cx25840 Conexant Systems, Inc., cx25840 Datasheet - Page 54

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

Detailed Functional Description
Table 3-21. Luma Output Ranges
3-28
RANGE[5:4]
00
01
1x
Nominal Range
After Color-
Conversion
(decimal)
64–940
64–940
0–1023
Space
Contrast
Contrast adjustment follows the peaking filter. This block multiplies the luma samples
by the contrast value that is programmed into the Contrast register (0x415). The
multiplication value is represented by a 2s complement number, thus values above
0x80 increase the contrast, and values below 0x80 decrease the contrast. This is
followed by color-space conversion, which converts the luma from YUV color space
to YCrCb color space. The function involves multiplying the incoming luma data by
the color-space conversion factor. The incoming video standard and the desired output
range of luma codes determine the multiplication factor. The RANGE bits of the
Luma Control register (0x416) determine the number of 10-bit codes, out of 1024,
that are allowed for the nominal luma range. This does not mean that the luma is
limited to this range. It just means that this range is used in the color space conversion
process.
Brightness
Following the contrast and color-space conversion multiplication operations, the
brightness adjustment is performed, which is an addition of the luma samples with 8
times the value found in the BRIGHTNESS register (0x414). This register value is
represented by a 2s complement number, allowing the overall video level to be
adjusted down by as much as 1024, or up by as much as 1016 on a 10-bit scale. This
results in a luma signal that has been offset by an amount equal to the brightness
value.
The next step is the coring operation, which serves to remove low-level noise by
zeroing-out any values whose magnitude is below a user-selected threshold. This
threshold is selected through the LUMA_CORE_SEL bits in the Luma Control
register (0x416). This coring threshold can range from 0, which would have no effect,
to 64, which would zero-out the 7 LSBs of the luma output.
The final process of the luma data path is the range saturation. The range adjustment
enables the user to select one of three allowed luma output ranges. The first range is
compliant to the SPI specification that allows for a nominal 64–940 range with
excursion up to 1016 but not below 16-black level. The second range is similar in that
it allows for a nominal 64–940 with excursions up to 1016 but in addition it allows for
excursions below 16. This is to support blacker-than-black values which are possible
on systems with a pedestal. The final range places no restrictions on the data in that it
allows all value from 0 to 1023 to pass.
luma data for the various RANGE bits in the Luma Control register (0x416).
Output Range
Allowed 8-bit
(decimal)
16–235
1–235
0–255
Conexant
Allowed 10-bit
Output Range
(decimal)
64–1016
4–1016
0–1023
Table 3-21
Nominal range with excursions allowed but
no blacker than black levels
Nominal range with excursions and blacker
than black allowed
0–1023 nominal with no limiting
illustrates the output range of the
Description
CX25840/1/2/3 Data Sheet
102284B
8/3/05