Patent GB-1266344-A (GB)
IPC: G08B13/194
Filed: 1968-03-20
Abstract: 1,266,344. Intruder alarms. EMI Ltd. 17 March, 1969 [20 March, 1968], No. 13417/68. Heading G4N. In an intruder alarm, radiation from a region repeatedly scanned (e.g. by a television camera) produces signals which are grouped according to the portion of the region from which they are derived and compared with corresponding signals from a subsequent scan. A plurality of individually adjustable threshold circuits is provided to initiate an alarm signal in the event of an output from any threshold circuit. The signals may be grouped before or after storage and comparison. In the arrangement shown in Fig. 3, signals from camera 2, 3 are digitized in a converter 5 whose output of three binary digits has a frequency corresponding to the resolving -power of the camera. The digits are written, preferably in parallel, on a magnetic drum 20 by head 21. Heads 21 and 22 and the speed of drum 20 are arranged so that a signal from  reading head 22 is fed into a subtractor 24 at the same time as a corresponding signal direct from converter 5 from a subsequent scan. The difference signals are distributed at 6 to accumulators 7, 8, 9, 10 under the control of signals from a track on drum 20 read by head 26. These signals also synchronize the operation of deflection circuits 4 in camera 2, 3. Each accumulator may group signals consecutively from a column of four rectangular portions constituting a vertical strip of the surveyed region. The grouped difference signals are passed to threshold devices 16 ... 19 which determine the level at which an alarm 13 is operated. The thresholds of devices 16 ... 19 are variable, and the devices comprise rectifying circuits and transistor switches. Alternatively, variable means are provided so that, the region may be divided into portions of equal brightness, with parts of the region whose brightness fluctuates excessively being included in a single portion. The threshold for this portion is set at a high level to prevent false alarms. Operation of the alarm may also switch on a monitor so that the region may be viewed. The alarm may be made responsive to movement across the region, which results in the thresholds for adjacent portions being progressively exceeded, by including logic circuits in alarm 13. Alternatively, the difference signals may be weighted before grouping by the sum of the rates of change of the signals from camera 2, 3 and reading head 22 (Fig. 4, not shown). The camera may scan the region obliquely (Fig. 5, not shown) so that the weighting gives an indication of both horizontal and vertical movement. The weighted signals may be used to control camera 2, 3 to prevent drift and spurious signals. If the camera 2, 3 scans horizontally, the alarm may be made sensitive to vertical movement by weighting the difference signals before grouping with the sum of the differences between adjacent lines of the scan of the signals from camera 2, 3 and reading head 22 (Fig. 6, not shown). In a second embodiment (Figs. 1, 7, not shown). The signals are grouped as in Fig. 3 before being delayed and compared in a subtractor with a subsequent signal. The difference is converted to analogue and fed to threshold circuits. Several cameras, operating sequentially, may be used. In a third embodiment the signal from two camera tubes are compared, the tubes having a different response lag.