Green dominates the luma channel: about 59% of the luma signal comprises green information. Therefore it is sensible, and advantageous for signal-to-noise reasons, to base the two colour channels on blue and 1red. The simplest way to remove luma from each of these is to subtract it to form the difference between a primary colour and luma. Hence, the basic video colour-difference pair is (B-Y), (R-Y) [pronounced "B minus Y, R minus Y"].
The (B-Y) signal reaches its extreme values at blue (R=0, G=0, B=1;
Y=0.114; B-Y=+0.886) and at yellow (R=1, G=1, B=0; Y=0.886; B-Y=-0.886).
Similarly, the extrema of (R-Y), +-0.701, occur at red and cyan. These
are inconvenient values for both digital and analog systems. The colour
spaces YPbPr, YCbCr, PhotoYCC and YUV are simply scaled versions of (Y,
B-Y, R-Y) that place the extrema of the colour difference channels at more
convenient values.
Bitrate
The rate at which the compressed bitstream is delivered from the storage
medium to the input of a decoder.
Block
An 8-row by 8-column matrix of
pels,
or 64
DCT
coefficients (source, quantised
or dequantised).
Bottom field
One of two fields that comprise a
frame
of interlaced video. Each line of a
bottom field is spatially located immediately below the corresponding line
of the top field.
Bridge
Bridges are devices that connect similar and dissimilar LANs at the data link
layer (OSIlayer 2), regardless of the physical layer protocols or media being
used. Bridges require that the networks have consistent addressing
schemes and packet frame sizes. Current introductions have been termed
learning bridges since they are capable of updating node address (tracking)
tables as well as overseeing the transmission of data between two Ethernet
LANs.
Brouter
Brouters are bridge/router hybrid devices that offer the best capabilities of
both devices in one unit. Brouters are actually bridges capable of
intelligent routing and therefore are used as generic components to integrate
workgroup networks . The bridge function filters information that
remains internal to the network and is capable of supporting multiple
higher-level protocols at once.
The router component maps out the
optimal paths for the movement of data from one point on the network to
another. Since the brouter can handle the functions of both bridges and
routers, as well as bypass the need for the translation across application
protocols with gateways, the device offers significant cost reductions in
network development and integration.
Byte aligned
A bit in a coded bitstream is byte-aligned if its position is a multiple of
8-bits from the first bit in the stream.