WO2003096295A1

WO2003096295A1 - Verified alarms

Info

Publication number: WO2003096295A1
Application number: PCT/GB2003/001979
Authority: WO
Inventors: Peter Dickinson; Nigel Couch; Alan Ruffell
Original assignee: Central Research Laboratories Limited
Priority date: 2002-05-13
Filing date: 2003-05-09
Publication date: 2003-11-20
Also published as: GB0210887D0

Abstract

An apparatus (1) for providing a verified alarm indication in response to a specified event occuring within a scene, said apparatus comprising a video unit for visually monitoring the scene and providing a first analogue output (300), a passive infrared unit (2) for thermally monitoring the scene and providing a second analogue output (200), and at least one logic element (7) adapted to receive and combine said first and second analogue outputs (200, 300) and provide an alarm indication when the combination of said outputs satisfies parameters indicative of said specified event.

Description

VERIFIED ALARMS

Field of the Invention

This invention relates to the visual and thermal monitoring of a scene, and in particular to the use of a passive infrared sensor in combination with a video camera in which the resultant video signals are processed to determine and verify if movement or change is taking place in the viewed scene.

Description of the Prior Art

Alarm systems which are typically used to protect a building employ a series of "make and break" contacts strategically positioned at doors, windows and other potential entry points. When a contact is broken, an alarm is sounded and relayed to a central control station located within the building, nearby to the building or remotely to a central control station of a security company or the police. Alarm systems may also include sensors such as vibration sensors, microwave sensors, laser sensors and radar, and the most common Passive Infrared sensors (PIR), which respond to heat differences caused predominantly by animate objects such as humans or animals.

False alarms plague the security industry. Sensors and "make and break" contacts may fail to take into account changes in a scene which are not due to the movement or the presence of an intruder. For example, inclement weather conditions may rattle doors and windows or cause items such as fans in a room to oscillate breaking contacts or initiating responses from movement sensors. Passive infrared sensors also have a propensity to give such false alarms. For example, when sunlight penetrates through a window, or a warm draft enters from a ventilation or heating system into a scene that is being thermally monitored, a hot spot is generated to which the sensor may wrongly respond

While the situation is annoying when the false alarm is relayed to a local monitoring station the situation is exacerbated when the alarm is relayed several miles to a central monitoring facility responsible for a large number of widely separated facilities. In the past, operators have been required to use their experience of the circumstances surrounding the alarm (i.e. local weather conditions, past alarm history) to determine whether or not the alarm is real. However, as a consequence of false alarms, police often require confirmation or evidence of an intrusion before investigating, or else priority is given to those situations where they have greater certainty an intrusion has occurred.

Alarm systems in which the possibility of false alarms is minimised have been developed. For passive infrared devices false alarms have to an extent been limited by dividing the coverage area of the infrared device into a sectoral views; an alarm is only given when more than one of the sectors senses an alarm event. A closely analogous solution has been to use multiple beam PIR devices wherein the probability of the alarm being a response to a real event is again increased as more than one sensor must triggered before a response is given. However, such devices either require a complex lens structure and processing capability or require very involved installation and wiring techniques which increase the difficulty and cost of fitting and maintaining such systems. Additionally hot spots which generate false alarms can be of sufficient size to be detected by more than one sector or beam in the coverage area of the PIR device; the problems which affect individual undivided beam passive infrared devices are not completely negated. As a consequence of these problems alternative alarm developments focused predominantly on the use of camera surveillance which is capable of providing real time evidence of an event.

The problem with camera surveillance is that one needs a constant communications channel between the sensor (camera) and the operator at the monitoring site. If video is continuously required for a properly functioning system a communications channel having sufficient bandwidth must be constantly connected to the site and the monitoring station, from the time the alarm system is energized to the time the alarm system is turned off. As the monitoring period is typically of the order of several hours, communication costs are high and one way of minimising these costs has been to use slow scan cameras whose output is compressed to transmission rates of 1 frame of video over a 1 to 5 second interval.

Importantly, when monitoring video for isolated, infrequent events the operator will see nothing out of the ordinary for most of the time. This is a serious problem as it has been estimated that after watching a camera for as little a five minutes an operator's performance diminishes rapidly to the point where the operator is essentially ineffective after 30 minutes.

To minimise this human error, video systems having the image processing capability to independently detect motion or other changes in a viewed scene have been developed for applications in security and safety monitoring and industrial process controls. Such systems compare information derived in a television image frame with analogous information derived during a previous frame, and trigger an alarm if the detected difference is greater than a pre-determined threshold.

