CCTV Systems Go Digital

Digital video delivers improved quality, which enables more details and changes in images.

With the introduction of network cameras and PC-based image processing and storage, Closed Circuit television systems are destined to become 100 percent digital. As modern security management insists on more video surveillance, and legacy CCTV systems wear out and require repair and upgrades, the...

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VGA is an abbreviation of Video Graphics Array, a graphics display system for PCs originally developed by IBM. The resolution is defined at 640x480 pixels, a very similar size to NTSC and PAL. The VGA resolution is normally better suited for network cameras since the video in most cases will be shown on computer screens, with resolutions in VGA or multiples of VGA. Quarter VGA (QVGA) with a resolution of 320x240 pixels is also a commonly used format, very similar in size to CIF. QVGA is sometimes called SIF (Standard Interchange Format) resolution, which can be easily confused with CIF.

Other VGA-based resolutions are XVGA (1024x768 pixels) and 1280x960 pixels, four times VGA, providing megapixel resolution.

Network Video Compression Standards

Without the use of image compression, most local area networks (LANs) are incapable of managing or transporting video data. Digital video is always compressed in order to speed up transmission and to save space on hard disks. Selection and use of the right compression is critical. Here is a listing of compression options.

MPEG-1: 352 x 240 pixels; 30 fps VCR Quality, MPEG-1 was released in 1993 and intended for storing digital video.

MPEG-2: 720 x 480 pixels/ 1280 x 720; 60 fps TV quality. MPEG-2 was approved in 1994 as a standard and was designed for high quality DVD, HDTV, interactive storage media (ISM), digital broadcast video (DBV), and cable TV (CATV). The MPEG-2 project focused on extending the MPEG-1 compression technique to cover larger pictures and higher quality at the expense of a lower compression ratio and higher bit-rate. The frame rate is locked at 25 (PAL)/30 (NTSC) fps, just as in MPEG-1.

MPEG-4: Wavelet based files designed to transmit video over less bandwidth and can combine video with text, graphics and animation. MPEG-4 is a major development from MPEG-2. There are many more tools in MPEG-4 to lower the bit-rate needed to achieve a certain image quality for a certain application or image scene. Furthermore, the frame rate is not locked at 25/30 fps. However, most of the tools used to lower the bit-rate are today only relevant for non real-time applications. This is because some of the new tools require so much processing power that the total time for encoding and decoding (i.e. the latency) makes them impractical for applications other than studio movie encoding, animated movie encoding, and the like. In fact, most of the tools in MPEG-4 that can be used in a real-time application are the same tools that are available in MPEG-1 and MPEG-2.

Motion-JPEG (M-JPEG): Compresses individual jpeg images at 16 - 30 images per second full motion video, depending on available bandwidth. (Individual image quality does vary with bandwidth, but frame rate does.)

Advanced Video Imaging: Mega pixel technology that permits more data and control features such as perspective correction; light level compensation; fixed field pan-tilt-zoom.

Megapixel Resolution

The higher the resolution, the more details can be seen in an image. This is a very important consideration in video surveillance applications because a high-resolution image can enable a criminal to be identified.

As a means of comparing technologies: the maximum resolution in NTSC and PAL, after the video signal has been digitized in a DVR or a video server, is 400,000 pixels (704x576 = 405504). 400000 equals 0.4 Megapixel.

Using the CIF format, i.e. a quarter of the image, the resolution is down to a mere 0.1 Megapixel.

Megapixel network cameras also bring the benefit of different aspect ratios. In a standard TV, an aspect ratio of 4:3 is used, while movies and wide-screen TV use 16:9. In a network camera, any aspect ratio can be used.

In addition, digital pan/tilt/zoom can be achieved, where the operator selects which part of the megapixel images should be shown. This does not imply any mechanical movement from the camera. It ensures much higher reliability and makes it possible for different operators to pan and tilt to different areas of the image simultaneously.

There are two different approaches to compression standards: still image compression and video compression.

All still image compression standards are focused only on one single picture at a time. The most well known and widespread standard is JPEG, short for Joint Photographic Experts Group international — a good and popular standard for still images that is supported by many modern programs. With JPEG, decompression and viewing can be done from standard Web browsers.

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