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Sony’s Motion Adaptive Field Noise Reduction uses new
technology to overcome the old compromises. Thanks to the
new circuit, the DVP-S9000ES can achieve very high signal-to-
noise ratio, very high resolution and very accurate motion
— all at the same time!
To overcome motion artifacts, the Sony design actively analyzes
the fields for movement. The system divides the image into
blocks of 4 pixels high by 16 pixels wide. This specific
rectangular shape is based on the understanding that most
movement in video images is horizontal. In blocks where no
large changes are detected between fields, the system applies
conventional noise reduction.
When large changes are detected between fields, the system
automatically searches for block movement. The search
“window” is 11 pixels high by 31 pixels wide.
By finding matches for blocks that have moved, Sony’s Motion
Adaptive Field Noise Reduction makes it possible to apply noise
to areas of screen movement, in addition to the still background.
The system compares data from the two fields by a mathematical
method called the Hadamard transform. Noise, in the form of
differential luminance (Y) signals, is analyzed in blocks one
pixel high by 8 pixels wide. The Hadamard transform converts
the noise into an easily processed frequency distribution. To
reduce errors, eight operations are performed for each pixel. The
average of the eight values is then used. A limiter is then used to
extract noise from the converted signals. Than a reverse
Hadamard transform generates an error-correction signal that is
added to the video signal to suppress noise.
In addition, Sony’s Motion Adaptive Field Noise Reduction
performs the same process for noise in the color difference
channels, C
B
and C
R
. Since luminance and color difference
signals are separately processed, their noise reduction can be
individually optimized.
The mathematics of this noise reduction process may be
complex, but the results are easy to appreciate. The circuit
accomplishes three formerly elusive goals simultaneously:
1. Clean noise-free images on both moving portions and still
portions of the screen.
2. The full resolution of the DVD.
3. Clear and natural image movement, without ghosting or other
motion artifacts.
Another characteristic limitation of MPEG-2 compression is
block noise. This is the tendency for delicately shaded picture
areas to be rendered as solid rectangles of color. Instead of a
continuous contour of color on the cheek of an actress, fading
softly into shadow, you see subtle squares of approximately
correct color. Block noise is most apparent at the edges where
squares meet. This effect is also called mosquito wings, because
it can appear as subtle discoloration in tiny slices of the picture
and just as quickly disappear.
Sony’s MPEG Image Processor mounts a sophisticated, compre-
hensive attack on block noise. The aim of Sony’s system is to
identify those areas of subtle gradation of tone — the areas most
prone to block noise — analyze the gradations and reconstruct
the abrupt steps of tone as more linear, more gradual slopes.
The principal challenge here is to distinguish legitimate steps in
the picture tone (signal) from the unwanted, artificial steps
(block noise). This separation of wanted and unwanted steps is
made easier because the block borders area always at the same
places on the screen. Their location is a fixed attribute of the
MPEG-2 compression used in the DVD format.
The DVD format divides the screen into 2,700 blocks (90 hori-
zontal by 30 vertical). Each block measures 8 pixels horizontal
by 8 pixels vertical. They’re shown as vertical rectangles because
of interlace scanning. To judge for block noise, the MPEG Image
Processor establishes the three pixels to the left and right of the
Block Noise Reduction
Fig. 7: Sony’s Motion Adaptive Field noise reduction identifies and suppresses noise
across the entire screen area — in moving parts of the picture as well as the motionless
background. The process uses advanced motion detection and a Hadamard transform to
suppress noise without sacrificing detail or generating motion artifacts.
Fig. 6:
Sony analyzes the video fields for movement by dividing the fields
into blocks that are 4 pixels high by 16 pixels wide. When big changes
are detected between two fields, Sony’s MPEG Image Processor
determines that there is screen movement. The system attempts to match
the block across a search window that extends four pixels up, three pixels
down, eight pixels to the left and seven to the right.