US Patent No. 5,937,092 (Wooton et al.) describes such a video security system which attempts to eliminate human error in the recognition of an alarm condition and limit the power consumption associated with the communications channels it includes. The video security system comprises an imaging means which continually views a scene and produces a signal representative of that scene. This signal is compared using a processor means with a signal representing the scene at a previous point in time to identify those segments of the scene at said point in time which differ from segments of the scene at earlier point in time. A discriminator means evaluates any differences to establish whether these are caused by the presence of an intruder. The discriminator means uses an absolute difference technique with pixel by pixel subtraction which is sensitive to surface differences between the images, insensitive to light-on-dark or dark-on-light changes and therefore sensitive to intrusion. Each pixel represents a grey level measure of the scene intensity that is reflected from that part of the scene, the most important reason for such a change in intensity being the presence of an intruder. Once the probability of an intrusion has been established in this way without a human operator monitoring images, a communications path is established which enables high frame rate and high quality video to be transferred from the site to the operator which can then evaluate the video and concur with the evaluation of the intrusion.

Typically video systems have utilised either analogue or digital circuitry in image processing. Digital processing of a video image has the advantage that sophisticated results can be achieved which may include object recognition or determination of the direction of movement of an object across a scene. However, digital processing requires a complex processor and concomitantly high power consumption. This means that this technique is not suited to be co-located in situ within the imaging means or sensor head as the unit size, power consumption and cost preclude this. Also the complexity of interfacing an analogue television system with digital equipment gives rise to a relatively high degree of difficulty in maintaining effective and reliable operation of the large number of system components which must co-operate to be effective.

The alternative analogue systems utilise the main characteristic of the video signal, which is an amplitude modulated signal, and process it in one or more of a variety of fashions to derive the desired information from frame to frame. Although such analogue processing systems are relatively simple and inexpensive, and additionally small enough to be co-located within the imaging means or sensor head, they have been regarded previously as lacking flexibility and accuracy.

It has been recognised that amalgamations of more than one type of sensor into a multi-sensor alarm system may negate the problems associated with each sensor type. However, few examples exist where such sensors are combined or collocated in a single apparatus because of the size constraints of the individual parts. Combining and collocating two different types of sensor is additionally problematic as there is an inherent incompatibility in the processing techniques, implementation and installation of the different sensor types. The apparatus sold by Detection Systems Inc. (New York) under the trade names CAM940 and CAM940E is a collocation of a passive infrared unit with a black and white video output. However, the video device does not contribute to the detection of an event in the system or the provision of a verified alarm response.

Consequently, there exists a need in the art to provide an apparatus which provides a verified and accurate alarm response by combining at least two detection methods, and for which the processing is sufficiently simple to enable it to be performed at least substantially in situ. There is also a need in the art to provide an apparatus which is flexible in the locations in which it can be used and which can be manufactured simply. Summary of the Invention

In accordance with a first aspect of the invention there is provided an apparatus for providing a verified alarm indication in response to a specified event occurring within a scene, said apparatus comprising: a video unit for visually monitoring the scene and providing a first output; a passive infrared unit for thermally monitoring the scene and providing a second output; and at least one logic element adapted to receive and combine said first and second analogue outputs and provide an alarm indication when the combination of said outputs satisfies conditions indicative of said specified event.

The video unit and the passive infrared unit can be mounted integrally within a single housing or as conjoined housings. In the latter case, for installing the apparatus it is important that the conjoining of the two units allows slight adjustments to the orientation of the lens of the imaging means or the aperture of the infrared sensor to be made. The field of visual monitoring (i.e. field of view) of the video unit and the coverage area of the infrared sensor can therefore be brought into conformity.

Both means of mounting have advantages. It is acknowledged that one means by which intruders can beat an alarm apparatus or a security system is to tamper with the apparatus or system. Mounting the passive infrared unit and the video unit in a single reinforced housing may deter tampering with the device. Conversely, where the PIR and video unit are mounted in separate housings within the sensor unit, the operator of the system may be able to establish more easily if tampering has occurred i.e. such a mounting provides greater tamper evidence and ease of replacement of that part of the sensor unit which has been tampered with. It is envisaged that in a security system comprising more than one apparatus, different mountings for the units in the apparatus can be used for different locations within the observed area dependent on inter alia the likelihood of an intruder gaining access to the apparatus. Optionally the housings in which the video unit and the passive infrared unit are mounted may be fitted with anti- tamper mechanisms as are known in the art.

The mounting of the passive infrared unit and the video unit in a single housing has the additional advantage that the field of view of the imaging means and the coverage area of the passive infrared unit can be brought into closer alignment. In any event it is preferable that the two units are mounted such that as far as possible substantially all of the scene is monitored simultaneously by both units.

The passive infrared unit to be incorporated in the invention may be any known sensor that is capable of monitoring a scene and which is responsive to movement of an animate object in that scene. Typically such units will comprise a plurality of pyroelectric sensor elements on which infrared radiation from the scene is incident via lenses (such as Fresnel lenses) covering the scene. Movement of objects in the scene produces electrical signals in sensor elements. The sensor elements are connected to a signal processor and a logic unit to process the signals according to selectable algorithms. On the basis of these determinations, the second output of the apparatus is selectively issued, or its magnitude is determinable, when the sensor signals are above a pre-set threshold within a set time interval.

