
Bt848/848A/849A
Single-Chip Video Capture for PCI
Brooktree
38
F
UNCTIONAL
D
ESCRIPTION
Video Data Format Conversion
L848A_A
Video Data Format Conversion
Pixel Data Path
The video decoder/scaler portion of the Bt848 generates a video data stream in
packed 4:2:2 YCrCb format. The video data is then color space converted and for-
matted in a 32-bit wide DWORD. Figure 21 shows the steps in converting the vid-
eo data from packed 4:2:2 YCrCb to the desired format. The YCrCb 4:2:2 data is
up-sampled to 4:4:4 format prior to conversion to RGB. It can then be dithered,
have gamma correction removed, or be presented directly to the byte swap circuit.
In the case where 4:1:1 data is desired, the 4:2:2 data is first down sampled, then
packed into BtYUV format or converted to planar format and vertically subsam-
pled to achieve the YUV9 format. Alternatively, packed 4:2:2 data may be convert-
ed to planar 4:2:2 and vertically sub-sampled to YUV12 format. The vertical
subsampling is achieved via the appropriate DMA instructions (see the DMA con-
troller section).
Bt848 also offers a Y8 color format, in which the chroma component of the
packed 4:2:2 data is stripped and the luma component is packed into 8 bits. This
format is otherwise known as gray scale. Table 7 shows the various color formats
supported by the Bt848 and the mapping of the bytes onto 32-bit DWORDs.
Video Control Code
Status Data
In addition to the pixel information, the Bt848’s Video Data Format Converter pro-
vides four bits of video control status code to the FIFO. These four bits of status
code STATUS[3:0] are based on inputs from the video decoder/scaler block of the
Bt848, and convey information about the pixel data and the state of the video tim-
ing (Figure 21). STATUS[3:0] are used to specify the FIFO mode (packed or pla-
nar), provide information regarding the pixel data (respective position of the pixel
and number of valid bytes), to indicate if the pixel data is valid, and to signal the
end of a capture enabled field. See Table 9 in the FIFO section for a full list of the
status codes and descriptions.