61
7.3
Variable and constant bit rates
With MPEG-4 and H.264, users can allow an encoded video stream to have a variable or a
constant bit rate. The optimal selection depends on the application and network infrastructure.
With VBR (variable bit rate), a predefined level of image quality can be maintained regardless of
motion or the lack of it in a scene. This means that bandwidth use will increase when there is a lot
of activity in a scene and will decrease when there is no motion. This is often desirable in video
surveillance applications where there is a need for high quality, particularly if there is
motion in a scene. Since the bit rate may vary, even when an average target bit rate is defined, the
network infrastructure (available bandwidth) must be able to accommodate high throughputs.
With limited bandwidth available, the recommended mode is normally CBR (constant bit rate)
as this mode generates a constant bit rate that can be predefined by a user. The disadvantage
with CBR is that when there is, for instance, increased activity in a scene that results in a bit
rate that is higher than the target rate, the restriction to keep the bit rate constant leads to a
lower image quality and frame rate. Axis network video products allow the user to prioritize
either the image quality or the frame rate if the bit rate rises above the target bit rate.
7.4
Comparing standards
When comparing the performance of MPEG standards such as MPEG-4 and H.264, it is impor-
tant to note that results may vary between encoders that use the same standard. This is because
the designer of an encoder can choose to implement different sets of tools defined by a stan-
dard. As long as the output of an encoder conforms to a standard’s format and decoder, it is
possible to make different implementations. An MPEG standard, therefore, cannot guarantee a
given bit rate or quality, and comparisons cannot be properly made without first defining how
the standards are implemented in an encoder. A decoder, unlike an encoder, must implement all
the required parts of a standard in order to decode a compliant bit stream. A standard specifies
exactly how a decompression algorithm should restore every bit of a compressed video.
The graph on the following page provides a bit rate comparison, given the same level of image
quality, among the following video standards: Motion JPEG, MPEG-4 Part 2 (no motion compen-
sation), MPEG-4 Part 2 (with motion compensation) and H.264 (baseline profile).
VIDEO COMPRESSION - CHAPTER 7