It is preferable that the pyroelectric sensor elements within the passive infrared unit are connected such that the background radiation signal can be subtracted from the signals of each element. Typically such sensor elements will be connected in pairs or in equal numbers. Although not precluded by this invention, it is also preferable for the signals to be processed by simple algorithms. The passive infrared unit provides a response which is verified by the second video sensor and therefore algorithms which are selective on the basis of size of the radiating object, its speed and / or direction are not necessary.

The parameters of the coverage angle of the passive infrared unit, its range of temperature sensitivity and the humidity tolerance will be dependent on the use to which the apparatus is put and in particular its location within any premise monitoring system. For the envisaged applications an operational temperature range of 0° to 50°C is preferred.

The apparatus preferably further comprises a first auxiliary alarm circuitry into which is inputted the first output from said video unit, and wherein the circuitry is capable of providing a first auxiliary alarm indication. Similarly, the apparatus may either additionally or alternatively further comprise second auxiliary alarm circuitry into which is inputted the second analogue output from said passive infrared unit, with this circuitry being capable of providing a second auxiliary alarm indication.

The provision of auxiliary alarm indications provides checks in the apparatus. Where the operation of the apparatus is to be supervised or overseen by an operator it has already been shown that such an operator is unlikely to monitor the output of the video unit continually or efficiently over a long period. These auxiliary signals would make such an operator immediately aware of an event that he has to closely review, prior to the verified alarm indication derived from the combination of the video and infrared unit outputs. Such auxiliary indications also enable the video unit or the infrared unit to operate independently (as non-verified alarm systems) in the event of malfunctions in either of these units or the logic element.

The alarm indication of the apparatus, and the first and second auxiliary alarm indications of its substituent units may be given by any suitable visible or audible alarm signal generator, either alone or in combination and may be provided either at the apparatus or remote therefrom. Suitable audible alarms include a siren, a sound recording of barking dog, and a steady, pulsed or temporal bell pattern. A preferred visible alarm generator is a light emitting diode (LED) or an area illumination device. A person skilled in the art would also be aware of other alert-oriented responses to specified events which do not include sound or light, such as the engagement of

(extra) locking mechanisms in the vicinity of detected movement. Alternatively, when the first and second analogue outputs reach a threshold combination value this may result in the establishment of a communications path with a third party or operator along which high frame rate, high quality video of the scene is transferred directly.

It is envisaged that the alarm indication given in this apparatus could sent by a wireless communications network interconnected with the alarm system. Indications such as voice messages, data, video and other images could be transmitted by communications enabled by means such as packet radio, spread spectrum, cellular technologies, satellite and microwave towers. At the operator or remote station the wireless communications network over which the alarm indication is to be sent could then be further interfaced with a regular computer network. An actuator means, such as a swivel bracket, for displacing the field of view of the video unit and the coverage area of the passive infrared unit is preferably included in the apparatus. Further, the actuator means is preferably operated under the control of a microprocessor which is arranged to displace the apparatus so that the objects of interest remain in the field of view thereby allowing tracking of a moving object.

Preferably the apparatus further comprises at least one monitor which receives a signal from the video unit via a communications path enabling it to visually represent the scene on which the unit is trained. Such a monitor can therefore be viewed by a guard, operator or third party.

The apparatus of the present invention may further include means to discriminate the type of motion which caused the alarm signal or the object which has caused the motion. Some alarm systems which are to be used to protect highly sensitive locations where an invasion of any kind must be indicated may not require such object recognition means. However, in applications where invasions due to animals can be tolerated or where alarms due to object vibrations would be undesirable object recognition means would be preferred. A suitable recognition means include intrinsic sensors having active laser beams which measure the height of a person or object as they move in a location. The output from the object recognition means is then used to gate the output from the video based unit, the passive infrared unit or the combined output from the two said units which feeds the alarm indication means.

In accordance with a second aspect of this invention, the video unit to be included in the apparatus for providing the verified alarm indication comprises an imaging means continuously viewing the scene and providing a signal representative of the scene; a processing circuit for providing at least one first division of the signal representative of the scene at a first point in time and at least one second division of the signal representative of the scene at a later second point in time; and at least one comparison circuit for identifying differences between the provided divisions of the signal; and wherein said first output is dependent on the identified differences. Preferably, the imaging means of the video unit comprises a video camera. Such a camera may operate in the visual or infrared portions of the light spectrum but it is preferred if the scanning system of said camera is preferably 2: 1 interlaced. The parameters of video camera sensitivity, electronic shutter speed, lens focal length and field of view are dependent on the particular use to which the apparatus or video unit is to be put.. However, it is preferred that the axial position of the camera lens is adjustable so that an image in a required plane can be sharpened.

In accordance with a first preferred embodiment of the second aspect of the invention the composite video waveform is subjected to at least one modifying step prior to its being inputted into the processing circuitry. To minimise the complexity of the modifying circuitry included in this apparatus these steps occur in the analogue domain. Accordingly the apparatus preferably comprises an analogue modifier circuit comprising at least one assembly selected from the group consisting of a buffer, a filter and an auxiliary amplifier.

