AXIS Media Control - FAQ

Bit rate control for MPEG-4 streams

Question:

Could you please describe in details how to set Bit Rate Control parameters i.e. which are the advantages and disadvantages of different bit rate modes?

Answer:

When streaming MPEG-4 in Axis products, it is possible to control the bit rate (bandwidth) from the server by specifying the bit rate mode (VBR or CBR), the maximum and the target bit rate (for Constant bit rate only).

VBR (Variable Bit Rate)
When using VBR, the quality of the video stream is kept as constant as possible, at the cost of a varying bit rate, and regardless of whether or not there is motion or not in the image. This is often desirable in security and surveillance applications, where high quality is a requirement, especially if there is motion in the picture. Since the bit rate in VBR can vary, it is important that the network infrastructure has the necessary capacity to provide the extra bandwidth. It is however, also possible to set a maximum bit rate, for more exact bandwidth control when the bit rate is allowed to vary.

CBR (Constant Bit Rate)
CBR is used to maintain a specific bit rate by varying the quality of the MPEG-4 stream. With CBR, it is possible to use a target bit rate, towards which the bit rate should always converge, with +/-10% as the divergence.
As for VBR, the maximum bit rate can be configured to prevent the frame rate deviating from the set target.

With limited bandwidth available, the preferred mode is CBR, since this mode generates a constant and predefined bit rate. The disadvantage is that video quality will vary, and while it will remain relatively high as long as there is no motion in the image scene, the quality may decrease significantly with increased motion.

As the bit rate would usually need to increase for increased activity in the image, but in the case of CBR cannot, the frame rate and image quality will be affected negatively. To go some of the way towards compensating for this, in Axis products it is possible to prioritize either the frame rate or the image quality whenever the bit rate would normally need to be increased. Not setting a priority means the frame rate and image quality will be affected approximately equally.

Maximum bit rate (MBR)
For more exact bandwidth control in Axis products, a maximum bit rate can be configured. This can be set when using VBR or CBR.

This setting is not the same as the network "maximum bandwidth" setting, which arbitrarily discards packets to enforce the bandwidth limit for the whole server, resulting in corrupt MPEG-4 streams. The MPEG-4 MBR (Maximum Bit Rate) setting is individual for each MPEG-4 source. After a frame has been compressed, the MBR will check if sending the frame would exceed the bit rate limit and, if this is the case, the frame is dropped. Since the MBR operates on whole frames, the stream will still be intact, although the frame rate will be lower when frames are dropped.

Note that in order to maintain the Profile@Level limit, MBR and CBR are always active. When VBR is enabled, the CBR controller increases the compression level if the bit rate exceeds 80% of the Profile@Level limit, although it will not adjust the frame rate. If the limit is exceeded, the MBR controller will start to drop frames.

It is recommended to set the MBR limit to at least 10-20% above the CBR limit or profile@level limit, to reduce the number of dropped frames.

Profile@Level is set by specifying Maximum bit rate in the AXIS 207 Series, the AXIS 212 PTZ and the AXIS 216FD.

For other products Profile@Level corresponds to 1500 kbps when ISMA Compliant streaming is set in the product's configuration and 8000 kbps when this is disabled. In this case, additional Profile@Levels are available through System Options > Advanced > Plain Config > Image
More about Profiles@Levels can be found at:
http://www.m4if.org/resources/profiles/index.html

This FAQ applies to the AXIS 207/207W/210/210A/211/211A/212 PTZ/213 PTZ/214 PTZ/216FD/221/225FD/241S(Blade)/Q(Blade)/241SA/QA.