GB2103901A

GB2103901A - Television surveillance systems

Info

Publication number: GB2103901A
Application number: GB08221536A
Authority: GB
Inventors: Michael Waywood
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1981-07-30
Filing date: 1982-07-26
Publication date: 1983-02-23
Also published as: GB2103901B

Abstract

The invention relates to television surveillance systems, for example, for observing terrestrial scenes. A television surveillance system in accordance with the invention includes a television camera 1, a video frame store 3 connected to receive video signals from the camera, a dual input switch 4 connected to receive video signals from the camera and video signals from the store, wherein the switch is arranged to output signals corresponding to the received camera and stored signals alternately at a switching frequency greater than 3 Hz to a television monitor 5. <IMAGE>

Description

SPECIFICATION ImRrovements in or relating to television surveillance systems The present invention relates to television surveillance systems and particularly, though not exclusively, relates to monochrome television systems for observing terrestrial scenes.

A known television surveillance system employs a closed-circuit television camera and monitor. The camera is directed at a scene using a remote control facility. Video signals from the camera are recorded on video tape for subsequent surveillance evaluation.

A disadvantage with such known television surveillance systems is that small changes in a scene over periods of several hours are often not noticeable. Long spells of continuous scrutiny of monitor sets leads to user fatigue and important scene changes may be missed. In particular, the introduction of a small object which contrasts poorly with its background is likely to be unobserved.

According to the present invention a television surveillance system comprises a television camera, a video frame store connected to receive video signals from the camera, a dual input switch connected to receive video signals from the camera and video signals corresponding to selected frames from the store, wherein the switch is arranged to output signals corresponding to the received camera and stored signals alternately at a switching frequency greater than 3 Hz to a television monitor.

Preferably the switching frequency is less than 20 Hz.

The system may include means for manually adjusting the switching frequency of the switch.

The system may further include variable positioning means arranged to vary the direction of the camera and to generate for each direction a signal indicative of direction, means for labelling each frame with a corresponding one of the said signals, and means for selecting a frame from the store which corresponds to the camera direction at the time of selection.

According to another aspect of the invention a television surveillance system comprises a television camera, a shutter for the camera, first and second video frame stores connected to receive video signals from the camera, a dual input switch connected to receive video signals from the frame stores, wherein the switch is arranged to output signals received from the first store which correspond to the latest frame period and signals received from the second store which correspond to an earlier frame period alternately at a switching frequency greater than 3 Hz to a television monitor.

Embodiments of the invention will now be described, by way of example only, with reference to the drawings of which Figure 1 is a schematic diagram of a television surveillance system in accordance with the invention; Figure 2 is a circuit diagram of a dual input video switch circuit which forms part of the system of Figure 1; Figure 3 is a schematic diagram of a further embodiment of the invention; Figure 4 is a schematic diagram of another embodiment of the invention.

Figure 1 shows a television camera having a video output which is carried on two lines 2a, 2b.

Line 2a is connected to an input terminal of a video frame store 3. Video signals from the store 3 corresponding to a selected frame and signals direct from the camera 1 are received on input terminals 31 and 32, respectively, of a video switch circuit 4. The output from the video switch circuit is displayed on a television monitor 5. In operation the signals received direct from the camera 1 and from the frame store 3 are switched alternately on a frame by frame basis by the switch 4, so that signals corresponding to frames from camera line 2b and the store 3 are displayed alternately at a frequency of 10 Hz.

On the monitor 5, if there is a difference between the viewed scene from the camera and the stored scene the difference is seen on the monitor screen as a flickering element. For example, if the stored frame corresponds to a view of eighteen dark targets on a light background and a nineteenth dark target is introduced into the scene viewed by the camera, the presence and position of the new target will be noticeable on the monitor screen.

Figure 2 shows details of the video switch circuit 4 of Figure 1. The circuit receives video signals at the input terminals 31 and 32. The two signals share the same sync. signals and the frame sync. pulses are separated and used to lock a free running astable multivibrator 8, the frequency of which is capable of being varied in discrete steps over a wide frequency range. The output from the multivibrator 8, termed the "locked frequency" signal, is used to switch, during frame blanking periods, inputs from terminals 31 and 32.

A frame pulse separation sub-circuit 33 includes a buffer transistor 25, and transistor amplifier 30, and a variable threshold comparator 6. The video signal input on terminal 32 is buffered by transistor 25 and the signal integrated by a capacitor 29, highlighting negative going sync. pulses. The integrated signals are then amplified by transistor 30 and the amplified signal input to the threshold comparator 6 which produces a squared sync. signal. The comparator threshold is adjustable by varying a resistance 7 to provide accurate settings on the negative going edge of the frame blanking pulse.

