CX25870/871
1.0 Functional Description
Flicker-Free Video Encoder with Ultrascale Technology
1.3 Device Description
However, the trade-off is that as flicker is reduced, more and more information
is being altered by the encoder and potentially lost from the original picture.
Vertical resolution is therefore sacrificed and text clarity suffers especially for
small fonts below 10 points in size. For this reason, the amount of flicker filtering
is programmable and should be controllable by the end user. Finding an optimal
standard flicker filter setting for Luma and Chroma is somewhat subjective in
nature and ensures that a pleasing image is seen on the television.
Unlike other encoders, the CX25870 integrates both a standard flicker filter
and additional adaptive flicker filter. This implementation allows for the
preservation of small font text clarity and other challenging video images lost
with only one filtering step. The adaptive feature eliminates more flicker with less
loss of resolution because it is able to selectively apply more aggressive flicker
reduction only to those portions of an image where the effect will be beneficial.
Encoders lacking this adaptive filter apply the standard flicker filtering process to
the entire screen. Small text and icons often become unreadable and thin,
horizontal lines may completely disappear. The CX25870’s adaptive flicker filter
prevents this from happening and is described in its own section within this
document.
So long as progressive RGB or YCrCb data is received, the CX25870’s flicker
filter is effective with any active resolution from 320x200 to a maximum of 1024
x 768. The flicker reduction is present on any interlaced video output such as
NTSC, PAL, or SECAM. The DIS_FFILT register bit turns off the standard
flicker filter. The vertical scaling can be disabled by setting the internal
V_SCALE register to 4096 for a noninterlaced input. Finally, the CX25870
supports up to 24-bit color processing, meaning that the converted image will
feature the same depth of color as the original computer picture.
1.3.19 Adaptive Flicker Filter
Adaptive Flicker Filtering is a new feature included with the CX25870/871. It
allows the encoder to automatically alter the amount of flicker filtering based on
the image being processed. The result is a high-quality optimized image because
the perfect balance between vertical resolution and flicker reduction has been
achieved. The adaptive flicker filter is enabled via the ADPT_FF bit. There are
four possible settings ranging from 2-line (most observable flicker, greatest
vertical resolution) to 5-line (minimal observable flicker, moderate vertical
resolution). The luminance and chrominance outputs are independent in terms of
the level of adaptive flicker filtering. When the adaptive flicker filter is on, the
manual flicker filter is off and vice versa.
Vertical filtering in the CX25870/871 serves three purposes:
•
Vertical polyphase interpolation filtering to upsample the image data
vertically. This increases the resolution and accuracy of the subsequent
vertical downsampling required to fit the entire image into the visible
region of the television.
•
•
Anti-alias filtering to reduce aliasing artifacts when downsampling
vertically.
Flicker filtering to reduce the flicker produced when vertical high
frequency content is displayed on an interlaced device.
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Conexant
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CONEXANT [ CONEXANT SYSTEMS, INC ]
