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Progressive Scan Explained

Progressive or noninterlaced 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 CCTV systems where only the odd lines, followed by the even lines of each frame (each image now called a field) are drawn alternately.

The system was originally known as "sequential scanning" (actually a more technically correct description) 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.

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.)

Indian Head Interlace example

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 artifacts associated with interlaced video of the same line rate, such as interline jitter.

No necessity in intentional blurring (sometimes referred to as anti-aliasing) of video to reduce interline jitter 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 artifacts 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 Brawl, where a "Deflicker" option exists. Ideally it would be turned on when played on an interlaced display to reduce interline jitter, 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 upconverting 480p to display on a 1080p HDTV.

Scaling works well with full frames, therefore interlaced video must be deinterlaced before it is scaled. Deinterlacing can result in severe "combing" artifacts.

Frames have no interlace artifacts and can be used as still photos.

Disadvantages of progressive scan.

A disadvantage of progressive scan is that it requires higher bandwidth than interlaced video that has the same frame size and vertical refresh rate. However with modern compression techniques and transmission systems the higher bandwidth is far outweighed by the increase in quality achieved by using Progressive Scan.

For more information on Progressive Scan and Interlaced Scan see the wiki online encyclopedia.

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