Filtering is achieved by selectively attenuating those components of the input signal which are undesired. Accordingly, the analogue filter circuit may comprise a combination of inductances, capacitances, resistor and capacitors and gyrators. It is preferable in this invention that the filtering of the video signal is adapted to provide a signal which when represented on a screen, for example, has an averaged greyscale level which thereby reduces pixel level noise.

A buffer is an amplifier with unity gain. The auxiliary amplifier receives the signal from the filter and preferably the filter performance of the amplifier is set so as to average the amplitude of the output. The buffer and / or the auxiliary amplifier combine with the filter components to enhance the fixed or tunable passband or rejection characteristics of the filter.

The imaging means or video camera will transmit continuously for the period in which it is used. The processing circuitry of the video unit must be resettable such that the provision of the division of the divisions of the signal at first and second points in time can be repeated. This allows for the differences in the scene to be identified continuously during the monitoring period. When an image from such a video camera is represented on a screen or other display medium, what is seen is a multiplicity of frames of video transmitted sequentially. Each frame constitutes two fields. The representation on the screen (or picture) is the two fields interlaced; the screen displays line 1 from field 1, line 1 from field 2, line 2 field 1 and so forth, wherein line means the lines of the screen which for industry standard televisions would number 405 or 625.

The actual output from the video camera which would be translated by a display medium to give the representation of the scene comprises a composite video signal. This is a signal conveying the information, including colour coding and synchronisation for each field of a frame.

A normal video camera will transmit a new frame every fraction of a second. The magnitude of the time difference between the first and second points in time at which the composite video signal is sampled will be at least equal to the time difference between successive frame transmission by the video camera, and is indeed typically chosen to be a multiple of this latter time difference. Whether sampling occurs in an adjacent frame or in a frame removed from the originally sampled frame is dependent on the type of use to which the apparatus is to be put or the type of motion it is required to sense. Sampling in adjacent fields would produce an apparatus that is sensitive to fast moving objects for example. For the envisaged applications of the apparatus the time difference between sampling points is preferably an integer multiple of the time difference between successive frame transmission, and more preferably said integer is between 2 and 8.

The division of the composite video signal on the basis of time difference can be achieved using simple counting circuitry and as such the processing circuitry preferably comprises first counting circuitry for generating two gating signals and two sample and hold amplifiers into which is input said gating signals and said filtered composite video signal. The sample and hold amplifiers combine said inputs to divide said filtered composite video signal into the two divisions representative of the scene at said first and second points in time. When the comparison identifies the differences between the sample of the whole composite video signal for the given times, the apparatus cannot easily discriminate against whole scene changes, such as ambient light levels. The division of the composite video signal enhances the sensitivity of the video unit to the events it is to monitor. Also, where the filtered composite video signal is divided into levels representative of a portion of a scene, some or each of different divisions of the signal can be compared utilising different time difference between the first and second points in time. This is particularly advantageous when the apparatus is required to be sensitive to different types and rates of motion in different parts of the observed scene.

Therefore, it is preferable that the filtered composite signal be divided into a plurality of levels for each point in time, wherein each level is representative of a segment of the scene. For each of the first and second points in time, the processing circuitry preferably provides at least three divisions of the composite video signal and more preferably the filtered composite video signal is divided into four levels each representative of a quarter of the scene. Although the signal could be split into more than four levels the circuitry required to do this becomes concomitantly more complex and expensive. Each signal division representative of a scene fraction at the first point in time and the division representative of that same fraction for the second point in time are then compared using comparison circuitry.

In accordance with a preferred embodiment of the second aspect of the invention, the processing circuitry comprises first counting circuitry for generating at least one pair of gating signals, second counting circuitry for generating n timing signals, where n > 3; a logic array for combining one pair of gating signals with each of said timing signals to generate 2n trigger signals; 2n sample and hold amplifiers into each of which is input one said trigger signal and said filtered composite video signal; and wherein the sample and hold amplifiers combine said inputs to divide said filtered composite video signal into 2n divisions of which n divisions are representative of n fractions of said scene at said first point in time and n divisions are representative of n fractions of said scene at said second point in time. It is preferable that any counting circuitry included in the processing circuitry can utilise synchronisation information carried by the composite video signal itself to divide this signal into a plurality of sample pulses (at each of the sampling points) and to determine the time difference between the first and second sampling. The value of this is that the synchronisation information ensures that the signal provided by the processing circuitry is representative of the same point in the observed scene for both the first and second points in time.

