Last month I described the interlaced video standard that has been, in North America and in other countries, the television display standard since television became a commercial entity in the late 1940s. If you aren’t familiar with the interlaced video standard, I would recommend reading that article before reading this one. Broadcast television cameras, home television sets, VHS, Beta, laserdisc, and, until just recently, DVD players have all output interlaced video. Computer monitors and a small number of expensive home-theater display devices derived from computer displays are progressive scan. U.S. television is mandated to change to digital-only for broadcast within this decade. The digital television standards, HDTV and DTV, support both interlaced and progressive-scan formats. So, a definition of progressive scan is needed before we go any further.

Progressive scan refers to the way the image is displayed. The first pixel in the first line is in the upper-left corner of the image. The entire first line is scanned to the right. The first pixel in the second line is next, followed by the rest of the second line and then the third line starting at the left, etc. until all of the lines of the image are displayed in sequence within 1/60 of a second. The next image starts in the upper-left corner again. It is called "progressive scan" because the display of the image "progresses" across and down the screen uninterrupted. Remember that interlaced scanning displays the odd-numbered lines first and then the even-numbered lines.

From 0 to 60

Depending on how the progressive camera captures the original images, progressive scanning gets rid of all the interlaced artifacts with one possible exception. 1/60 of a second is not fast enough to freeze motion. If the progressive-scan video camera takes a full (or nearly full) 1/60 of a second to capture an image, motion will be blurred in proportion to the speed. Similarly, panning the progressive camera will also blur the images if a full 1/60 of a second is required to capture each image. However, a high-sensitivity progressive-video camera may be able to capture images at speeds fast enough so that there is no obvious motion blur. If this is the case, progressive-scan video can provide the most stunningly realistic images you may ever have seen, even more convincing than film, when displayed using the best possible devices.

Commercial movies are shot on film and projected at 24 frames per second. Projectors actually pull a trick and flash each frame two times at 1/48 of a second to reduce the fairly obvious flicker your eye/brain mechanism would notice if there was a single 1/24 of a second flash for each frame. The IMAX HD movie format shoots and projects large format film at 48 frames per second in order to achieve a more convincing illusion of reality (the effect is quite stunning if you’ve ever had the opportunity to see it). So, you can imagine what 60 frames per second can look like -- even more convincing than IMAX HD in respect to seamlessness of the images from frame to frame. However, there are no high-resolution video displays that can challenge the sheer size of the projected IMAX HD image.

From then to now

Progressive-scanning video cameras and display devices used to be impossible to build because the parts needed to make the display or camera operate fast enough to scan 60 full-resolution frames every second did not exist. Once parts began to appear that were fast enough, cost was prohibitive. Today we have plenty of parts that are fast enough and cheap enough to produce things like the $200 17" progressive-scan computer monitor. It wouldn’t be much of a leap to think you might be able to get a pretty large display device that did what a computer monitor does for perhaps $2000, if it weren’t for a couple tricky problems that I’ll touch on now.

Tricky problem #1 involves all the old interlaced video programming. This is a big problem. You could just take every interlaced image and display it two times. But this will look terrible because many of those frames are made up of two images that are somewhat dissimilar. When you display a somewhat compromised image two times, the problems are pretty obvious and the images don’t look very good. Doing this job correctly requires sophisticated video processing so that the progressive images are "corrected" to remove some or most of the interlace artifacts. This processing is not inexpensive or easy. In fact, this interlaced source to progressive output processing is one of the most important differences between various progressive-scan products. There are sub-$400 DVD players that produce progressive-scan images when set to do so. They don’t do anything very sophisticated, yet the results are not bad -- certainly better than the interlaced version of the same movie. If you pay over $700 or so for a progressive-scan DVD player, more sophisticated (expensive) technology produces nearly ideal results.

Tricky problem #2 deals with converting film sources originating at 24 frames per second, which were converted to interlaced video at 30 frames per second, and putting that into a progressive-scan format at 60 frames per second. This requires powerful image processing to produce the best results with the least amount of telltale artifacting. 3-2 pulldown may be a clever trick for getting film to interlaced video, but it is a royal pain when trying to get interlaced movies into progressive-scan mode. It wouldn’t be so bad if the 3-2 pulldown was consistent, but this cadence can change many times during a movie in order to avoid some sort of artifact. These changes are incredibly difficult to detect when the source is analog videotape or analog broadcast of a movie. The DVD format makes this sort of thing easier to deal with via the presence of "repeat" flags in the bitstream. However, to make use of these flags for the new generation of progressive-scan displays, new DVD chip sets will need to be developed since the existing MPEG video chip set does the interlacing internally, making it impossible to access the digital frame data on the DVD directly. In the absence of those chips, current progressive-scan DVD players take the interlaced output of the MPEG decoder and de-interlace it again before display. When there is a new generation of MPEG chips, progressive-scan DVD players will have a much easier job of producing high-quality images from the DVD image data.

Conclusion