separate from the descriptions. 
         [0017]    Optionally, the rules are modified while the monitors are transmitting the descriptions. 
         [0018]    Preferably, the monitors transmit the descriptions by broadcasting the descriptions. To each monitor, that has been allocated a portion of the communication channel for transmitting the monitor&#39;s descriptions, is assigned a respective terminal, and the descriptions that are broadcast by the monitor are displayed using the monitor&#39;s respective terminal. Note that the assigning of a terminal to a monitor need not be for the entire duration of that monitor&#39;s sharing of the communication channel. A terminal may be reassigned from one monitor to another at any time. Also note that more than one terminal may be assigned to the same monitor. That a terminal is “respective” to a monitor means that while the terminal is assigned to the monitor the terminal displays only descriptions that are broadcast by that monitor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
           [0020]      FIG. 1  is a high-level block diagram of a video system of the present invention; 
           [0021]      FIG. 2  is a high-level block diagram of one of the video cameras of  FIG. 1 ; 
           [0022]      FIG. 3  is a high-level block diagram of the control station of  FIG. 1 ; 
           [0023]      FIG. 4  is a high-level block diagram of one of the personal terminal of  FIG. 1 ; 
           [0024]      FIG. 5  illustrates an exemplary deployment of a system of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    The present invention is of a surveillance system which can be deployed temporarily for surveillance of a region of interest. Specifically, the present invention can be used for local reconnaissance in military operations. 
         [0026]    The principles and operation of a surveillance system according to the present invention may be better understood with reference to the drawings and the accompanying description. 
         [0027]    Referring now to the drawings,  FIG. 1  is a high-level block diagram of a system  10  of the present invention, and  FIGS. 2-4  are high-level block diagrams of the components of system  10  that are illustrated in  FIG. 1 . 
         [0028]      FIG. 1  shows that system  10  includes a set of video monitors  12 , a control station  14  and a set of portable user terminals  16 . 
         [0029]    Video monitors  12  are deployed in a region of interest. Each video monitor  12  acquires successive video frames of its own field of view in the region of interest and broadcasts those video frames in a common wireless video channel  18 . The broadcasts are received by control station  14 . As discussed below, there is not enough bandwidth in video channel  18  for all video monitors  12  to broadcast simultaneously, so control station  14  decides which video monitors  12  are allowed to broadcast and allocates respective subchannels of video channel  18  to those video monitors  12 . Control station  14  uses wireless transmissions in an RF channel  20  to inform video monitors  12  which video monitors  12  are allowed to broadcast and in which respective subchannels of video channel  18 . The decisions of control station  14  as to which video monitors  12  are allowed to broadcast are based in part on information transmitted back to control station  14  by video monitors  12  via RF channel  20 . 
         [0030]    The function of each user terminal  16  is to allow a user of user terminal  16  to view the broadcasts of one of video monitors  12 . Each user terminal  16  is capable of receiving and displaying video frames that are broadcast in one of the subchannels of video channel  18 . Control station  14  determines for each user terminal  16 , via transmissions in RF channel  20 , which subchannel that user terminal  16  receives and displays. 
         [0031]      FIG. 2  is a high-level block diagram of a video monitor  12 . In  FIG. 2 , the flow of video signals is indicated by thick arrows and the flow of control signals is indicated by thin arrows. The flow of video signals is from a video camera  22  to a processor  24  to a compressor  26  to a video recorder  28  to a video transmitter  30 . Video camera  22  captures images of its field of view, which also is the field of view of video monitor  12 , as video frames. The video frames are processed as described below by processor  24 , compressed by compressor  26 , optionally recorded by video recorder  28  and optionally broadcast in video channel  18  by video transmitter  30  using an antenna  38 . Processor  24  preferably is a general-purpose programmable computer that, apart from processing the video frames, also exercises general control over video monitor  12 . Compressor (e.g. a MPEG4 compressor)  26  preferably is implemented in firmware. Video recorder  28  preferably is based on a memory, such as a RAM, with sufficient capacity to hold the compressed video frames that video recorder  28  needs to store. 
         [0032]    Video monitor  12  also includes a RF transceiver  32  that transmits and receives in RF channel  20  via an antenna  40 . Transceiver  32  receives from control station  14  instructions of whether, and on which subchannel of video channel  18 , to transmit video frames, and operates video transmitter  30  in accordance with those instructions. Transceiver  32  also receives from processor  24  information, derived from the video frames as described below, that control station  14  needs to decide whether to allow video monitor  12  to broadcast, and relays that information to control station  14 . 
         [0033]    These components are housed together in a common housing  36  that is equipped with a mechanism  34  for mounting and dismounting video monitor  12  as needed in the region of interest. Mechanism  34  also is operative to tilt housing  36  so as to change the field of view of video monitor  12  in response to commands received from control station  14  via RF channel  20 . The field of view of video monitor  12  also is modified by commands received from control station  14  to video camera  22  to zoom in and out. 