To provide this synchronisation information the processing circuit of the video unit may further comprise a sync separator into which is inputted a parcel of the composite video signal, said separator splitting said parcel into a composite synchronisation wave parcel and a frame synchronisation wave parcel. The composite video signal inputted into the sync separator may itself be filtered or modified as described hereinbefore. The first counting circuitry provided in the processor uses the frame synchronisation wave parcel to generate the gating signals described above. More preferably, the first counting circuitry comprises a 4-bit cyclic counter into which the frame synchronisation wave parcel is inputted, said counter generating four binary outputs, and a line decoder into which said binary outputs are inputted to derive 16 gating signals. As only two points in time are to be used for signal comparison, the processing circuit is also provided with means for selecting two of these gating signals.

Furthermore, using such inherent synchronisation information, the composite video signal can also be divided into a number of pulses carrying the colour coding and synchronisation information for a horizontal band of each frame. Each such band consists of a fixed number of lines of both the interlaced fields, and whereby, preferably each field is divided into 4 bands. To achieve this the second counting circuitry may be adapted to process the composite synchronisation wave parcel to generate said timing signals. Preferably the second counting circuitry comprises a first cyclic counter having a count value of 77 in to which is inputted said composite synchronisation wave parcel; a second cyclic counter into which is inputted the terminal count output of said first cyclic counter; and a multiple line decoder into which is inputted the binary count output of said second cyclic counter to generate said timing pulses. (The count value of 77 equates to a period of 4.9mS as the counter is clocked by the composite synchronisation wave parcel as a line rate of 64uS.)

As with the filtering or more particularly the active filtering of the composite video signal, the comparison of the composite video signal (or its divisions) for different times is conducted in the analogue domain. The comparison circuit preferably consists of a differential amplifier, a non-inverting amplifier and two comparators. One division of the (modified) composite video signal at the first time and the identical division at the second point in time are inputted into the differential amplifier, the output of which will be proportional to the differences between the two inputs. The output is then fed into the non-inverting amplifier of which the output is fed into each of the two comparators.

The first comparator is provided with a positive reference voltage and the second is provided with a negative reference voltage. The first comparator is therefore configured to give a high output if the input signal exceeds its positive reference voltage. Similarly, if the input signal of the second comparator goes below the negative reference voltage, the output of this comparator goes high.

Obviously the sensitivity requirements of a surveillance system will be dependent on the use to which it is put. In the main, the sensitivity to differences in the video signals sampled at different times is controlled by the respective gains of the differential amplifier and the non-inverting amplifier of the comparison circuit and the reference voltages at which the comparators switch to high outputs.

The output of the two comparators can be summed in accordance with procedures known in the art to provide the a single output. Where the (modified) video signal has not been divided on the basis of bands or other field divisions, this single output will comprise the output of the entire video unit. However, in the alternative case, each signal representative of a scene fraction will produce an output from each comparison circuit dependent on the differences in that signal with time. The outputs from each comparison circuit must then be summed provide the overall output of the video unit which is fed into the main alarm circuit for combination with the analogue output of the passive infrared unit. The algorithms on the basis of which summing operation is performed can be used to control the sensitivity of the alarm apparatus. The overall output of the video unit can be dependent on one comparison circuit output being high or can be made dependent on more than one such output being high as required.

In accordance with a third aspect of the invention there is provided a system for monitoring a premises comprising at least one apparatus as described above. The positioning of each apparatus within a premises will be made on the basis of the function which the system is to serve. The coverage area of each passive infrared device and the field of view of the video unit would preferably be trained on entry points to a premises, such as doors, windows and gates, where the system is to be used for a security function.

Prior art security apparatus which rely entirely on thermal detection of intrusion using passive infrared devices cannot be mounted in positions with large temperature fluctuations as this would substantially increase the incidence of false alarms. In the present invention where the passive infrared unit provides a confirmation that movement is occurring the positioning of the alarm apparatus within a premises is less critical although preferably positions where the apparatus is in direct sunlight, near ports to an air conditioning system or other points exposed to hot or cold drafts should be avoided.

In accordance with a fourth aspect of the invention there is provided a method for determining the occurrence of a specified event in a scene, comprising the steps of visually monitoring the scene using a video camera and developing a composite video signal representative of said scene; processing said composite video signal to provide at least one sample level, wherein each sample level is representative of a segment of said scene; cyclically sampling each said sample level at a first and second point in time and identifying differences between the sampled levels; evaluating said differences and providing a first output dependent on said differences; and further comprising the simultaneous steps of; thermally monitoring said scene using a passive infrared unit; providing a second output dependent on movement of thermal bodies within the coverage area of the passive infrared unit; with the method also comprising the step of processing said first and second outputs to provide an alarm indication when both first and second analogue outputs indicate the occurrence of said specified event.

Brief Description of the Drawings

A number of embodiments of the invention will now be described with reference to the Figures, wherein:

Figure 1 illustrates an alarm circuit for combining the output from the video unit with the output from the passive infrared unit and the circuitry used to provide main and auxiliary alarm indications.

Figures 2a and 2b are schematic diagrams of those part of the processing circuit of the video unit which generate the timing and gating signals and also of the analogue circuitry of the video unit used for modification of the signal output of a video camera.

