How Video Rates are Configured
(Example is pertinent to MPEG2 (MP2))
Video rates can be configured manually or automatically. The factory default is to automatically
set the video rate based on TMR. This option is referred to as VAF (Video Autofill). VAF
determines what the TMR is. It reserves 3.5% for null packets. It detects the video input
resolution for each channel. It automatically calculates the required headroom for audio pids. It
sets the video bit rate for each channel accordingly to maximise available bandwidth.
SD encode bitrate = 1.00 to 15.00 Mbps
HD encode bitrate = 7.00 to 60.00 Mbps
Example:
TMR is set to 40 Mbps.
Channel 1 has a SD 720x480i source.
Channel 2 has a HD 1920x1080i source.
15.00 Mbps will be allocated to channel 1 because the source is standard definition. 19.10Mbps
will be allocated to channel 2 (this is the available bandwidth after VAFs calculation)
If VAF is set to OFF, the video rate can be manually adjusted for each channel. If the manual
configured video rate would cause an egress overflow, the VAF logic will constrain the
misconfiguration and reconfigure the video rate for both channels.
Example:
TMR is set to 40 Mbps.
Both channels have a HD1920x1080i source.
Both channels audio = 256 kbps
Video fill is set to off.
If both encoders are set to 19.10 Mbps and you want to reconfigure them to say 15.00 Mbps for
channel 1 and 25.00 Mbps for channel 2. This would cause an egress overflow condition
because you have not factored in the 3.5% reserve and bandwidth occupied by audio pids.
Even though VAF is set to off it’s logic will override the configuration request and constrain the
video rates back down to 19.10 Mbps for both channels. To obtain the desired configuration,
the user would need to recalculate rates, then configure channel 1 to 15.00 Mbps and channel 2
to 23.00 Mbps.
