Deinterlacing
Deinterlacing is the process by which
interlaced video is converted to progres-
sively scanned video. Progressive scan-
ning 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-quali-
ty techniques or for many thousands of
dollars for very sophisticated techniques.
The low-cost techniques are frequently
used in progressively scanned TVs or pro-
jectors. 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 described above.
To avoid these artifacts, some deinterlac-
ers 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 and a 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 soften-
ing filter only to portions of the image
that are in movement. This is referred to
as “motion adaptive” deinterlacing.
Many line doublers can also take advan-
tage 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
lines, 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 dein-
terlacer can examine a series of fields to
detect this sequence and thereby deter-
mine 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 and with no introduc-
tion of motion artifacts.
DVDO’s PureProgressive deinterlacing
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 Ultra 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 pro-
gressive image with maximum picture
detail and minimum motion artifacts.
18
How It Works
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