Figures 3A and 3B illustrate circuits for the generation of trigger signals from the timing and gating signals.

Figures 4A and 4B illustrate two circuits combining the modified, analogue video signal with the digital trigger signals to generate signal divisions on the basis of time and field position.

Figure 5 illustrates a comparison circuit for combining the primary response signals to generate the analogue output of the video unit in accordance with an embodiment of the invention.

The circuit components illustrated in the figures have each been denoted using conventional symbols in this art. Throughout the drawings positive and negative indications of voltage inputs and outputs are given as these are deemed necessary to the understanding of the drawings. Where "+N" or "-V" is denoted this does not mean that each voltage value is the same, merely that the choice of voltage is left to the operator in accordance with inter alia the desired sensitivity of the alarm unit and the required gain of amplifiers The same consideration applies to the choice of resistor and capacitor values as no values have been given in the figures.

Description of the Preferred Embodiment

Figure 1 provides an overview of the operation of the alarm apparatus (1). A passive infrared sensor circuit (2) outputs a signal (200). Simultaneously, the video unit of the apparatus is shown as providing 4 inputs (AL1 to A-L4). Each of these inputs represents the output of one comparison circuit (described below) which has compared four divisions of a video signal representative of a quarter fraction of the picture at a first time with identical divisions of the signal at a second point in time (the production of which is described herein below). These four signals are summed to provide the output signal (300) of the video unit. As the inputs (Al to A4) are analogue signals, the summing circuit comprises a line decoder (4), the output of which is fed into a NAND logic gate (5).

The outputs (200, 300) are themselves combined at an AND gate (7). Consequently the output of this gate (7) will only be high when both the outputs (200,300) are high which thereby provides a verification within the alarm apparatus as a high output from only one of the substituent units will not give a high output. The output (if any) of this AND gate (7) is passed through a transistor (acting as a current amplifier) to a light emitting diode (8) which thereby provides the alarm indication.

As shown in figure 1 both the video unit output (300) and the passive infrared unit output (200) are also independently fed into auxiliary alarm circuits (3,6). The alarm providers in these circuits (3,6), here illustrated light emitting diodes, will issue when either or both of the respective outputs (200,300) are high.

The generation of the inputs (AL1 to AL4) shown in figure 1 will now be explained with reference to the remaining drawings. As shown in figure 2a, a video camera (9) generates a composite video signal (10), such as a IV p-p signal conveying the information (colour coding and synchronisation) for each field of a picture. This signal is inputted into three different circuit channels. In the first of these channels a parcel (11) the composite video signal (10) is fed into a monitor (12). This enables the scene which is being surveyed to be visually represented on a screen monitor for actual physical viewing by a operator or third party as a supplementary check on the existence of an alarm state in the scene.

The second channel feed the composite video signal into an assembly of components which act to modify the signal in the analogue domain. This signal (10) is first inputted into a buffer (13) and then passed through a single-pole filter network (14) with a cut-off frequency of approximately 16Hz. This equates to a response time constant of approximately half a field period. After filtering the video waveform is amplified in amplifier (15) to provide an average level; for a IN p-p signal, this level can reach a value of around + 5 V for a completely peak white picture. In reality most cameras will have an iris control system fitted which means that peak white across the whole picture is rarely achieved. By amplifying this averaged level the sensitivity of the system to picture content changes is maintained. The modified video signal (100) is fed directly into the sample and hold amplifiers (40,41) shown in figures 4A and 4B for combination with the timing and gating pulses as described herein below .

The sample and hold amplifiers (40,41) are controlled by timing signals which are generated from synchronisation information inherent in the composite video signal. A section of the logic circuitry which generates these timing signals is shown in figure 2 as the third channel into which the composite video signal (10) is inputted. A composite signal parcel (16) is first inputted into a sync separator (17) that generates a composite line sync pulse train (171) and a Frame sync pulse train (172).

The composite line sync pulse train (171) is fed into a first cyclic counter (18) for which the count period equates to the time period for four bands in each field. The terminal count output from this first cyclic counter (18) is then fed into a second cyclic counter (19), followed by a 2 to 4 line decoder (20) the outputs of which comprise equi-spaced gating pulses occurring in every field scan of the video waveform. These outputs of the decoder (20) are fed into four OR gates (21a,21b,21c,21d) together with the terminal count of the first cyclic counter to create four separate timing pulses one line scan, or 64uS, wide (SI, S2, S3, S4), synchronised to a quarter of the picture. As each pulse has a very small gating width it will effectively take a "snapshot" of the modified video waveform at the end of each quarter of the field. If each of these signals are combined with the composite or modified video signal (10,100) they would then divide this continuous waveform into four continuous (bands) divisions, representative of the four sections of the screen.

