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Scan Signals
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Interlaced and Progressive Scan Signals
Standard 480-line NTSC TV
broadcasts (incl. cable television), VCR, DVD and laserdisc signals are sent in
an "Interlaced Scan" format. A TV screen first draws the image's odd lines, one
at a time sequentially from top to bottom (which takes 1/60 of a second), and
then fills in the even lines (taking another 1/60 of a second). That is, the
full picture (top to bottom) is first drawn with half its information hollowed
out, and then the other half is filled in -- the entire process taking 1/30 of a
second.
A newer and superior scanning method called "Progressive" permits the entire
picture to be drawn sequentially from top to bottom without the odd/even
interlacing. Some newer DVD players now have outputs for both an interlaced and
progressive scan image. And HDTV signals are now being broadcast in both
progressive and interlaced formats: 720p (720 lines of resolution in progressive
scan format) and 1080i (interlaced).
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Progressive Scan:
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Interlaced
Scan:
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1st Scan of even lines |
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2nd Scan of odd lines |
Improving Picture Quality with Line Doublers
and Scalars
Unlike TV picture tubes and
computer monitors, projectors don't actually "draw" the picture. Rather, at any
given split-second in time they are either projecting image or not (i.e., the
pixels are either "on" or "off"). Thus, an attempt to project an "interlaced"
signal would result in every other line (the "odd" lines) being projected by
themselves for 1/60 of a second, followed by just the even lines, resulting in a
picture worse than any big screen TV.
To accommodate incoming interlaced signals such as from a TV broadcast (incl.
cable), VCR, DVD or laserdisc, most projectors contain deinterlacer or "Line
Doubler" circuitry that changes the interlaced signal into a progressive EDTV
format. This is accomplished by waiting a full 1/30 of a second to receive both
the odd and even lines before projecting them together onto the screen. During
this split-second wait, the previous image frame continues to be projected a
second time, so there is a fully formed image being displayed at all times.
Despite its name, there is not actually a "doubling" in the number of lines of
resolution. But there is a doubling of the amount of time that each image frame
is displayed, resulting in a picture that not only is devoid of "flicker," but
which is also brighter.
With a high-quality line-doubler (and not all of them are), the resulting
picture quality from an "interlaced" source is absolutely superb.
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