Every digital video surveillance system uses compression in order to manage file size when transporting video over the network for storage and viewing. Bandwidth and storage requirements render uncompressed video impractical and expensive, so compression technologies have emerged as an efficient way to reduce the amount of data sent over the network. In short, compression saves money.
Today there are many kinds of compression available. Compression technology can be proprietary - invented and supported by one only vendor - or based on a standard and supported by many vendors. Selecting the right compression is vital to ensuring the success of a video surveillance installation. It provides the appropriate quality at the budgeted cost and ensures the system is future proof. Selecting the right compression can even determine whether video is admissible in court cases, an important consideration for security and surveillance installations.
The effectiveness of an image compression technique is determined by the compression ratio, calculated as the original (uncompressed) image file size divided by the resulting (compressed) image file size. At a higher compression ratio, less bandwidth is consumed at a given frame rate. If bandwidth is kept consistent, the frame rate is increased. A higher compression ratio also results in lower image quality for each individual image.
[See Images A, B, C and D at right to compare how different compression formats can affect your final image quality.]
There are essentially two approaches to compression: lossless or lossy. In lossless compression, each pixel is unchanged, resulting in an identical image after the image is decompressed for viewing. Files remain relatively large in a lossless system, which makes them impractical for use in network video solutions. A well-known lossless compression format is the Graphics Interchange Format , better known as a .GIF image.
To overcome these problems, several lossy compression standards have been developed, such as JPEG and MPEG. The fundamental idea in lossy compression is to reduce portions of the image that appear invisible to the human eye, thereby decreasing the size of the data transmitted and stored.
A Note on Still Images
Video is essentially a stream of individual images. The most widely accepted standard for still image compression is the Joint Photographic Expert Groups (JPEG) standard. It was developed in the 1980s and has been integrated into standard Web browsers. JPEG decreases file sizes by making use of similarities between neighboring pixels in the image and the limitations of the human eye. Other lossy image compression techniques include JPEG2000 and Wavelet. JPEG is by far the most common and most widely supported compression standard for still images.
Motion JPEG is the most commonly used standard in network video systems, however it is technically a still-image compression technique. When employing Motion JPEG compression, network cameras capture individual images and compress them into JPEG format - similar to a still picture - and there is no compression between the individual frames. If a network camera captures and compresses 30 individual still images per second, it makes them available as a continuous flow of images resulting in full-motion video. As each individual image is a complete JPEG compressed image, they all have the same guaranteed quality, determined by the compression ratio for the network camera or video server.