Digital video compression techniques are prevalent throughout a variety of markets due to a finite amount of storage and bandwidth coupled with the necessity for the highest possible image quality. This is especially important for IP video surveillance networks where security departments require a happy medium between usable video records and adequate bandwidth throughput of the computer network. As it were, there are various compression standards that each have its advantages, giving security system design engineers the additional responsibility of determining which compression technology is best for the organization’s video system.
One of the most broadly-used compression formats used for many years is the Motion JPEG (M-JPEG) standard, which is unlicensed and compatible with a wide variety of technology, allowing for easy integration. M-JPEG compression is unique because it does not technically compress video sequences but rather image sequences, which is what comprises video anyway (albeit in varying frame rates). M-JPEG is able to reduce the image data by making small reductions in various aspects of the image, such as color nuances and light intensity, or removing parts of the image entirely since they would not be visible to the average user.
- The upside to this method? Each frame is compressed individually, meaning that if one happens to be dropped during transmission, other frames within the video sequence will not be affected.
- The downside is that the compression ratio is low in comparison with other techniques, and since every image is transmitted, the bit rate can be higher if the frame rate of the captured video sequence is also high (more than 16 images per second).
Other compression technologies, such as MPEG-4 and H.264, actually compress video sequences rather than image sequences using techniques called difference coding and interframe prediction. This method compares frames to each other to determine where differences occur. If, for example, there is footage of a person running by a building, the first image is coded in its entirety, and then the following frames reference the first coded image to see where there are redundancies in the sequence; in this case, it would be most of the building, as those parts of the image are static. Since that portion of the following images are already encoded, they don’t need to be sent and stored; only the portions of the image that are moving (the person) are encoded during the compression process (illustrated below).
|Illustration courtesy of Axis Communications|
- Advantages: H.264 is able to reduce the size of a digital video file by 50 to 80% compared to M-JPEG compression, drastically reducing the amount of network bandwidth and storage space needed.
- Disadvantages: These standards are licensed, so a fee per monitoring station is required for users to pay. Additionally, H.264 in particular requires higher performance network cameras and monitoring stations.
Of course, there are multitudes of other compression technologies throughout the security industry, one of which being H.265, which may be helpful for encoding and transmitting 4K and 8K video in the future. Because of all these different compression techniques, comprised of both proprietary and industry standards, utilize different methods of reducing data, and, consequently, differ in results of bit rate, quality and latency, it’s important to take such factors into consideration when designing and/or upgrading a video surveillance solution.