The multivibrator 8 is a standard (NE 555) timer connected in its astable mode, the frequency of which is varied over the required range by sequentially switching between a bank of eight capacitors 9 connected into the timing circuit of the multivibrator 8. The output signal from multivibrator 8 is received by a monostable 10 to produce a very short duration output pulse which is used to set a latch 11. The latch output is input to a NAND gate 13 and is used to gate the frame sync. pulse on line 34.

The first frame sync. pulse after the multivibrator 8 has output a pulse to set the latch 11 appears at the output of the gate 13. The negative edge of the sync. pulse triggers a monostable 14 which produces an output pulse which is used to reset the latch 11 and inhibits gate 13.

The circuit is provided with power from a main supply via lines 27, a master switch 28 and a power supply unit 26 which produces 12 V, -12 V and 0 V as shown in Figure 2. A time delay unit 1 5 produces a short pulse when the switch 28 is closed to ensure that the latch 11 is reset.

A NOR gate 1 6 is used to provide the monostable 14 with an input comprising either a power-on reset pulse or a normal reset pulse from the latch 11, the normal reset pulse being inverted by a NOR gate 1 7 before input to NOR gate 1 6. The output signal from NOR gate 1 6 is inverted by a further NOR gate 1 8 and the inverted signal input to the monostable 14.

A flip-flop 1 9 configured as a divide-by-two circuit controls a pair of video switches 20 so that the switches are in a stable position for the period between consecutive pulses from the multivibrator 8.

Antiphase Q and Q outputs from the flip-flop 1 9 drive the video switch pair 20.

The output from the video switch pair 20 is buffered by a transistor 23 and appears on an output terminal 24.

The embodiment shown in Figure 3, includes a television camera 40 mounted on a circular arc track 46 with opto-electrical pick-offs, 47, for providing signals on a line 41 indicative of the azimuth angle of the camera axis. Thus a unique signal will be generated for, for example, positions A and B of the camera as shown in the drawing. A video frame store 42 is arranged to receive signals from the camera and the signals on line 41 which are used to 'label' each stored frame.

The store 42 includes an automatic frame selection circuit which selects only those frames which have labels which correspond to the direction of the camera at the time of selection. A video switch circuit 44 which is identical to the circuit of Figure 2 receives signals from the frame store 42 and from the camera 40 on a line 43 and provides an output to the monitor 45 in the same way as that described for the embodiment of Figure 1.

Figure 4 shows a further embodiment of the invention which includes a television camera 51 and an electromagnetic shutter 52 placed over the camera lens so that the camera is protected at least in part against damage caused by exposure to intense sources of radiation such as lasers. The camera 51 has a video output which is carried on two lines 53, 54 connected to video frame stores 55, 56 respectively.

A video switch circuit 57 controls the periodic operation of the shutter 52. in one mode of operation the shutter cycle is open for one frame period and closed for one minute. In operation, the switch circuit selects signals from the store 55 which correspond to the frame obtained during the latest shutter open period and selects signals from the store 56 which correspond to a frame obtained, for example, one hour before the latest shutter open period. The switching operation of the circuit 57 is similar to the circuit of Figure 2. The circuit 57 provides an output of the selected signals to a television monitor 58 at a switching frequency of 10 Hz.

Claims

1. A television surveillance system comprising a television camera, a video frame store connected to receive video signals from the camera, a dual input switch connected to receive video signals from the camera and video signals corresponding to selected frames from the store, wherein the switch is arranged to output signals corresponding to the received camera and stored signals alternately at a switching frequency greater than 3 Hz to a television monitor.

2. A television surveillance system comprising a television camera, a shutter for the camera, first and second video frame stores connected to receive video signals from the camera, a dual input switch connected to receive video signals from the frame stores, wherein the switch is arranged to output signals received from the first store which correspond to the latest frame period and signals received from the second store which correspond to an earlier frame period alternately at a switching frequency greater than 3 Hz to a television monitor.

3. A television surveillance system as claimed in claim 1 or claim 2 wherein the switching frequency is between 3 Hz and 20 Hz.

4. A television surveillance system as claimed in any of the preceding claims including means for manually adjusting the switching frequency of the switch.

5. A television surveillance system substantially as herein described with reference to the drawings.