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Silicon Image, Inc. iScan Pro User Manual
17
Deinterlacing
Deinterlacing is the process by which interlaced video is converted to progressively scanned video.
Progressive scanning paints all of the lines of a frame in one top to bottom pass. This is used where
transmission bandwidth is not an issue and where the highest quality image is required. None of the
interlaced side effects are present with progressive scanning.
Devices for performing deinterlacing are available for tens of dollars for low quality techniques or
many thousands of dollars for very sophisticated techniques. The low cost techniques are frequently
used in progressively scanned TVs or projectors. High quality algorithms capable of generating very
high quality video are typically used in Line Doublers designed for high-end home theater markets.
Some very inexpensive deinterlacers simply put fields together, creating an output frame containing
even lines from one point in time and odd lines from 1/60 second later. Any motion between these
two fields will result in the motion artifacts illustrated above.
To avoid these artifacts, some deinterlacers simply scale each of the fields up to the entire frame
size, interpolating between the existing lines. Unfortunately, this also significantly reduces the
vertical resolution of the image, resulting in softening of the picture with loss of image detail.
One method of avoiding this softening is to determine if there is any movement between fields by
comparing each of the fields with its counterpart in a previous frame. Further refinement of this
algorithm would be to apply the softening filter only to portions of the image that are in movement.
This is referred to as “motion adaptive” deinterlacing.
The most advanced and best quality Line Doublers are designed to also take advantage of the “3:2
pulldown” technique that is used to transfer film to video. During this transfer, the first film frame is
captured onto 2 video fields (first even, then odd lines are scanned) then the second film frame is
captured onto 3 video fields (even, odd, even). As this is repeated, you can see that two 24fps film
frames (for a total of 1/12 of a second) are captured onto five 60fps video fields (for a total of 1/12 of
a second). A deinterlacer can examine a series of fields to detect this sequence and thereby
determine that the original, pre-video source of this sequence was film. It can then reassemble the
original progressive frames from the partial interlaced fields with no loss of resolution or with no
introduction of motion artifacts.
Silicon Image’s DVDO technology performs even more advanced techniques than those described
above. Performing over six billion arithmetic operations per second on the incoming video stream,
the iScan Pro uses the data from four video fields during its processing. It can determine not only
which portions of the image are in motion, but also what type of movement this is, and how best to
generate a progressive image with maximum picture detail and minimum motion artifacts.
The iScan Pro performs excellent 3:2 pulldown detection. In addition, it also recognizes the 2:2
pulldown sequence used for converting PAL film and computer graphics to video. For these film and
computer graphics sources, the iScan Pro will reassemble the original progressive frames with no
unnecessary filtering of image detail.