Sample Chapter: 18 Video motion detection

From CCTV Information

Sample chapter from Mike Constant's book: The Principles & Practice of CCTV

Video Motion Detection

There are many methods of detecting intruders into premises. These include such systems as:

• Intruder alarms.
• Fence mounted detectors.
• Buried vibration or electric field devices.
• Active infrared devices.
• Passive infrared devices.
• Microwave devices.
• Video motion detection devices.

This chapter is concerned with Video Motion Detection devices. (VMD). These may be within or outside the premises and, besides detecting intruders, can be used as part of a building management system. VMD may often be used either as a stand-alone system or integrated with other detection systems. In an ideal world, detection devices would give no false alarms and 100% of genuine alarms. Unfortunately, this is not an ideal world, and a certain amount of compromise is necessary. This compromise must be reduced to the most effective and acceptable level to achieve the system objectives.

There are really only two types of alarm, genuine alarms and false alarms. Sometimes mention is made of 'spurious alarms', unexplained alarms and system failures. These must only be considered as false alarms because the system has alarmed for no apparent reason. A genuine alarm is one created by deliberate nefarious human action, e.g. by movement of a person or vehicle into the detection field or disturbance of the alarm system. A false alarm is one that has no deliberate human input, such as those caused by animals, birds or any malfunction of equipment. One measure of the efficiency of a system is the 'False Alarm Rate' (FAR). This is the ratio of false alarms to a time scale, i.e. five per day. The FAR level will depend on many local site considerations. The objective is to reduce this to the minimum without missing any real alarms. Another measure is the 'probability of detection' (PD) rate, which is the ratio of detections to the number of attempts in controlled tests. The ideal for PD is 100%.

Uses Of VMD

The primary function of a VMD system is to relieve CCTV operators from the stress of monitoring one or many screens of information that may not change for long periods. The VMD system will be monitoring all the cameras in its system, and only reacting when there is suspicious activity in one of the scenes. During the long periods of inactivity the operator can continue with other tasks, secure in the knowledge that when something occurs the system will immediately respond. Even a moderate sized system, with eight cameras, would prove impossible for an operator to monitor. Eight monitors could not be viewed with any degree of concentration for more than about twenty minutes. If the monitors were set to sequence, then activity on seven cameras is lost for most of the time and would be totally ineffective to detect intruders. With more cameras in a system, the task of detecting intruders becomes impossible and technology must take over the strain.

The idea of VMD systems is that the processor is continuously monitoring all the cameras in the system. During this time, the, operator may select or sequence cameras using the conventional switching system. The system may include an additional monitor connected to the VMD system that will normally show a blank screen. When activity in any camera occurs that the VMD system interprets as an intruder, the alarmed camera is immediately switched to the blank monitor and a warning sounded to alert the operator. The operator's attention, is therefore, immediately focused on the camera covering the alarm. The detection of an intruder can also set off further events, such as setting a video recorder to real time recording, setting a matrix switching system to sequence through a specific series of cameras, etc. The operator can analyse the scene and take the appropriate course of action.

An intruder could generate an alarm and be out of view of the camera before it is displayed. The operator would therefore see just a blank screen and be unsure about what to do next. To overcome this, at the time of detection, many VMD systems will capture an alarm image sequence containing one or more freeze frames. This may be displayed as the first view on the previously blank screen. The operator may then examine the scene at the instant of alarm in more detail.

Principle of operation

In the descriptions that follow reference is made to a 'frame' of video. Some systems use frames and some use fields, some systems can select between the two. This also applies to storage devices. For ease of description, the term frame is used for consistency but the actual method used should be checked for the system being considered.

Video Motion Detection is an electronic method of detecting a change in the field of view of a camera. In its simplest form, this is achieved by storing one frame of the video information and then comparing the next frame with this to decide whether there has been a change. The change detected would be a difference in the video voltage, indicating a change of brightness within the scene. This would be initially ignored as an alarm until a further frame confirmed the change, or not. If confirmed as a change of brightness in the scene, then an alarm would be generated. This could cause a contact to close and activate some warning device such as a buzzer, or cause the switcher to select the camera that detected the motion. The sampling process may take somewhere between one fiftieth of a second and one second to detect a change, depending on the method of sampling. This simple detector could be used in an environment where all conditions were absolutely stable and the only possible change in brightness would be due to an intruder. However, the intruder could be a mouse or a person. The system couldn't differentiate between the two. In addition, by the time the alarm is displayed on a monitor, the cause of it could be out of view. If the scene were being continuously recorded, the event could be reviewed but this may be too late to take effective action.

Detection Cells

For the purposes of this chapter the following definitions are used although there are no standard terms used at present. A CELL is a single detection block that is analysed electronically for brightness changes. A cell may be a single pixel, a block of pixels, or the whole screen. A ZONE is a group of cells that have been defined as an active area. The exact meaning of 'zone' must be checked with a manufacturer's specification before assuming what area is covered and to what degree of definition. This method of comparing complete frames therefore has severe drawbacks. The next development was to divide the picture into a number of separate areas or cells. This was refined by being able to switch cells on or off to define the area of the scene that is of interest. Diagram 17.1 illustrates a VMD system that divides the picture into cells, and how only a selected part of the scene can be set for motion detection. The shaded areas are inactive and the clear parts are the active cells. In this case, only activity in the area of the car will create an alarm. The cells are only displayed as such during setting up the system. Once the set-up mode is exited, the complete picture is displayed as normal and it is not possible to see any of the cells.

The sensitivity of the cells can be adjusted to take into account local conditions. This control though is applied across all cells to the same extent. Some systems can be pre-set to different sensitivity levels, for instance, to make allowance for day or night operation when the lighting levels may be different.