Interlacing, Deinterlacing, and Scaling
Until recently, the basic technology behind television images was pretty cut and dried. In our analog system, the video stream consists of 30 complete frames per second (fps), a frame being a complete still picture created by 480 active scan lines running horizontally across the screen. (The total number of lines in a frame is 525, but that includes lines in what is known as the vertical blanking interval, or VBI, which carry no picture information.) Each frame is split into two fields, each of which contains every other scan line. The fields are transmitted and displayed sequentially, so that the first field of a frame is completely scanned, and then the lines of the second field are scanned between those of the first. This is known as interlaced scanning. Interlacing is surprisingly effective. However, if you compare a scene shot and displayed in standard analog format, now known as 480i, with the same scene displayed using progressive scanning — in which all the lines in each frame are displayed sequentially instead of being divided into two interlaced fields — the progressive-scan version will look cleaner and smoother. (This is how all computer monitors work.) That format is called 480p, the number indicating the active scan lines and the “i” or “p” the scanning method.

The process of converting video from interlaced to progressive scanning is known as “deinterlacing.” Although it seems straightforward enough to combine two fields of a frame and display them all at once, there are a couple of gotchas. The first is that when video is shot in interlaced format — which is how most TV and video cameras operate — the fields are shot sequentially, with the second field of a frame acquired a sixtieth of a second after the first. Any motion that occurs between the two will cause “jaggies” if the two fields are just slapped together and displayed simultaneously. Consequently, a deinterlacer must incorporate sophisticated motion-compensation techniques to achieve good results with typical video-originated material, and some are distinctly better than others in this regard.

The second potential issue arises with material originally shot on film. When film is transferred to video, it is normally converted from its native frame rate of 24 fps (frames per second) to the interlaced 30 fps employed by most TV sets, using a method called 2:3 (or 3:2) pulldown to pad out the sequence with a repeated field every other frame. Video originated from film can be deinterlaced perfectly to 60-fps progressive-scan format, but only if the deinterlacer correctly detects and compensates for the 2:3 pulldown. If it doesn’t, it will create some video frames out of fields from two different film frames, causing an ugly artifact called “combing” if there is any motion between those original frames. This is why 2:3 pulldown compensation is so important in a progressive-scan DVD player.

With the advent of HDTV, it has become important in TV sets as well. An HDTV set has to handle at least four basic video formats: regular old 480i standard-definition (SD) for conventional analog broadcasts and videotapes, 480p SD (mainly from progressive-scan DVD players), and the two widescreen high-definition (HD) formats, 720p and 1080i, which provide much greater picture detail. An HDTV set should, therefore, be able to accommodate inputs in a number of scan formats and in both 4:3 and 16:9 aspect ratios for standard-definition signals (4:3 is not used for high-definition broadcasts).

It’s possible to design a CRT display to handle all of those formats directly, which is what high-end CRT front projectors typically do. But since it’s cheaper to convert some formats to others than to make a full-bore multiscanning monitor, most rear-projection and direct-view CRT sets take the conversion approach. And in the case of fixed-pixel displays, such as LCD, DLP, LCoS, and plasma, all incoming signals must be converted to a progressive-scan format that exactly matches the display’s pixel array. Most digital CRT sets work at 480p and 1080i and convert every other incoming signal to one of those native formats. That usually means 480i gets bumped up to 480p and, if the set has a built-in HDTV tuner, 720p gets converted to 1080i. (Because 720p actually has the highest data bandwidth and horizontal scan rate, it is easier from the display-design standpoint to convert it “up” to 1080i than to step 1080i “down.”) The process of converting between scan formats is known as scaling, and interlaced signals must be deinterlaced prior to any other processing. So deinterlacing is a critical function in all HDTV sets, especially those based on fixed-pixel displays, which in turn means that 2:3 pulldown compensation is important, since much of what is broadcast on TV was originally shot on film.

Scaling is hard to do, and bad scaling can look really, really bad (especially if it starts off with mediocre deinterlacing). Historically, good scalers have been very expensive, even if all they did was line-double 480i to 480p. And the very best scalers, from companies such as Faroudja, Key Digital, and Runco, are still very pricey. The good news is that the growing need for video scaling has led to substantial progress further down the food chain — a trend that will surely continue. Still, before you buy any set with built-in scaling — a category that includes all HDTV sets and all fixed-pixel displays — cast a critical eye on how it looks with a variety of input signals. Pay special attention to what the set does with ordinary analog signals from cable or broadcast TV, which tend to give crummy scalers the biggest fits. Look particularly at what happens around the edges of moving objects. (Problems are often most apparent on slowly moving objects in the background.) Jagged or fuzzy edges or halos around objects are a bad sign. Poor handling of analog TV signals might not matter much ten years from now, but it could make you pretty unhappy in the meantime. You should also make sure the set provides 2:3 pulldown detection and compensation for film-originated programs.

Understanding Our Lab Data
How to read our lab tests for TVs.

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