Axis Communications FD-R M12 - Manual (Page 17)

17 AXIS 209FD/FD-R/FD-R M12
Advanced - MPEG-4 Settings
These are the tools for adjusting the MPEG-4 settings and controlling the video bit rate.
GOV Settings
The GOV structure describes the composition of the MPEG-4 video stream. Setting the GOV-length to a high
value saves considerably on bandwidth. The Cyclic refresh parameter gives a smoother bit rate and can be
useful if bandwidth is limited.
Bit Rate Control
Limiting the maximum bit rate is a good way of controlling the bandwidth used by the MPEG-4 video
stream. Leaving the Maximum bit rate as unlimited will provide consistently good image quality, but at the
expense of increased bandwidth usage whenever there is more activity in the image. Limiting the bit rate to
a defined value will prevent excessive bandwidth usage, but images will be lost when the limit is exceeded.
Note that a maximum bit rate can be used for both variable and constant bit rates.
The bit rate type can be set as Variable Bit Rate (VBR) or Constant Bit Rate (CBR). VBR will adjust the bit
rate according to the images' complexity, thus using a lot of bandwidth for a lot of activity in the image
and less for lower activity in the monitored area.
CBR allows you to set a fixed Target bit rate that will consume a predictable amount of bandwidth, and
which will not change whatever happens in the image.
As the bit rate would usually need to increase for increased image activity, but in this case cannot, the
frame rate and image quality will be affected negatively. To go some of the way towards compensating for
this, 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.