If a camera is viewing a car park, the only part of the scene that is likely to change will be cars or people moving around. The system will grab a sample of the signal from the camera and compare it with the previous sample. If there is a difference in the signal, then only the differences will be transmitted.
This reduces the amount of data sent and, therefore, improves the update time. (That concludes Part 6)
Camera Housings
For CCTV applications there are a number of different types of housings and enclosures available. The basic choice is between housings for external use or housings for internal use.
Criteria for selecting Internal housings are:
Protection (anti tamper fastenings, locked, weather-proof, dustproof, insulated, cooled); Covert (satellite, disguised housing, inset housing); Environment (satellite, ceiling mount, disguised, covert, inset).
Criteria for selecting External housings are:
Protection (weather proof) and Covert (Disguised weather-proof housings, external satellites)
Internal housings
Internal housings have four main applications:
- To protect the camera against the environment
- To protect the camera against unauthorised tampering
- To provide a discreet enclosure for covert surveillance
- To enhance or maintain the décor of the site.
Housings may fill one, some or all of these requirements, depending on their design and construction. Care should be taken when selecting the housing for an application to ensure that it has the qualities that are required. For example, if it is necessary to protect a camera against tampering, it will need to be fitted with either anti tamper screws or a lock.
In addition to housing the camera, some internal housing enclosures, mainly dome types, include movement. This may be a panning facility only or a pan and tilting facility. These types are normally referred to as "satellites" within the industry. There are a range of satellites available in different sizes which offer a choice of discreet surveillance or high profile. Some of the larger enclosures can house a zoom lens (Fig 1).
The pan and tilt satellite
This unit (Fig 2) is normally in two parts, the enclosure which may be a half or full dome and the frame containing the pan and tilt mechanism. This would consist of a cradle on which the camera would be fixed. A pan motor is connected to the centre via a gearing mechanism. This will turn the satellite frame. A tilting motor would be attached to the cradle via a gear wheel to tilt the camera. Because of the amount of movement involved in these units, the wiring of them has to be carefully considered to avoid excessive cable failure. In some types all the cables to the unit are fed down the centre support on to the frame, including the coaxial signal cable, the control cable and power supply. Cables must have plenty of room in the support pole so that they can flex with the unit's movement. If not, cable breaks will occur. For safety reasons low power should be used in all these units and most can be supplied with 24V motors. The camera should, if possible, operate off the same supply, reducing the number of cables required (Fig 3). Cables are fed down the centre support tube on to connectors in the frame. As an alternative to wiring the satellite directly, some manufacturers use a supply of slip rings. These allow the control voltages and signals to be passed to the unit without the need for cables (Fig 4). While this does not reduce the fault incidence due to broken cables, it can result in problems due to faulty spring contacts and dirt on the tracks.
External housings
External housing are constructed from either steel, aluminium or moulded plastic. Metal housings should be treated against the weather, being either galvanised, anodised or painted with a weatherproof coating. They come in a range of sizes to accommodate the different cameras and their attachments. Some manufacturers offer a range of weatherproof ratings from low level for sheltered locations to high level for exposed positions. It is important to ensure that you choose the correct housing for the application.
Some important things to consider when choosing a housing are:
Wash and wipe facilities
There are situations when the camera position is fully exposed to the weather and it may be necessary to fit a wiper on to the housing to clean the viewing window. This is particularly important if the camera is fitted with a zoom lens because any blemishes on the glass will be magnified when the lens is zoomed in. This may also affect the selection of the housings as not all housing can accept wiper motors. The wiper motor is a separate unit which bolts on to the front underside of the housing. The motor is normally wired directly into the camera housing and controlled by the operator. Under normal conditions a wiper is sufficient to keep the viewing window clean. In dusty and dirty environments it may be necessary to add a washer facility. In addition to these standard housings, there are some applications where specialist units will be required. In unfriendly environments, CCTV cameras can play a vital role in reducing danger.
Water cooled housings
In glass and steel making for example, monitoring the furnace can be important but dangerous. A camera fitted either inside or on the wall of the furnace will provide constant monitoring. Because of the severity of the environment, the camera must be placed inside a special housing. Water cooled housings have two walls with water circulating between them. This will be pumped around the housing and onto a cooling assembly. A fan may also be used to blow cool air into the housing itself. A Pyrex glass window would normally be used for extremely high temperatures and some housings incorporate an ir-absorbing filter within the window which reduces infra-red light given off by an intense radiant source.
Heated housings
At the other extreme, it may be necessary to monitor a refrigerated room with temperatures as low as –5 deg centigrade. CCTV can provide the solution by using a heated glass to prevent frosting. The housing will have an inner and outer wall with insulation between the walls. A fan may also be fitted with heated air being blown in.
Explosion proof housings
Other environments may have different dangers, such as the risk of explosion. Cameras can be used to monitor danger areas in petroleum, armaments and chemical industries. The cameras can be placed in explosion proof housings. These housings are designed to protect the external environment for the camera and the danger of a flame or spark from causing an explosion. The units are sealed to required standards (BS5501). In some situations, health and safety regulations restrict the access to cameras in these environments. Occasionally the camera must be powered off for a set time period before the cover is removed and the camera cannot be powered on without the cover in place.
In our next session: CCTV ancillary & peripherals continued.
Source
Security Installer
