Deinterlacing
Deinterlacing is the process by which interlaced video is converted to progressively
scanned video. Devices for performing deinterlacing are available for tens of dollars for
low quality techniques or for many thousands of dollars for very sophisticated techniques.
The low cost techniques are frequently used in progressively scanned TVs or projectors.
High quality algorithms are used typically in Line Doublers designed for high-end home
theater markets and generate very high quality video.
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 when it isn't needed 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 "2:3 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 frames (for a total of 1/12 of a second). A
deinterlacer can examine a series of frames 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 frames with no loss of resolution
or with no introduction of motion artifacts.
DVDO's PureProgressive™ technology performs even more advanced techniques than those
described above. Performing over six billion arithmetic operations per second on the
incoming video stream, PureProgressive™ uses the data from four video fields to
determine not only which portions of the image are in motion, but also what type of
movement exists in each (using our proprietary modified Fourier technique), and how best
to generate a progressive image with maximum picture detail for that portion.
PureProgressive™ also performs excellent 2:3 pulldown detection. In addition, it also
recognizes the "2:2 pulldown sequence" used for converting computer graphics to
video. For film and computer graphics sources, PureProgressive™ flawlessly
reassembles the original progressive frames. Unlike some other processors that advertise
similar capabilities, PureProgressive reliably detects the source type even with noisy
signals and poor video sources.
One problem with 2:3 pulldown detection is that if the 2:3 sequence changes it will be at
least several frames before most processors will detect the change. In the meantime
though, the processor has been blindly and incorrectly assembling frames, resulting in
severe motion artifacts in the output. PureProgressive avoids this by immediately
detecting when the source type changes preventing it from ever generating these artifacts.
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