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Besides difference coding and motion compensation, other advanced methods can be employed
to further reduce data and improve video quality. H.264, for example, supports advanced tech-
niques that include prediction schemes for encoding I-frames, improved motion compensation
down to sub-pixel accuracy, and an in-loop deblocking filter to smooth block edges (artifacts).
For more information on H.264 techniques, see Axis’ white paper on H.264 at www.axis.com/
corporate/corp/tech_papers.htm
7.2
Compression formats
7.2.1 Motion JPEG
Motion JPEG or M-JPEG is a digital video sequence that is made up of a series of individual JPEG
images. (JPEG stands for Joint Photographic Experts Group.) When 16 image frames or more are
shown per second, the viewer perceives motion video. Full motion video is perceived at 30
(NTSC) or 25 (PAL) frames per second.
One of the advantages of Motion JPEG is that each image in a video sequence can have the same
guaranteed quality that is determined by the compression level chosen for the network camera
or video encoder. The higher the compression level, the lower the file size and image quality.
In some situations, such as in low light or when a scene becomes complex, the image file size
may become quite large and use more bandwidth and storage space. To prevent an increase in
the bandwidth and storage used, Axis network video products allow the user to set a maximum
file size for an image frame.
Since there is no dependency between the frames in Motion JPEG, a Motion JPEG video is robust,
meaning that if one frame is dropped during transmission, the rest of the video will not be
affected.
Motion JPEG is an unlicensed standard. It has broad compatibility and is popular in applications
where individual frames in a video sequence are required—for example, for analysis—and where
lower frame rates, typically 5 frames per second or lower, are used. Motion JPEG may also be
needed for applications that require integration with systems that support only Motion JPEG.
The main disadvantage of Motion JPEG is that it makes no use of any video compression tech-
niques to reduce the data since it is a series of still, complete images. The result is that it has a
relatively high bit rate or low compression ratio for the delivered quality compared with video
compression standards such as MPEG-4 and H.264.
VIDEO COMPRESSION - CHAPTER 7
