Closed-circuit television camera
From Wikipedia, the free encyclopedia
CCTV cameras can produce images or recordings for surveillance purposes, and can be either video cameras, or digital stills cameras.
Contents |
[edit] Video Cameras
Video cameras are either analogue or digital, which means that they work on the basis of sending analogue or digital signals to a storage device such as a video tape recorder or desktop computer or laptop computer.
[edit] Analogue Video Cameras
Can record straight to a video tape recorder which are able to record analogue signals as pictures, but these devices are becoming obsolete as more cameras are being manufactured as digital devices.
If the analogue signals are recorded to tape, then the tape must run at a very slow speed in order to operate continuously. This is because in order to allow a 3 hour tape to run for 24 hours, it must be set to run on a time lapse basis which is usually about 4 frames a second. In one second, the camera scene can change dramatically. A person for example can have walked a distance of 1 meter, and therefore if the distance is divided into 4 parts i.e. 4 frames or 'snapshots' in time, then each frame invariably looks like a blur, unless the subject keeps perfectly still.
Analogue signals can also be converted into a digital signal to enable the recordings to be stored on a PC as digital recordings. In that case the analogue video camera must be plugged directly into a video capture card in the computer, and the card then converts the analogue signal to digital. These cards are relatively cheap, but inevitably the resulting digital signals are compressed 5:1 (MPEG compression) in order for the video recordings to be saved on a continuous basis.
Digital storage (and sometimes compression) of the video does not improve the image quality, because it depends heavily on the optics (i.e. lenses and video sensor). It can, however, decrease the quality further.
Another way to store recordings on a non-analogue media is through the use of a digital video recorder (DVR). Such a device is similar in functionality to a PC with a capture card and appropriate video recording software. Unlike PCs, most DVRs designed for CCTV purposes are embedded devices that require less maintenance and simplier setup than a PC-based solution, for a medium to large number of analogue cameras.
Some DVRs also allow digital broadcasting of the video signal, thus acting like a network camera. If a device does allow broadcasting of the video, but does not record it, then it's called a video server. These devices effectively turn any analogue camera (or any analogue video signal) into a network camera.
[edit] Digital Video Cameras
These cameras do not require a video capture card because they work using a digital signal which can be saved directly to a computer. The signal is compressed 5:1, but DVD quality can be achieved with more compression (MPEG-2 is standard for DVD-video, and has a higher compression ratio than 5:1, with a slightly lower video quality than 5:1 at best, and is adjustable for the amount of space to be taken up versus the quality of picture needed or desired). The highest picture quality of DVD is only slightly lower than the quality of basic 5:1-compression DV.
Saving uncompressed digital recordings takes up an enormous amount of hard drive space, and a few hours of uncompressed video could quickly fill up a hard drive. Holiday uncompressed recordings may look fine but one could not run uncompressed quality recordings on a continuous basis. Motion detection is therefore sometimes used as a work around solution to record in uncompressed quality.
However, in any situation where standard-definition video cameras are used, the quality is going to be poor because the maximum pixel resolution of the image chips in most of these devices is a mere 320,000 pixels (analogue quality is measured in TV lines but the results are the same); they generally capture horizontal and vertical fields of lines and blend them together to make a single frame; the maximum frame rate is normally 30 frames per second.
[edit] Network Cameras
Network cameras are analogue or digital video cameras, plus an embedded video server having an IP address, capable of streaming the video (and sometimes, even audio).
Due to the fact that network cameras are embedded devices, and do not need to output an analogue signal, resolutions higher than CCTV analogue cameras are possible. A typical analogue CCTV camera has a PAL (768x576 pixels) or NTSC (720x480 pixels), whereas network cameras may have VGA (640x480 pixels), SVGA (800x600 pixels) or quad-VGA (1280x960 pixels, also referred to as 'megapixel') resolutions.
An analogue or digital camera connected to a video server acts as a network camera, but the image size is restricted to that of the video standard of the camera.
However, optics (lenses and image sensors), not video resolution, are the components that determine the image quality.
Network cameras can be used for very cheap surveillance solutions (requiring one network camera, some Ethernet cabling, and one PC), or to replace entire CCTV installations (cameras become network cameras, tape recorders become DVRs, and CCTV monitors become computers with TFT screens and specialised software. Digital video manufacturers claim that turning CCTV installations into digital video installations is inherently better.
[edit] Digital Still Cameras
These cameras can be purchased in any high street shop and can take excellent pictures in most situations.
The pixel resolution of these cameras are usually in excess of 4 million pixels, Some point and shoot models like those produced by Canon or Nikon boast resolutions in excess of 6 million pixels.
At these resolutions, and with high shutter speeds like 1/125th of a second, it is possible to take jpg pictures on a continuous or motion detection basis that will capture not only anyone running past the camera scene, but even the faces of those driving past.
These cameras can be plugged into the USB port of any computer (most of them now have USB capability)and pictures can be taken of any camera scene. All that is necessary is for the camera to be mounted on a wall bracket with two screws and then pointed in the desired direction.
Modern digital still cameras can take 500kb snapshots in the space of 1 second, and these snapshots are then automatically downloaded by the camera software straight to the computer for storage as timed and dated JPEG files. The images themselves don't need to stay on the computer for long. If the computer is connected to the Internet, then the images can automatically be uploaded to any other computer anywhere in the world, as and when the pictures are taken.
The user doesn't need to lift a finger except to simply plug the camera in and point it in the desired direction. The direction could just as easily be the street outside a house, or the entrance to a bank or underground station.
Digital still cameras are now being made with in-built wireless connectivity, so that no USB cable is required; images are simply zapped through walls or ceilings to the PC.
