Television-based alarm system
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an alarm system for detecting a movement or a change in a field of view supervised by a television camera. 
     2. Description of the Prior Art 
     In German Published Patent Specification No. 22 55 876 there is disclosed a television-based alarm system wherein uniform brightness regions within the scene viewed by a television camera are suppressed. For this purpose the video signal produced by the television camera is taken through a resonance filter. The signal thus obtained is rectified and transferred to a capacitively coupled band-pass filter. The band-pass filter serves for eliminating very slow or very quick changes. Thereafter any resulting single variation indicates a scene variation in the supervised field of view and causes a signal to be emitted. The response reliability of such an alarm system is small because it reacts only to relatively large dynamic changes of scene. A further disadvantage is that the response sensitivity does not vary and cannot therefore be suited to the detection of different scenic events. 
     Furthermore there is disclosed in German Published Patent Specification No. 20 02 478 an arrangement for supervising objects with the use of a television camera, wherein there is connected to the television camera an amplitude filter combined with an electronic counter. Upon a prescribed count being exceeded an alarm is actuated. In this arrangement also the reliability of response is low. This is so primarily because it is not possible to adapt to alternating light conditions. Furthermore variable components such as brightness, structure, movement or direction of movement cannot be evaluated differentially. 
     It is an object of the invention to provide an improved television-based alarm system which has, or may be designed to have, a response sensitivity and response characteristic which can be varied. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided an alarm system for detecting a movement or change in the field of view supervised by a television camera, the system comprising a discriminator responsive to the video signal produced by the camera and adapted to produce a signal upon the occurrence of a predetermined characteristic in the picture content of the video signal, at least one means for providing in synchronism with the video signal from the camera a masking signal defining one or more portions of the field of view from which discriminator signals are not required, means for logically coupling the at least one masking signal and the discriminator signals in order to remove any discriminator signals derived from the said one or more portions of the field of view, and means for evaluating the discriminator signals remaining after said logical coupling according to a predetermined criterion to produce an alarm signal when said criterion is satisfied. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     An embodiment of the invention will now be described by way of example, with reference to the accompanying drawing whose single FIGURE is a block schematic diagram of a television-based alarm system. 
    
    
     DESCRIPTION OF A PREFERRED EMBODIMENT 
     For the purpose of extending the field of protection to larger objects, or to a plurality of objects remote from one another, the alarm system comprises a plurality of television cameras 1 1 , 1 2  . . . 1 n . The video signals generated by these television cameras are sequentially evaluated in time-multiplex mode. For this purpose the individual video signals are delivered to a controlled multi-position switching device 2, the position of the movable contact of which is controlled by a control signal (n) generated by a control device 3. Although the drawing shows the switch 2 symbolically as a mechanical switch, it is to be understood that the switch 2 (and the switches 4, 7, 12 and 15 to be described later) is in fact an electronic switch, for example a semiconductor switch. The cycle time of the control device 3 is so designed that each video signal is passed for evaluation for the duration of a plurality of fields before the video signal generated by another one of the cameras is switched in. The signal available at the output of the first switching device 2 is delivered to a second controlled multi-position switching device 4. To the outputs of the second switching device 4 there are connected a plurality of discriminators 5 1 , 5 2  . . . 5 m , any of which can be switched to receive the available video signal by a further control signal (m) generated by the control device 3. The discriminator 5 1  to 5 m  possess different discriminating functions. The selection of the particular discriminating function at any particular time depends upon the desired characteristic to be detected in the particular scene in the field of view of the television camera whose video signals are at that time passed through the switch 2. The connection sequence of discriminators 5 bears a predetermined relationship to the connection sequence of the cameras 1 and is &#34;programmed&#34; into the control device 3 so that as each camera 1 is switched in the appropriate discriminator 5 is also switched in to receive the video signal from the selected camera. 
     Each discriminator 5 1  to 5 m  serves to a first approximation to convert the analog video signal into a series of counting pulses representing the scene viewed by the camera, each pulse being produced upon the occurrence of a particular characteristic in the picture content of the received video signal. For example, one of the discriminators may be an edge discriminator, which, upon a predetermined rate of signal level variation being exceeded in the picture signal delivers a pulse independently of the absolute signal level. For particular scenes other types of edge discriminator may, however, be used, for example discriminators which detect a particular magnitude of edge being exceeded or which detect the repetition of edges above a particular frequency. Because an edge discriminator only registers structural variations, it is particularly suited for evaluating pictures which are characterised by relatively rapid temporary fluctuations such as brightness, e.g. open air scenes (with rapidly vairable lighting caused by moving clouds) but which are irrelevant for the alarm system. 
