cx25840 Conexant Systems, Inc., cx25840 Datasheet - Page 49

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

CX25840/1/2/3 Data Sheet
102284B
8/3/05
information. The 2ωt components of the U and V are then removed by a low-pass
filter, resulting in baseband chroma components.
The Y/C separation block implements a 5-line adaptive comb filter, with adaptive
fallback to a chroma notch filter. The comb filter implements two algorithms, one for
NTSC and one for PAL. The two algorithms are required because the NTSC color
carrier has a 180-degree phase shift between adjacent lines, while the PAL color
carrier has only a 90-degree phase shift. However, the comb filter takes advantage of
these phase shifts in the color subcarrier from line to line to recover luminance data
from the bandwidth occupied by the chrominance data.
The CX25840/1/2/3 separates luminance and chrominance components of the
composite video signal with a flexible combination of a luma comb, chroma comb
and notch filter. A total of five lines are used in the chroma comb filter algorithm.
Four lines are stored in on-chip memory, and the fifth line is the currently decoded
line. The adaptation algorithms take advantage of the two additional lines to provide
pixels that are in phase with the current pixel in NTSC mode, or to provide additional
comb filter options in PAL mode. The luma comb uses only three lines, but which
three depends on whether the signal is NTSC or PAL.
The chroma comb filter uses addition and subtraction to determine the lines with the
highest degree of correlation with the current pixel and extract the chrominance from
these pixels. Since any sample consists of both luma and chroma components, the
difference between two pixels could be due to either luminance or chrominance
difference, and there is not enough information to tell them apart. In other words,
since on any adjacent line, the chroma is out of phase with the current line, subtracting
the two pixels is not the difference between the pixels, but rather chroma plus the
difference. For NTSC systems, the addition of another pair of lines above the usual
three gives access to two lines that are in phase with the current line, making a true
comparison possible. In PAL systems, the additional lines are 180 degrees out of
phase with the current line, making it similar to NTSC with three lines. However,
using a combination of all five lines, a comparison between the current line and the
outer lines is possible. So the comb filter finds the combination of lines in which this
difference is the smallest. If all the differences are above the programmed
CCOMB_ERR_LIMIT threshold in the Chroma Comb Error Limit Max (0x49E)
register, the chroma is passed through unaltered and the luminance is notch filtered.
The luma comb uses only the lines which have chroma 180 degrees out of phase, but it
first low pass filters the signal to remove the chroma, then compares the remaining
luma components. In addition, the chroma comb feeds the difference it has computed
to the luma comb. If both errors are acceptably low (below LCOMB_ERR_LIMIT)
the luma comb averages the pixels over the entire signal bandwidth. This greatly
reduces cross luma in color transitions and high-frequency chroma sources such as
DVD players.
In SECAM mode, or when the adaptation algorithm determines that an unacceptable
error would be created by the comb filter algorithm, it simply notch-filters the color
subcarrier out of the luma. Since SECAM uses an FM modulated signal, a carrier is
always present, regardless of whether or not there is color information being
broadcast. This results in a visible artifact in the luminance at the carrier frequency. To
minimize this effect, an “inverse bell” filter is applied at the encoder to attenuate color
frequencies near the Dr and Db carriers. Thus, if little or no color information is
present in the signal, the carriers will be reduced in amplitude. In order to properly
decode the color information, a “bell” filter is applied to the chroma data when in
SECAM mode.
Conexant
Detailed Functional Description
3-23