Gating pulses are required to control the two fields from which the sampled and compared signal will be generated. These gating pulses are taken from the frame sync pulse train (172) the processing of which is illustrated in figure 2b. This train (172) is used to clock a 4-bit cyclic counter (22), the four binary output of which goes into a 4 - 16 line decoder (23). The output from this decoder results in 16 lines, each of which has a field pulse in a given time position in every block of 16 fields. By selecting tapping points "0" and "8" using the two pins (24a,24b) of the decoder (23), this will select as the output two gating pulses denoted herein as Fl and F8. The gating signals have a "high" output for the time duration in the domain equal to the complete duration of the selected field and only for those selected fields. Consequently, if combined with the composite video signal (10) or the modified signal (100) by themselves they would divide the continuous signal (10,100) into signals conveying only single frame information.

Figure 3A and 3B illustrate the combination of the timing pulses (S1,S2,S3,S4) and the gating (F1.F8) to provide trigger signals (F1,Q1 which, in effect, combine the gating effects of both the timing and gating signals. The quarter screen pulses (SI to S4) are combined with signal Fl (figure 3 A) at a system of four NOR gates (30a,30b,30c,30d) and combined with signal F8 (figure 3B) at second set of four NOR gates (31a,31b,31c,31d). The output trigger signals of each of these gates has been respectively denoted as F1Q1 and so forth in these figures (Q designating quarter).

These trigger signals (F1Q1) are combined with the modified composite video signal (100) as illustrated in figures 4 A and 4B which denote only the trigger pulses for field 1, quarter 1 and field 8, quarter 1 respectively. Each of these trigger signals is fed into a sample and hold amplifier (40,41) at the logic input of this integrated circuit with the modified composite video signal input at the analogue input. The output is the sample division of the modified video signal (100) for field 1, quarter 1 and field 8, quarter 1 (as shown) but this course will be repeated analogously.

In this preferred embodiment there will be four sets of sample and hold amplifiers. Each set analogue processes the information gathered from two fields to produce an output for each quarter section within a picture.

These sample pulses are then processed using the comparison circuitry illustrated in figure 5.

The two sample divisions for each quarter (i.e. SAMPLE_F1Q1 and SAMPLE_F8Q1) are fed into a differential amplifier (50), the output of which is fed into a non- inverting amplifier (51) and two comparators (52a,52b). The output of the differential amplifier (50) will be proportional to the differences between the two sample inputs (SAMPLE_F1Q1 to SAMPLE_F8Q1). The output of the non-inverting amplifier (51) is fed into each of the two comparators (52a,52b).

The first comparator (52a) is provided with a positive reference voltage (+V) and the second is provided with a negative reference voltage (-V). The first comparator (52a) is therefore configured to give a high output if the input signal exceeds its positive reference voltage (+V). Similarly, if the input signal of the second comparator (52b) goes below the negative reference voltage (-V), the output of this comparator (52b) goes high.

The output of the two comparators is passed through a diode and a transistor to amplify its current and thereby provide one of the video unit alarm indications for feeding into the alarm circuit illustrated in figure 1. As shown in figure 5 a supplementary light emitting diode (53) is provided in the comparison circuit to provide an additional or preliminary indication when one of the comparator outputs is high (this being indicative of a difference between the representative sample pulses input to the comparison circuit). Industrial Application

Although the above invention has been described in respect of a security system it will be acknowledged that a number of other uses could be made of it. The system could for example be used simply as a motion sensor which can be used for a variety of non-security functions such as monitoring the flow of a production line to identify potential equipment malfunctions. In addition, it could be used as an arrival or occupancy sensor that monitors the flow of individuals to and from a location, the verified system then controlling heating and lighting systems for that location or providing access control.

Claims

1. An apparatus (1) for providing a verified alarm indication in response to a specified event occurring within a scene, said apparatus comprising: a video unit for visually monitoring the scene and providing a first analogue output (300); a passive infrared unit (2) for thermally monitoring the scene and providing a second analogue output (200); at least one logic element (7) adapted to receive and combine said first and second analogue outputs (200,300) and provide an alarm indication when the combination of said outputs satisfies parameters indicative of said specified event.

2. An apparatus according to claim 1, wherein said video unit comprises an imaging means (9) continuously viewing the scene and providing a signal representative of the scene; processing circuitry for providing at least one first division of the signal representative of the scene at a first point in time and at least one second division of the representative of the scene at a later second point in time; and at least one comparison circuit for identifying differences between said at least first and second divisions of the signal; and wherein said first analogue output (300) is dependent on said differences.

3. An apparatus according to claim 2, wherein said imaging means comprises a video camera (9) and said signal comprises a composite video signal (10).

4. An apparatus according to claim 3, further comprising an analogue modifier circuit for modifying said composite video signal (10) provided by the video camera (9), said modified signal being divided by said processing circuitry.

5. An apparatus according to claim 4, wherein said analogue modifier circuit comprises a filter (14).

6. An apparatus according to claim 5, wherein said analogue modifier circuit comprises at least one amplifier.

7. An apparatus according to claim 6, wherein the filter performance of said amplifier is set so as to average the amplitude of the output.

