Interlaced or Progressive

It is earnestly discussed recently whether television in the digital broadcasting age should be of I = Interlaced or P = Progressive scans.

The NTSC system, used in the States and in Japan, has 525 horizontal scanning lines, among which about 490 are actually visible on the screen. While horizontal scans are repeated 525 times, vertical scans are performed twice. In the first vertical scan, 262 and a half horizontal scanning lines are drawn. This one half is very important because horizontal scans in the second vertical scanning come exactly between the horizontal scanning lines in the first round. Because of this reason, the number of horizontal scanning lines must be, until recently, an odd number. HiVision, the Japanese analog HDTV, has 1125 lines and European televisions have 625 or 819 lines. From the historical reason, the vertical scanning rate is the same as the AC power frequencies; i.e. 60 Hz in the States and Japan, and 50 Hz in Europe. Therefore 30 frames or 25 frames a second respectively are drawn on the screen. These are interlaced scanning, while PCs use progressive scanning in which all the horizontal scanning lines are completed within one vertical scan.

Human eyes recognize a moving picture from images shown more than 15 frames a second, but a safer specification is more than 24 frames a second. In order to minimize fatigue of the eyes, twice as many frames are desirable. The NTSC interlaced scheme sends signals only for 525 lines x 30 frames a second, but gives the fatigue-less effect almost equivalent to that by 60 frames a second. This is a very clever information compression method which makes use of human illusion.

Then, why do PCs use progressive scanning ? Historically, PCs didn't have to worry about the signal band width as in the case of broadcasting, and only characters used to be displayed. Therefore the progressive scanning has been the natural choice. Use of interlaced scanning only after VRAM in PC doesn't help anyway or doesn't decrease the band width. When the interlaced signal is displayed on an interlaced screen, the screen must have a fixed size and a fixed number of scanning lines. This requirement squarely contradicts with the idea of adjustable size Windows in PCs. Therefore PCs don't like interlaced scanning.

According to the amateurs' thought, conversion from interlaced to progressive scanning seems simple, by overlaying one half interlaced frame on top of another. It may be indeed simple for a still image but experts say it is not that simple for a moving image. The odd and even scanning lines of the overlaid screen represent images of different timings. Simply overlaying two different interlaced half-frames makes an unbearable image. Therefore conversion requires intricate image processing involving movement prediction, which is an expensive technology often not necessarily for satisfaction. This is why the PC industry led by Microsoft earnestly proposes progressive scanning to broadcasters.

There is no problem if one doesn't care about the image quality. I watch television at home often with my PC. I notice strange distortion on equal space horizontal lines caused by adjustment of differences of the numbers of scanning lines. Movement is strange. But it is sufficient for an individual to enjoy television in a small PC screen in a private room. Important point is that PCs can not make a large screen family television unless broadcasters send out signals according to PC's specifications.

Then why do broadcasters stick to interlaced scanning ? Because less amount of information is needed in the signal and, therefore, the facility equipment in the station is cheaper. In addition, HiVision cameras, developed in Japan for considerable number of shipment so far, can be used almost as it is for digital HDTV if interlaced scanning is used.

FCC in the States has decided the scanning standard for the digital television. But being impatient of the controversy for many years, it decided as many as 18 kinds of scanning standards as follows;

Among these, 480 x 704 has a strange aspect ratio of 13.2 : 9 = 4.4: 3 and is questionable if it can draw the full moon round. But this pixel size supports two aspect ratios and there are 18 in total.

To cope with this, Microsoft, Compaq and Intel formed a Digital TV Initiative and proposed priorities of implementation. Namely they proposed

@ in the above table is realized in early 1998, # in 1999-2000 and * after 2001.

Among broadcasters, ABC is said to be saying that it can use 720 x 1280 24P, and all the others including NBC, CBS and PBS would not concede from 1080 x 1920 60I. But Fox is more practical and says the number of channels is more important than screen quality of HDTV and that it will use 480 x ? 60I. Japanese broadcasters seem prepared to go with 1080 x 1920 60I.

It is a very interesting era right now, to see what Japan, US and Europe will do in the digital broadcasting.

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