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The progressive scan versus interlaced signal debate takes a frame by frame look at video. As more progressive scan television and video equipment pops up, the question of which is better, and why, is bound to surface. In looking at the differences between the two, it is important to recognize the fact that progressive scan devices still involve interlacing. This also explains why progressive scan DVD players and televisions still have some “glitches” associated with interlaced signals.
It is important to understand how interlacing works, and what it is. It is, essentially, just as its name implies. It is a video image made up of interlaced lines. Motion pictures are not truly in motion, but instead are made up of many still images flashed quickly to give the illusion of movement. These images are flashed at a rate of 24 frames per second (fps). When looking at interlaced video the idea of frames per second is replaced with fields per second. Each field is made up of lines drawn on the CRT face from left to right and top to bottom. The fields are alternated, odd and even. To get the whole picture the fields must be interlaced. This gives 60 unique fields per second.
When the image is still, the alternating fields complement each other and the resultant image is clear, although some flicker and scan lines may be seen. Since most video consists of movement, interlace artifact becomes an issue. “Combing” (or “feathering“) and “line twitter” are two common artifacts that effect image quality in interlaced signals.
As stated, when an image is still the two fields interlace nicely and the image appears smooth. However, when the image is in motion the object in the field will be in a slightly different position so the images will not complement each other. This leads to some “bleeding” of the image and since the two fields are alternating odd and even lines the edges will appear jagged. When looking at a smaller display or when further back from the image this artifact, along with others, is less noticeable. Increased bit rate decreases this artifact as well. To get a clearer picture of this, get up close to an interlaced television and watch the edges of objects in motion.
Line twitter comes into effect with finer lines and images with greater detail. If a fine detail appears only in the even lines, when the odd lines are alternated the detail will disappear. So half the time it will be there and the other half it will not. It will “twitter” in an out. This artifact can be present even in still images. Due to this artifact many finer details are filtered out before encoded for delivery by an interlaced device.
Now, going back to the motion picture. It is shot in 24 fps. It is difficult to get a nice even 60 fields from 24 frames, since 24 doesn’t go nice and neatly into 60. To do this a little math is required. Breaking this down to its simplest numbers we see that 4 frames must be made into 10 fields. To do this a “3-2 pull down” is used. Simply put, three fields are created from the first frame and then two from the second, three from the third and finally two from the fourth. The drawback to this is that the artifacts mentioned above are a part of these images as well. When a field is made from a different frame, where movement has occurred, the potential for “combing” and “line twitter” exists.
So, how does progressive scan deal with these issues? Progressive scan takes a “scan” of the CRT after the lines have been interlaced. Instead of showing 60 alternating half fields, the progressive scan shows 60 complete fields, or frames, per second. This eliminates the flicker associated with interlaced pictures. It also makes the scan lines less visible and helps eliminate the other artifacts associated with interlaced signals.
Since truly progressive devices are difficult to come by, the video from many sources must be deinterlaced to be shown as a progressive scan. This involves coupling two interlaced fields to create one progressive frame. This would reduce the overall number of frames, but since progressive scan devices still operate at 60 frames per second they must get back to the same rate as the interlaced. This is accomplished by alternately doubling and tripling the frames and showing them for 1/30th and 1/20th of a second. This unfortunately does lead to some “judder”, or hitching, in the playback.
The actual quality of the picture from a progressive scan device will depend on the deinterlacing chip. Due to limitations in digital video equipment, very few videos, television shows, and other media are produced in progressive format, so some deinterlacing is required.
Progressive signals can eliminate some of the flicker, scan lines, and artifact associated with interlaced signals. Currently, it must rely on deinterlacing to do this. As more devices are produced to record and copy in progressive format this will eliminate the need for much of the deinterlacing, and lead to truly progressive scans.