8. An apparatus according to any one of claims 4 to 7, wherein said analogue modifier circuit comprises a buffer (13).

9. An apparatus according to any one of claims 3 to 8, wherein said processing circuitry comprises first counting circuitry for generating two gating signals; two sample and hold amplifiers (40,41) into which is input said gating signals and said composite video signal (10); and wherein the sample and hold amplifiers (40,41) combine said inputs to divide said composite video signal (10) into the two divisions representative of the scene at said first and second points in time.

10. An apparatus according to any one of claims 3 to 8, wherein said processing circuitry provides at least three said divisions of the composite video signal

(10) at said first and second points in time, and wherein each of said at last three divisions is representative of a fraction of the scene.

11. An apparatus according to claim 10, wherein said processing circuitry provides four divisions of the composite video signal (10), and wherein each division is representative of a quarter of said scene.

12. An apparatus according to any one of claims 10 to 12, wherein said processing circuitry comprises first counting circuitry for generating at least one pair of gating signals; second counting circuitry for generating n timing signals, where n > 3; a logic array for combining one pair of gating signals with each of said timing signals to generate 2n trigger signals; 2n sample and hold amplifiers into each of which is input one said trigger signal and said composite video signal (10); and wherein the sample and hold amplifiers (40,41) combine said inputs to divide said composite video signal (10) into 2n divisions of which n divisions are representative of n fractions of said scene at said first point in time; and n divisions are representative of n fractions of said scene at said second point in time.

13. An apparatus according to claim 10, wherein said first counting circuitry generates more than one pair of gating signals.

14. An apparatus according to any one of claims 10 to 13, wherein said at least three divisions comprise bands.

15. An apparatus according to claim 9 or claim 14, wherein said processing circuitry further comprises a sync separator (17) into which is inputted a parcel of said composite video signal (10), said separator (17) splitting said parcel into a composite synchronisation wave parcel (171) and a frame synchronisation wave parcel (172); and wherein said first counting circuitry processes said frame synchronisation wave parcel (172) to generate said gating signals.

16. An apparatus according to claim 15, wherein said first counting circuitry comprises: a four-bit cyclic counter (22) into which the frame synchronisation wave parcel (172) is inputted, said counter (22) generating four binary outputs; a line decoder (23) into which said binary outputs are inputted to derive sixteen gating signals; and means (24a,24b) for selecting two of said gating signals.

17. An apparatus according to claim 15 or claim 16, wherein said second counting circuitry processes the composite synchronisation wave parcel (171) to generate said timing signals.

18. An apparatus according to claim 17, wherein said second counting circuitry comprises: a first cyclic counter (18) into which is inputted said composite synchronisation wave parcel (171); a second cyclic counter (19) into which is inputted the terminal count output of said first cyclic counter (18); and a multiple line decoder (20) into which is inputted the terminal count output of said second cyclic counter (19) to generate said timing pulses.

19. An apparatus according to any one claims 3 to 18, wherein said processing circuitry provides the or each division of the composite video signal (10,100) at a second point in time separated from the first point in time by a time difference equivalent to a multiple integer of the time difference between successive field transmission by the video camera.

20. An apparatus according to claim 15, wherein said multiple is between 2 and 8.

21. An apparatus according to any one of claims 1 to 16, further comprising first auxiliary alarm circuitry (6) into which is inputted the first analogue output

(300) from said video unit, said circuitry (6) capable of providing an auxiliary alarm indication.

22. An apparatus according to any one of claims 1 to 17, further comprising second auxiliary alarm circuitry (3) into which is inputted the second analogue output (200) from said passive infrared unit (2), said circuitry (3) capable of providing an auxiliary alarm indication.

23. An apparatus according to any one of claims 1 to 18, wherein said video unit and said infrared unit (2) are contained in a single housing.

24. An apparatus according to any one of claims 1 to 19, further comprising an actuator means on which at least said video unit and said passive infrared unit (2) are mounted.

25. A system for monitoring a premises comprising at least one apparatus (1) according to any one claims 1 to 20.

26. A method for determining the occurrence of a specified event in a scene, comprising the steps of visually monitoring the scene using a video camera (9) and developing a composite video signal (10) representative of said scene; processing said composite video signal (10) to provide at least one sample level, wherein each sample level is representative of a segment of said scene; cyclically sampling each said sample level at a first and second point in time and identifying differences between the sampled pulses; evaluating said differences and providing a first output (300) dependent on said differences and further comprising the simultaneous steps of thermally monitoring said scene using a passive infrared unit (2) providing a second output (200) dependent on movement of thermal bodies within the coverage of passive infrared unit (2); and also comprising the step of processing said first and second outputs (200,300) to provide an alarm indication when both first and second analogue outputs (200,300) indicate the occurrence of said specified event.

27. A method according to claim 26, further including the step of filtering the composite video signal developed by the video camera in the analogue domain before processing of said signal.