         [0034]    The recording by video recorder  28  and the broadcasting by video transmitter  30  are “optional” in the sense that video recorder  28  records video frames only if instructed to do so by processor  24  and video transmitter  30  broadcasts video frames only if instructed to do so by transceiver  32 . 
         [0035]    The processing by processor  24  is intended to identify video frames that indicate that something “interesting” is going on in the field of view of video monitor  12 , and to notify control station  14  of that fact using transceiver  32 . The kind of processing done by processor  24  depends on whether the “uninteresting” state of the scene that is observed by video monitor  12  is expected to be normally static or normally dynamic. For example, in some applications, such as perimeter defense, any significant motion in the scene is of interest, so that the “unteresting” state of the scene is a static state. The video frames of a normally static scene therefore are processed by the well-known and standard technique of video motion detection. In other applications, some motion is normally expected in the scene, so that the scene is normally dynamic. The video frames of a normally dynamic scene are processed to detect unusual motion of interest. Methods of detecting unusual motion are known in the art. See for example H. Zhong et al., “Detecting unusual activity in video”,  IEEE Computer Society Conference on Computer Vision and Pattern Recognition  (CVPR&#39;04), Washington D.C., vol. 2 pp. 819-826, 27 Jun.-2 Jul. 2004. Typically, in the present invention, the detection of unusual motion is based on statistics of low-level features such as optical flow direction and the amount of motion in the scene. In addition, video frames of both normally static scenes and normally dynamic scenes are processed to detect unusual low-level video frame feature statistics that are not related to motion. For example, a sudden overall brightening of a scene could indicate an explosion. 
         [0036]    The departure from the norm of a low-level statistic of a video frame may be a relative departure, such as a sudden brightening relative to a moving average of the preceding frames, or an absolute departure, such as brightening above a predetermined threshold. The detection of unusual low-level feature statistics require a learning period during which processor  24  learns the statistical distribution of “usual” feature statistics. The advantage of an absolute criterion is that no such learning period is required. 
         [0037]    When processor  24  identifies an unusual event in the field of view of video monitor  12 , processor  24  uses transceiver  32  to notify control station  14  of the occurrence and nature of the unusual event. 
         [0038]      FIG. 3  is a high-level block diagram of control station  14 . As in  FIG. 2 , the flow of video signals is indicated by thick arrows and the flow of control signals is indicated by thin arrows. Control station  14  includes several video receivers  44  and associated decompressors  46  and display screens  48 , as well as a RF transceiver  52 , all under the control of a controller  42 . Video receivers  44  share a common antenna  50 . Controller  42  instructs each video receiver  44  to receive video broadcasts in a respective subchannel of video channel  18 . Each video receiver  44  sends the compressed video frames thus received to a corresponding decompressor  46  that decompresses the video frames and sends the decompressed video frames to a corresponding display screen  48  for display. Controller  42  also uses transceiver  52  to send instructions to video monitors  12  regarding which video monitors  12  are to broadcast in which subchannels of video channel  18  and to receive information from video monitors  12 . Transceiver  52  transmits and receives the associated RF signals in RF channel  20  via an antenna  54 . 
         [0039]    Decompressors  46  preferably are implemented in firmware. 
         [0040]    Controller  42  is based on a conventional computer that includes a processor, a nonvolatile read/write memory such as a hard disk or a flash disk, and suitable peripherals for input and output. Controller  42  decides which video monitors  12  are allowed to broadcast, and in which subchannels of video channel  18 , according to rules that are stored in the nonvolatile memory of controller  42 . An example of a set of such rules is given below. The operator of control station  14  can modify or replace the rules at any time, even during an ongoing operation. The operator of control station  14  also can override the rules manually at any time, for example to tilt or zoom the field of view of a video monitor  12 . 
         [0041]    Controller  42  also uses transceiver  52  to assign each user terminal  16  to a respective video monitor  12 , by instructing each user terminal  16  which subchannel of video channel  18  to tune to. 
         [0042]      FIG. 4  is a high-level block diagram of a user terminal  16 . As in  FIGS. 2 and 3 , the flow of video signals is indicated by thick arrows and the flow of control signals is indicated by thin arrows. User terminal  16  includes a video receiver  56 , a decompressor  58 , a display screen  60 , a RF transceiver  66  and a control panel  68 . Video receiver  56  receives broadcasts on video channel  18  via an antenna  62 . Transceiver  66  transmits and receives in RF channel  20  via an antenna  64 . In particular, transceiver  66  receives, from control station  14 , instructions that assign user terminal  16  to a particular subchannel of video channel  18 . In response to these instructions, transceiver  66  tunes video receiver  56  to that subchannel of video channel  18 . Video receiver  56  then receives video broadcasts in that subchannel. The video frames thus received are decompressed by decompressor  58  and are displayed at display screen  60 . 
         [0043]    Preferably, decompressor  58  is implemented in firmware. 
         [0044]    The user of user terminal  16  uses control panel  68  to send to control station  14  requests to change the subchannel of video channel  18  that is assigned to user terminal  16  and to change the field of view of the corresponding video monitor  12 . In one exemplary configuration of system  10 , the user of user terminal  16  is free to override manually the subchannel selections and fields of view that are determined by the rules of control station  14  but control station  14  is free to override the subchannel and field of view that is selected by the user of user terminal  16 . 
