The Changing World Of Video Compression
Now that digital video recording and network video have entered the mainstream, attention has turned to methodologies for transmitting and storing digital data without sacrificing performance or cost. The industry is flooded with choices of digital video compression methodologies — including Motion JPEG (M-JPEG), MPEG-2, and MPEG-4, the latter of which has recently gained in popularity, yet remains largely misunderstood. End-users, security directors and IT administrators are quickly learning that one compression standard does not fit all.
In IP-based systems, data is transmitted over a network from a source to various recording and/or monitoring hardware, such as a PC server. The data (i.e. images and video) are compressed in order to ease transmission and storage of the series of data. The effectiveness of an image compression technique is measured by the compression ratio, calculated as the original uncompressed image file size divided by the resulting compressed image file size. With a higher compression ratio, less bandwidth is consumed at a given frame rate. Or, if the bandwidth is static, the frame rate is increased. At the same time, a higher compression ratio results in lower image quality for each individual image.
The trade-off among various digital video compression techniques is that the more sophisticated the compression technique used, the more complex and expensive the system. The savings on bandwidth and storage are paid for in system complexity, latency and decoding costs.
A still picture camera captures images and compresses them into a JPEG format. Likewise, a network camera captures and compresses multiple individual images and sends them in a continuous flow over a network to a viewing station. At a frame rate of about 16 fps and above, a viewer will perceive this flow of images as full-motion video. This method is called Motion JPEG or M-JPEG.
A better-known alternative compression technique is MPEG. There are a number of different MPEG standards, including MPEG-1, MPEG-2 and MPEG-4. These standards compare two compressed images to be transmitted over the network, using the first compressed image as a reference frame, and only sending the parts of following images that differ from the reference image. With increased complexity, MPEG enables the amount of data transmitted across the network to be less than that of M-JPEG.
The MPEG-1 standard was released in 1993 with the target application of storing digital video onto CDs. MPEG-2 was approved in 1994 and was designed for high-quality digital video (DVD), digital high-definition TV (HDTV), interactive storage media, digital broadcast video and cable TV.
In contrast to the first two MPEG standards, MPEG-4 does not have specific target applications and may be appropriate for applications demanding a range of image qualities. MPEG-4 covers applications from low bandwidth streaming to mobile devices, to applications with extremely high quality and almost unlimited bandwidth demands.
Due to its simplicity and ability to offer most image resolutions, M-JPEG is a good choice for many applications. The cost in both system time and money is relatively low when using M-JPEG, as it minimizes the delay time between when the image is captured by the camera and when it is displayed over the viewing station — otherwise known as latency. This makes M-JPEG ideal for an application where image processing must be performed, such as video motion detection. M-JPEG also provides relatively high image quality using less bandwidth. On the flipside, users will find that M-JPEG has high bandwidth consumption and storage requirements at frame rates greater than 5 fps, and no support for synchronized sound.
MPEG-2 and MPEG-4 offer users a constant frame rate, meaning that if bandwidth availability goes down, frame rate is maintained at the cost of image quality. The formats also offer high compression capabilities, with low bandwidth and storage requirements at frame rates greater than 5 fps. However, the techniques have a variety of disadvantages, including complex processing requirements on PCs performing decompression, enabling few channels to be shown live and making off-line image analysis slower. In addition, MPEG-2 and MPEG-4 are not very robust — if bandwidth goes below a certain threshold, all video is lost. Users will also find a longer latency time when using MPEG-2 and MPEG-4.
As the general manager for Axis Communications, Fredrik Nilsson oversees the company’s operations in North America‥
