Variable and constant bit rates

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.

Variable bit rate

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.

Constant bit rate

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.

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