         [0045]    Note that in contrast to video channel  18 , that lacks sufficient bandwidth for all video monitors  12  to broadcast their video frames simultaneously, RF channel  20  preferably is wide enough to accommodate simultaneously the wireless transmissions of all transceivers  32 ,  52  and  66 . 
         [0046]    System  10  is intended to be used by a team of military or law enforcement personnel. Typically, the team leader is the operator of control station  14  and every other team member is given a respective user terminal  16 . 
         [0047]      FIG. 5  illustrates an exemplary deployment of system  10 . In this example, a squad of soldiers is assigned to intercept a suspicious vehicle that is expected to travel from right to left on a road  84 . Four video monitors  12 A through  12 D are deployed around the squad&#39;s position to guard against hostile forces approaching four sectors A, B, C and D. Two video monitors  12 E and  12 F are deployed to monitor, respectively, traffic approaching on road  84  from the left and traffic approaching on road  84  from the right. Video monitor  12 A also monitors traffic on road  84 . The squad commander  70  is stationed in the middle of the squad&#39;s position along with a control station  14  (not shown). Three soldiers  72 ,  74  and  76  are stationed at road  84  to intercept the suspicious vehicle. Three soldiers  78 ,  80  and  82  are stationed in sectors B, C and D, respectively, to guard against a counterattack. Each soldier has his own user terminal  16  (not shown). In this example, video channel  18  is wide enough for only two video monitors  12  to broadcast simultaneously and control station  14  has only two sets of video receivers  44 , decompressors  46  and video screens  48 . In other words, video channel  18  includes only two video broadcast subchannels, which are designated subchannels  1  and  2 . 
         [0048]    The following are the rules that control station  14  uses to allocate subchannels  1  and  2  among video monitors  12 . 
         [0049]    Regime I: Default: Subchannel  1  cycles through video monitors  12 A,  12 B,  12 C and  12 D. Subchannel  2  cycles between video monitors  12 E and  12 F. Each video monitor  12  is given the same broadcast time. Soldier  72 , the second-in-command, receives broadcasts from video monitors  12 E and  12 F on his user terminal  16 . Soldiers  74  and  76  receive broadcasts from video monitor  12 A on their user terminals  16 . Soldier  78  receives broadcasts from video monitor  12 B on his user terminal  16 . Soldier  80  receives broadcasts from video monitor  12 C on his user terminal  16 . Soldier  82  receives broadcasts from video monitor  12 D on his user terminal  16 . 
         [0050]    Regime II: Movement detected by video monitor  12 A,  12 B,  12 C or  12 D: Subchannel  1  is dedicated to the video monitor  12  that detected movement. Subchannel  2  cycles through the other three video monitors that are deployed around the squad&#39;s position. Each video monitor of subchannel  2  is given the same broadcast time (e.g., five seconds each per cycle). Soldier  72  receives broadcasts from the video monitor  12  that detected movement. Soldiers  74  and  76  receive broadcasts from video monitor  12 A. Soldier  78  receives broadcasts from video monitor  12 B. Soldier  80  receives broadcasts from video monitor  12 C. Soldier  82  receives broadcasts from video monitor  12 D. 
         [0051]    Regime III: Leftbound traffic detected by video monitor  12 F: Subchannel  1  is assigned to one of the video monitors  12  other than video monitor  12 A, according to the motion detected by those video monitors  12 , with priorities determined as follows:
         12 F (leftbound traffic)&gt; 12 E (rightbound traffic)&gt; 12 B (any motion)&gt; 12 D (any motion)&gt; 12 C (any motion)
 
After the leftbound traffic that triggered regime III has departed the field of view of video monitor  12 F, subchannel  1  is assigned to one of the other video monitors  12  according to these priorities. Then, for example, if subchannel  1  is assigned to video monitor  12 B and video monitor  12 E subsequently detects rightbound traffic, subchannel  1  is reassigned to video monitor  12 E. Meanwhile, subchannel  2  is dedicated to video monitor  12 A. Soldiers  72 ,  74 ,  76  and  78  receive broadcasts from video monitor  12 A with high priority, for identification of the suspicious vehicle, and from video monitor  12 A with medium priority, to monitor vehicles approaching the squad&#39;s position. Soldier  82  receives broadcasts from video monitor  12 E with high priority and from video monitor  12 A with medium priority, to monitor vehicles approaching from the left. Soldier  80  receives broadcasts from video monitors  12 B,  12 C and  12 D with high priority and from video monitor  12 A with medium priority.
       
 
         [0053]    Both regimes II and III have priority over regime I. Regime III has priority over regime II. In other words, receipt by control station  14  from any of video monitors  12 A,  12 B,  12 C or  12 D of notification of an unusual event causes control station  14  to switch from regime I to regime II, and receipt by control station  14  from video monitor  12 F of notification of an unusual event causes control station  14  to switch from either regime I or regime II to regime III. 
         [0054]    While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.