     A different kind of discrimination function may be achieved with amplitude discriminators. For example one of the discriminators may be an amplitude discriminator which always delivers a counting pulse if the brightness of the picture exceeds a predetermined threshold value. However other amplitude-dependent discriminators may be used for the detection of certain scenic events, for example a discriminator having a locally-dependent variable threshold or one having several thresholds which are evaluated either additively or selectively on a weighted basis. By means of a discriminator using frequency discrimination, such as frequency selection by filtering or spectral analysis of the video signals, it is possible to derive counting pulses representing further characteristics of the scene under consideration. As mentioned before the particular discriminator 5 selection by the control device 3 for any particular camera 1 is determined beforehand preferably individually and depends upon the nature of the scene viewed and the variations thereof which are to be expected and which it is desired to detect. It is to be noted that discriminators having the functions previously defined are well known in the television art and therefore no further description thereof is deemed necessary. 
     The counting pulses generated at the outputs of the discriminators 5 1  to 5 m  are delivered to one input of an AND gate 6. The other input of the AND gate 6 is connected to the output of a third multi-position controlled switching device 7. At the inputs of the third switching device 7 there are applied masking signals from a plurality of mask generators 8 1 , 8 2  . . . 8 i , which signals serve for the detection of certain directions of movement in the scene under consideration. The third switching device 7 is also controlled by a control signal (i) generated by the control device 3. 
     The masking signals are synchronous with the video signals and each is arranged to allow passage through the AND gate 6 of only those counting pulses derived in respect of predetermined portions of the field of view of the particular camera 1 connected to the switching device 4. 
     One of the mask generators 8 is preferably arranged to provide a masking signal which limits passage through the AND gate 6 only to those counting pulses derived from vertical strips of the scene being viewed. In combination with a suitable discriminator 5 this enables detection of horizontal movements in the scene. 
     Another masking generator 8 is preferably arranged to allow passage of pulses only from horizontal strips of the scene, whereby vertical movements may be detected. Yet another masking generator 8 may be arranged to allow passage of counting pulses from portions of the scene arranged in the manner of a checker board, whereby directionally independent movements may be detected. 
     As in the case of the discriminators 5, the connection sequence of the mask generators 8 bears a predetermined relationship to the connection sequence of the cameras 1 and is programmed into the control device 3 so that as each camera 1 and discriminator 5 is switched in the appropriate mask generator 8 is also switched in. Mask generators are well known in the television art and do not require detailed description here. 
     The counting pulses which are passed by the selected masking signal at the AND gate 6 are delivered to the input of a second AND gate 9. To the other input of the AND gate 9 there is delivered a further masking signal derived from the output of a further multi-position controlled switching device 12. The purpose of the further masking signal is to limit passage through the AND gate 9 only to counting pulses derived from predetermined portions of the total field of view, for example one or more rectangular portions. The masking provided by the masking signal at AND gate 9 is thus superimposed on that provided by the masking signal at AND gate 6, whereby motion in only a small part of the field of view may be detected, for example. 
     The masking signal at AND gate 9 is synchronous with the video signal and is produced by whichever of the mask stores 13 1  . . . 13 n  is connected to the output of the switching device 12, each of the mask stores 13 being connected to a respective input of the device 12. 
     As before, the connection sequence of the mask stores 13 to the output of the switching device 12 bears a predetermined relationship to the connection sequence of cameras 1 as determined by the programmed control device 3. As each camera 1 is switched in so is the appropriate mask store 13. 
     The masking signals stored in the respective stores 13 are generated by a common mask generator (not shown) selectively connectable to each store 13. This mask generator is adjustable thereby to generate for storage masking signals defining different sizes and/or different locations in the field of view for the rectangular portions of the field of view from which the counting signals to be passed by the AND gate 9 are to be derived. Thus each store 13 can be provided with a different masking signal and, furthermore, the adjustable masking generator permits the masking signal in each store 13 to be changed if desired merely by providing a new masking signal for storage. Adjustable mask generators and mask stores are known in the television art. 
     The counting pulses passed through the AND gate 9 pass through a sequence low pass filter 10 for the suppression of pulses which are too small, to one input of a counter-comparator combination 11. For each camera 1 in connection with the system the counter-comparator combination 11 sums for one field period the individual counting pulses representative of the detected characteristic in the picture content of the video signal and compares the sum with a stored sum derived from the same television camera but during an earlier field period. The previously derived sums are stored in stores 14 1 , 14 2  . . . 14 n . The allocation of stored sums for comparison is effected by means of a fifth multi-position controlled switching device 15, the movable contact path of which is switched in synchronism with that of the switching device 2 by the same control signal (n) generated by the control device 3 as is used for the switching of the video signal by the first controlled switching device 2. In the event of there being a predetermined excess or deficit in the sum as compared with the previously stored sum, the counter-comparator combination 11 delivers a so-called &#34;event&#34; pulse, which is delivered to an evaluation device 16. 
     In the evaluation device 16 the number of pulses relating to the interval of the event is compared with a preselectable value. If the number of pulses relating to the event exceeds this value then an alarm signal is emitted by the evaluation device 16. 
     By the choice of an appropriate discriminator function in combination with an appropriate movement detecting mask and an appropriate mask for limiting the field of view of a television camera there may be obtained a criterion for the production of an alarm in the event of almost any type of picture variation in a scene within the field of view of the television camera.