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Video Production Glossary
Progressive Scan
Progressive or non-interlaced scanning is a method for displaying, storing
or transmitting moving images in which all the lines of each frame are drawn
in sequence. This is in contrast to the interlacing used in traditional
television systems where only the odd lines, then the even lines of each
frame (each image now called a field) are drawn alternatively.
The system was originally known as "sequential scanning" when it was used in
the Baird 240 line television transmissions from Alexandra Palace, England
in 1936. It was also used in Baird's experimental transmissions using 30
lines in the 1920s.
Progressive scan is used for scanning
and storing film-based material on DVDs, for example, as 480p24 or 576p25
formats
Progressive scan (also known as: P-Scan) is used for most cathode ray tube
(CRT) computer monitors, all LCD computer monitors, and most HDTVs as the
display resolutions are progressive by nature. (Other CRT-type displays,
such as SDTVs, typically display interlaced video only)
Some TVs, and most video projectors have one or more progressive scan
inputs. Before HDTV became common, some high end displays supported 480p
(480 vertical lines of resolution with progressive scan.) This allowed these
displays to be used with devices which output progressive scan like
progressive scan DVD players and certain video game consoles. HDTVs support
the progressively scanned resolutions of 480p and 720p. 1080p displays are
available, but are usually more expensive than the comparable lower
resolution HDTV models. Computer monitors can use even greater display
resolutions.
Advantages of progressive scan
Higher vertical resolution than
interlaced video with the same frame rate.
The perceived vertical resolution of displayed video is traditionally
adjusted using a Kell factor coefficient. This coefficient has no fixed
value and depends on display device. Its value for interlaced video is
usually lower than for progressive video, when the same display device is
used. When interlaced video is compared to progressive video with the same
number of scan lines, interlaced video delivers lower perceived vertical
resolution at a lower frame rate.
Absence of visual artefacts associated with interlaced video of the same
line rate, such as interline twitter.
No necessity in intentional blurring (sometimes referred to as
anti-aliasing) of video to reduce interline twitter and eye strain.
In the case of most media such as DVD movies and video games, the video is
blurred during the authoring process itself to mask flicker artefacts when
used on interlace displays. As a consequence, recovering the sharpness of
the original video is impossible when the video is viewed progressively. An
excellent, but rarely employed countermeasure to this is when display
hardware and video games come equipped with options to blur the video at
will, or to keep it at its original sharpness. This allows the viewer to
achieve the desired image sharpness with both interlaced and progressive
displays. An example of a video game with such a feature is Super Smash
Bros. Melee, where a "De-flicker" option exists. Ideally it would be turned
on when played on an interlaced display to reduce interline twitter, and off
when played on a progressive display for maximum image clarity.
Offers much better results for scaling to higher resolutions than equivalent
interlaced video, such as up-converting 480p to display on a 1080p HDTV.
Scaling works well with full frames, therefore interlaced video must be
de-interlaced before it is scaled. De-interlacing can result in severe
"combing" artefacts.
Frames have no interlace artefacts and can be used as still photos.
A disadvantage of progressive scan is that it requires higher bandwidth than
interlaced video that has the same frame size and vertical refresh rate. For
explanations of why interlacing was originally used, see Interlace. For an
in-depth explanation of the fundamentals and advantages/disadvantages of
converting interlaced video to a progressive format, see De-interlacing
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