Video Port/VCXO Interpolated Control (VIC) Port User's Guide

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3.5.1 Input Filter Modes
3.5.2 Chrominance Re-sampling Operation
YCbCr 4:2:2 co-sited
input samples
chroma-resampled
capture results
Luma (Y)
sample
-
Chroma (Cb/Cr)
samples
a b c d e f g h i j k l
Cb’
ef
= (-3Cb
c
+ 101Cb
e
+ 33Cb
g
-3Cb
i
) / 128
Cr’
ef
= (-3Cr
c
+ 101Cr
e
+ 33Cr
g
- 3Cr
i
) / 128
3.5.3 Scaling Operation
Video Input Filtering
The input filter has four modes of operation: no-filtering, ½ scaling, chrominance re-sampling, and ½
scaling with chrominance re-sampling. Filter operation is determined by the CMODE, SCALE, and
RESMPL bits of VC xCTL.
Table 3-10 shows the input filter mode selection. When 8-bit BT.656 or Y/C capture operation is selected
(CMODE = x00), scaling is selected by setting the SCALE bit and chrominance re-sampling is selected by
setting the RESMPL bit. If 8-bit BT.656 or Y/C capture is not selected (CMODE ≠ x00), filtering is
disabled.
Table 3-10. Input Filter Mode Selection
VC xCTL Bit
CMODE RESMPL SCALE Filter Operation
x00 0 0 No filtering
x00 0 1 ½ scaling
x00 1 0 Chrominance re-sampling (full scale)
x00 1 1 ½ scaling with chrominance re-sampling
x01 x x No filtering
x10 x x No filtering
x11 x x No filtering
Chrominance re-sampling computes chrominance values at sample points midway between the input
luminance samples based on the input co-sited chrominance samples. This filter performs the horizontal
portion of a conversion between YCbCr 4:2:2 format and YCbCr 4:2:0 format. The vertical portion of the
conversion must be performed in software.
The chrominance re-sampling filters calculate the implied value of Cb and Cr in between luminance
sample points based upon nearby co-sited Cb and Cr samples. The resulting values are clamped to
between 01h and FEh and sent to the Cb and Cr capture buffers. Chrominance re-sampling is shown in
Figure 3-8 .
Figure 3-8. Chrominance Re-sampling
The 1/2 scaling mode is used to reduce the horizontal resolution of captured luminance and chrominance
data by a factor of two. For applications that require only CIF or lower resolutions, this reduces the video
capture buffer memory requirements (and the bandwidth needed to write the buffer) by a factor of two.
Vertical scaling must be performed in software. (The bandwidth to load in the buffer is again reduced by
50% over the non-horizontal scaled case.)
58 Video Capture Port SPRUEM1 – May 2007
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