
10-20
Video Interface
Since the rst eld is not available after it has been displayed, RMM
cannot fully support all of the freeze modes. In RMM, only Freeze Last
Field is supported on B pictures because the rst eld of data is
overwritten in memory. In addition, when performing pulldown the second
eld, instead of the rst eld, is repeated during the display of B-frames.
10.8 Horizontal Postprocessing Filters
The Display Controller integrates two separate horizontal postprocessing
lters, a simple 2:1 horizontal decimation lter and an 8-tap interpolation
lter. These lters are provided for scaling images horizontally along the
scan line.
The decimation lter is a simple bilinear averaging lter that decimates
two pixels down to one along the horizontal scan line. This lter may be
used in conjunction with the 0.5 letterbox lter for displaying pictures at
1/4 resolution. Such an application may require a four-frame store
system to account for the higher bandwidth requirements. The
decimation lter is enabled by setting the Horizontal Decimation Filter
Enable bit in Register 274
(page 4-82). When using the decimation lter,
the main reads per line should be programmed to the number of
frame-store reads required to reconstruct the picture, i.e., twice the
picture width.
Regardless of the input picture resolution, the horizontal interpolation
lter can provide up to 720 pixels on each line. In addition to its
interpolation features, the lter also provides ne-scale horizontal pan
and scan to within 1/8th of a pixel during pan and scan operation. This
lter is used for both luminance and chrominance data.
The interpolation lter is implemented using an 8-tap polyphase lter.
The lter is capable of generating up to eight, unique, subpixel values
between two consecutive pixels on a scan line. The generation of pixels
depends upon the ratio between the width of the source image and the
target image. Typically, the target image width is 720 pixels.