AU575194B2 - Intrusion alarm signal enhancement - Google Patents

Description

"A METHOD OF SIGNAL PROCESSING OF OPTICAL SIGNALS"

This invention relates to a method of signal processing which can be used in visual or infrared intrusion alarms or similar systems.

Infrared and visual scenes are generally so cluttered as to render the detection of targets such as intruders or vehicles, extremely difficult.

Various forms of movement detectors related to video images are already known and as an example the specification of PCT Application No. WO82/01454 by The Commonwealth of Australia refers to such a system where areas are scanned and movement in such areas is recorded, the scannings in that case being related to windows which can be selected to limit the clutter which could occur from adjacent zones.

The object of the present invention is to provide a method of and means for detecting change or movement in a simple effective manner.

A feature of this approach is to adapt to both spatial and tamporal changes in the background clutter.

The method comprises a simple processing technique of monitoring the background signals and the detection of any small changes to them.

The invention consists in the processing of an optical signal which can be visual or infrared to provide two channels, the one channel having a higher decay rate than the other, and the indication of change or movement is obtained from the difference in output between these two channels. The invention can be readily carried into effect by using an optical scanning device which produces a time-varying signal representing the spatial variations of radiance in the scene. This signal is amplified and passed through two profile integrating stages, the first of short term duration to provide signal to noise enhancement whilst maintaining a fast response time, the second of long term representing the averaged scanned scene over the long term. When these two signals are compared, the difference indicates a change or movement in any part of the scanned scene.

In order that the invention will be fully appreciated an embodiment thereof will be described with reference to the accompanying drawing which is to be taken as illustrative only and not as limiting the invention.

In the drawings:- FIG. 1 is a block diagram depicting the invention, and

FIG. 2 is a compound graph showing sequentially at A the output from a first profile integrator, at B the output from a second profile integrator and at C the output from a difference amplifier.

It will be realised that the output can be the signal from a single detector, or multiple detector signals can be combined and processed.

For a single detector, scanning is achieved opto-mechanically, as for example, achieved by a rotating reflecting polygon or vibrating mirror. For a detector array, scanning is achieved by multipl-exing the array output.

The signals from the detector 1 are fed to an A.C. coupled discrete low-noise, high gain preamplifier 2, then through a band-pass filter 3 centred on the scan frequency with a Q of 3. The scan frequency is typically 100 Hz. This signal is then passed through an initial stage of analogue integration in the first profile integrator 4. This consists of an amplifying stage 5 with a CCD analogue delay line 6 in a positive feedback configuration. The delay line is synchronised with the scan frequenny. The complete scan cycle is integrated, the time constant of which is in the order. of 4 cycles. This stage is used to enhance the signal to noise ratio as generated by the detector (short term store).

The enhanced signal is then fed to a second profile integrator 7 which comprises a synchronous integrating stage consisting of an amplifier 8 and associated analogue delay line 9 in a feedback path. Here the time constant is made much longer, in the order of one second (100 cycles). This stage functions as a longterm store.

The outputs of the first and second profile integrators 5 and 7 are then compared in a comparator 10. With a steady-state scanned profile no output is generated by the comparator. When a change of scene occurs due to a moving body within the field of view, the input signal is sensed to differ from the profile as stored in the long term store. The resultant output signal from the comparator has a large component synchronous with the scan frequency. A phase-locked tone detector 11 is used to detect output at this frequency and an alarm is triggered when a threshold level is exceeded.

The advantages of this technique lie in the high sensitivity, high clutter rejection, real time operation and low power consumption using low cost analogue hardware.

As defined earlier, the output from the first integrator 4 as shown at A in FIG. 2 has an envelope different than the envelope generated by the second stage integrator 7 as shown at B in FIG. 2 and this difference results in a signal as shown at C in FIG. 2, indicating a change in the data received by the detector 1. The first profile integrator 4 and the second profile integrator 7 are connected to a phase locked loop generator 12.

In operation as the target enters, the first profile integrator responds rapidly as shown in A in FIG. 2. The middle trace B shows the slower response of the second profile integrator. These two signals are compared in the difference amplifier. Here, the signal initially rises with the faster response of the first integration loop, then once again falls away as the longer term second profile integrator responds to the change.

A similar perturbation will occur when a target moves frcm the field of view. Alarm is initiat-sd when the output from the difference amplifier exceeds a set threshold.

Claims

THE CLAIMS DEFINING THE INVENTICN ARE AS FOLLCWS:

1. The method of signal processing of optical signals in which an area is scanned for change detection by visual or infrared beam, in which the said scanning beam is processed to detect changes in the image, characterised by dividing the signal into two channels, causing one channel to have a higher decay rate than the other, and comparing the two channels after an elapsed time to indicate change.

2. The method of processing optical signals according to Claim 1 to detect a change in such signal as brightness or motion in which an optical, signal from a scanning system is fed to a detector and the output of the detector is processed to detect change, characterised by the steps of; submitting the signal from the detector (1) to a first profile integrator (4), integrating the scan cycle to enhance the signal, feeding the enhanced signal to a second profile integrator (7) having a longer time constant than the said first integrator (4), submitting the outputs of the first profile integrator (4) and the second integrator store (7) to a comparator (10) and using the differential between the said signals in the comparator (10) to generate an output signal.

3. The method of Claim 2 wherein the signal from the said optical detector (1) is passed through a low noise preamplifier (2) and a bandpass filter (3), and the signal from the comparator (10) is submitted to a tone detector to generate said output signal. 44. The method of Claim 2 or 3 wherein the said first profile integrator (4) and the said second signal integrator (7) are connected to a clock generator (12) receiving a scanning signal from the said scanning optics.

5. A signal processing device for processing optical signals to detect a change in such signal as brightness or motion in which a scanner directs the said signal to an optical detector (1) characterised by a first profile integrator (4) and a second profile integrator (7) arranged to receive the signal from the said detector (1), means to have different time constants in the said first and second integrators (4-7), a comparator arranged to receive the signals from the said first and the said second integrators (4-7) to detect change in the signals from the said first comparator (4) and the said second comparator (7), and means to indicate such change.

6. A signal processing device according to Claim 5 characterise by means to enhance the said signal in said first integrator (4), and by means to pass the enhanced simal to the said seαond integrator (7) and by a clock generator connected to each said first and said second integrators (4-7) whereby to control the different time constants of the said integrators (4-7).

7. A signal processing device according to Claim 6 wherein the said clock generator (12) is conneccec to synchronise with the said scanner.

8. A signal processing device according to Claim 5 or 6 characterised by a low noise preamplifier (2) connected to receive the signal from the said detector, and a bandpass filter (3) connected to receive the signal from the optical detector (1) and pass the signal to the said first profile integrator.

9. A signal processing device according to Claim 5 or 6 characterised by a comparator (10) connected to receive the signal from the said first integrator (4) and the said second signal integrator (6) and to pass a processed signal to a tone detector (11).

10. A signal processing device constructed substantially as described and illustrated in the aaxnpanying drawings.