Progressive Scanning Explained

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Progressive Scanning Explained

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Oriphus

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4. March 2004 @ 07:07

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Please note: The following is for NTSC playback but does apply to PAL also. For PAL, change the 480i/480p to 576i/576p.

This is taken largely from Panasonic:

Progressive Scan vs Interlaced

Progressive scanning, otherwise known as 480P (p=progressive), creates a picture signal with double the scan lines of a conventional interlaced picture, 480I (I=interlaced), to create a noticeably sharper image. The 480P image offers higher picture resolution and eliminates virtually all motion artifacts. Even on large screens, the progressive scan lines are barely noticeable and picture flickering is greatly reduced, so you can enjoy extended viewing without eye fatigue. There are also many advantages to progressive conversion.

The Benefits of Progressive Vs. Interlaced Scanning
Standard DVD-Video players use interlaced scanning to produce a picture. Interlaced scanning combines two fields to generate a picture of 525 scan lines (480 of which are displayed). Your television projects an interlaced image by first scanning the 240 odd-numbered lines of one field (in 1/60th of a second), followed by the 240 even-numbered lines of the other field (in 1/60th of a second). So, it takes two fields to build one frame of video.

Progressive-scan DVD players have a progressive video processor, which enables all 525 lines (480 of which are displayed) to be scanned at the exact same time. A television with 480P component inputs can completely scan the entire image in 1/60th of a second, so it only takes one field to build one frame of video. The result is a smoother and sharper picture, with high resolution and no motion artifacts.

Films are normally shot at a frame rate of 24 frames per second. This is also the rate used to transfer the vast majority of films to DVD-Video. In other words, DVD-Video movies are usually recorded as 24-frame progressive scan images. So what happens when they are reproduced as an interlaced image? To match the TV field rate of 60 fields per second, each of the film's 24 frames must be shown either twice or three times in an alternating sequence. This kind of conversion sometimes causes two different film frames to overlap as they make up one TV frame, causing visible jaggedness along contours in the picture.

Converting film's 24 fps rate to video's 30 fps rate.
Conventional interlaced video results in "frame mixing" which can cause motion artifacts.

With progressive scanning, this kind of frame overlap doesn't occur. To match the TV field rate of 60 fields per second, it's still necessary to scan each film frame two or three times, as in interlaced scanning. But this is followed by a pattern detection algorithm, that determines which fields should be paired to reassemble the frames as they were on film. This process, called inverse 3:2 pull-down, results in a series of complete de-interlaced frames.

Inverse 3:2 pull down

When the video shows a still image, the common way of converting it to progressive-scan is to combine two fields. When the video shows motion, the common way is to interpolate the missing data between lines. However, the resulting difference between still and motion images is readily apparent on a large screen. This method also tends to misinterpret slow movement as still images, thereby causing jagged edges on moving objects.

Instead, progressive-scan DVD players compute the missing scan lines based on multiple line data from at least two fields. This VT processing method employs the same algorithm whether there is motion or not, so quality is consistently good and misdetection is not an issue. The result is natural image reproduction for large-screen viewing.

Advantages of Progressive Conversion

Line doublers are used in other high-end home entertainment systems to provide progressive scanning. These may be stand-alone devices or incorporated into a digital TV. Impressive as they may be, progressive-scan DVD players' in-player progressive conversion has three big advantages over line doublers:

1) High precision and stability
A DVD-Video disc mastered from a film holds all the data necessary to produce an accurate progressive image, whereas an external line doubler must take hints from the video source to determine the source material and frame allocation.

2) All-digital conversion minimizes signal degradation
Since the signal from the DVD-Video disc is digital, progressive conversion can be performed digitally inside the player. Signal quality is protected until it leaves the player's analog output. In contrast, a stand-alone or in-TV doubler first receives information from the analog output of the source device then converts this analog signal back to digital for processing. Finally, it must translate the signal back to analog before outputting it. All this back-and-forth translation is much more likely to degrade the signal.

3) Processing is optimized to DVD-Video's high image quality
Line doublers built into digital TV sets are designed to work with a variety of video sources, so their settings are not necessarily ideal for DVD-Video. The progressive conversion of progressive-scan DVD players is optimized for the high resolution and low noise of the DVD-Video format. This enables the unit to preserve DVD-Video picture quality for display on all screen sizes, from direct-view CRT to projectors.


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