cx25840 Conexant Systems, Inc., cx25840 Datasheet - Page 50

• Manufacturer Part Number: cx25840
• Description: Video Decoder And Broadcast Audio
• Manufacturer: Conexant Systems, Inc.

Detailed Functional Description
3.4.9
3-24
Luma Processing
The luma data path logic processes the luma waveform that comes from the Y/C
separation module to produce SPI luma samples. These decoded luma samples are
then sent through logic blocks that can perform post-processing functions such as
white crush, peaking for sharpness control, contrast, and brightness adjustment before
they are passed along to the scaling block.
White Crush
The “white crush” term refers to Conexant’s algorithm for adapting to luma overflow
in the presence of nonstandard sync-tip-to-white ratios. The luma samples are first
passed through the white crush circuitry, which works with the front-end to
automatically fine-tune the video levels. This circuitry adjusts the overall gain of the
input signal such that excursions above nominal 100 or 110 IRE peak white-level are
compensated for to reduce video-blooming artifacts. The white crush processing is by
default enabled but can be disabled by writing a 0 to the WCEN bit of Mode Control
register 2 (0x401). The 110 IRE threshold can be enabled by setting the WTW_EN bit
high in the White Crush Control 3 register (0x4A2).
Following white crush, the back porch of the luma waveform is removed, and for some
video formats, the setup (or pedestal) value is removed as well. The luma samples are
then multiplied by scale factors to match SPI defined luma levels. Setup removal is
determined by the video standard.
Luma Low-Pass Filtering
Next, the luma is optionally low-pass filtered to remove any high frequency content
that could cause alias artifacts associated with horizontal scaling. The filter has four
different transfer functions based upon the desired horizontal scaling factor. The
appropriate transfer characteristic is automatically selected by deriving from the value
that is programmed in the Horizontal Scaling registers (0x418 to 0x41A). One of
these four transfer functions is nothing more than a filter bypass. This is automatically
chosen when no scaling is enabled.
Conexant
CX25840/1/2/3 Data Sheet
102284B
8/3/05