Video compression uses a similar method as that of still image compression. However, it adds compression between the frames to further reduce the average file size. MPEG is one of the best-known audio and video compression standards and was created by the Motion Picture Experts Group in the late 1980s. MPEG compression utilizes one frame as a reference. Each additional frame saves and transports only the image information that is different from the original. If there is little change between the images, there will be few differences resulting in a high compression ratio. With significant movement in the images the compression ratio will be much lower. The video is then reconstructed at the viewing station based on the reference image and the "difference data." MPEG video compression leads to lower data volumes being transmitted across the network than with JPEG.
[Images E and F (above, at right) give an example of the difference between how a Motion JPEG storage format works and that of an MPEG format.]
The MPEG standard has evolved since its inception. MPEG-1 was released in 1993 and was intended for storing digital video onto CDs. For MPEG-1, the focus was on keeping the bit-rate (the amount of data transmitted via the network per second) relatively constant. However, this created inconsistent image quality, typically comparable to that of videotapes.
MPEG-2 was approved in 1994 and was designed for video on DVDs, digital high-definition TV, interactive storage media, digital broadcast video, and cable TV. The MPEG-2 project focused on extending the MPEG-1 compression technique to cover larger, higher quality pictures with a lower compression ratio and higher bit-rate.
For network video systems, MPEG-4 is a major improvement from MPEG-2. It was approved as a standard in 2000, and there are many more tools in MPEG-4 to lower the bit-rate needed and achieve higher image qualities. MPEG-4 comes in many different versions. Simple Profile is the lowest quality, while Advance Simple Profile (Part 2) provides much higher quality video. A newer version of MPEG-4 called Part 10 (or AVC - Advanced Video Coding, or H.264) is also available.
With a limited bandwidth available, users can opt for a constant bit-rate (CBR), which generates a constant, pre-set bit-rate. However, the image quality will vary depending on the amount of motion in the scene. As an alternative, users can use a variable bit-rate (VBR) where parameters can be set to maintain high image quality regardless of the motion in the scene. This option is generally preferred in surveillance applications. Because the actual bit-rate will vary with VBR, the network infrastructure must have enough capacity to transport the video.
The MPEG-4 vs. Motion JPEG Debate
As described above, MPEG-4 and Motion JPEG each employ a different technique for reducing the amount of data transferred and stored in a network video system. There are advantages and disadvantages to each, so it is best to consider the goals of the overall surveillance system when deciding which of the two standards is most appropriate.
Due to its simplicity, Motion JPEG is often a good choice. There is limited delay between image capturing, encoding, transfer, decoding, and finally display. In other words, Motion JPEG has very little latency, making it most suitable for real-time viewing, image processing, motion detection or object tracking.
Motion JPEG also guarantees image quality regardless of movement or image complexity. It offers the flexibility to select either high image quality (low compression) or lower image quality (high compression), with the benefit of smaller file sizes and decreased bandwidth usage. The frame rate can easily be adjusted to limit bandwidth usage, without loss of image quality.
However, Motion JPEG files are still typically larger than those compressed with the MPEG-4 standard. MPEG-4 requires less bandwidth and storage to transfer data resulting in cost savings. At lower frame rates (below 5 fps) the bandwidth savings created by using MPEG-4 are limited. Employing Motion JPEG network cameras with video motion detection built in, is an interesting alternative, if a higher frame rate is only required a portion of the time when motion is in the image. If the bandwidth is limited, or if video is to be recorded continuously at a high frame rate, MPEG-4 may be the preferred option. Because of the more complex compression in a MPEG-4 system, there is more latency before video is available at the viewing station. The viewing station needs to be more powerful (and hence expensive) to decode MPEG4, as opposed to the decoding of Motion JPEG streams.
One of the best ways to maximize the benefits of both standards is to look for network video products that can deliver simultaneous MPEG-4 and Motion JPEG streams. This gives users the flexibility to both maximize image quality for recording and reduce bandwidth needs for live viewing.