Progressive scan

Progressive scanning (alternatively referred to as noninterlaced scanning) is a way of displaying, storing, or transmitting moving images in which all the lines of each frame are drawn in sequence. This is in contrast to interlaced video used in traditional analog television systems where only the odd lines, then the even lines of each frame (each image called a video field) are drawn alternately.

The system was originally known as "sequential scanning" (a more technically correct description) when it was used in the Baird 240 line television transmissions from Alexandra Palace, United Kingdom in 1936. It was also used in Baird's experimental transmissions using 30 lines in the 1920s.

Contents

Example of interline twitter

Interline twitter, demonstrated using the Indian Head test card.

This rough animation compares progressive scan with interlace scan, also demonstrating the interline twitter effect associated with interlace. On the left are two progressive scan images. Center are two interlaced images. Right are two images with line doublers. Top are original resolution, bottom are with anti-aliasing. The interlaced images use half the bandwidth of the progressive ones. The images in the center column precisely duplicate the pixels of the ones to the left, but interlace causes details to twitter. Real interlaced video blurs such details to prevent twitter, but as seen on the bottom row, such softening (or anti-aliasing) comes at the cost of image clarity. A line doubler could not restore the previously interlaced image at bottom center to the full quality of the progressive image on the top left.

Note: Because the refresh rate has been slowed down by a factor of three, and the resolution is less than half that of typical interlaced video, the flicker in the simulated interlaced portions and also the visibility of the black lines in this image are exaggerated. Also, the images above are based on what it would look like on a monitor that does not support interlaced scan, such as a PC monitor or an LCD or plasma-based television set, with the interlaced images displayed using the same mode as the progressive images.

Usage in storing or transmitting

Progressive scan is used for scanning and storing film-based material on DVDs, for example, as 480p24 or 576p25 formats.

Usage in TVs, video projectors, and monitors

Progressive 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 horizontal lines of resolution with progressive scan). This allowed these displays to be used with devices that 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.

The 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 interlaced video. For an in-depth explanation of the fundamentals and advantages/disadvantages of converting interlaced video to a progressive format, see deinterlacing.

Advantages of progressive scan

  • Absence of visual artifacts 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 subdue interline twitter when played back on interlace displays. As a consequence, recovering the sharpness of the original video is impossible when the video is viewed progressively. A user-intuitive solution 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 this feature is Super Smash Bros. Brawl, where a "Deflicker" option exists.[1] Ideally, "Deflicker" 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 clearer and faster results for scaling to higher resolutions than its equivalent interlaced video, such as upconverting 480p to display on a 1080p HDTV.
HDTVs not based on CRT technology cannot natively display interlaced video, therefore interlaced video must be deinterlaced before it is scaled and displayed. Deinterlacing can result in noticeable visual artifacts and/or input lag between the video source and the display device.
  • Frames have no interlace artifacts and can be captured for use as still photos.

See also

References


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