What is Video Interlacing?

What does video interlacing matter to me? Not much, unless you want to understand what makes your DVD player software tick.

Both a TV screen and a computer monitor are drawn, or refreshed, with tiny horizontal lines made up of closely spaced red, green, and blue dots. An electron beam inside your TV or monitor draws these lines over and over again. The electrons glow for a short time, allowing the beam time to continuously redraw the entire screen–many times per second–before your eye can detect any part of the image fading away. This magnified image illustrates the fine lines.

The method of drawing the display has evolved over time. Interlacing is an alternating display style that helped make up for slow refresh speeds (slow drawing speeds). When interlacing, the electron beam draws odd lines first, then goes back and draws even lines. Because your eye tends to blend and balance little differences in light, color and intensity, this method made the changing image on the screen appear less noticeable.

Interlacing also saves on storage and so it’s considered a method of data compression–more data fits into a smaller space when interlaced. (But some of the data is lost in the process.

In the early 1990s, interlaced computer monitors (and video cards) were all the rage. But now we use progressive scan monitors (non–interlaced–lines are drawn top to bottom, one after another). Why? Because the refresh speed has increased to such a point that interlacing is no longer needed. At lower speeds, interlacing generally delivers the illusion of a better picture. Current computer monitors have been using the progressive scan method for years now. Except for some versions of HDTV, television screens and television broadcasts still use the interlace method to display video data.

Of course, by 2006, old NTSC television interlaced screens and receivers will be obsolete. TV broadcasters in the United States are adopting the new HDTV standard and will retire the current NTSC standard by mandate. Some portions of these new standards are interlaced, some are not. Some creators of the new standards look toward progressive scanning as the answer, but others disagree and promote an improved version of the interlaced standard. The battle rages on.

So for now, your old TV and your new computer monitor actually play the same DVD quite differently. Your NTSC (North America) TV screen displays a picture as 525 lines. Interlaced, this breaks down to two screens: 240 odd lines and 22 black lines for the first screen, and 240 even lines and 23 black lines for the second screen. (The black lines not only allow a black space at the bottom for closed captioning, but also contain display information and allow a buffer of refresh time so that your screen can stay in sync with live broadcasts.)

The two alternating sets–if you ever saw them separately–would appear as two squashed, half–height i of one picture. Each set is refreshed approximately 30 times per second, so the electron ray gun actually makes 60 passes of the entire screen per second.

By contrast, a computer monitor with XGA resolution has a whopping standard of 768 horizontal lines that are redrawn continuously from top to bottom, left to right. These lines are refreshed much faster–60 to 80 times per second (60 to 80 hertz) or more–depending on your video card. Even at 60 hertz, you monitor refreshes every line with every pass, and your TV only hits half the lines in each pass. Monitors and HDTVs with tighter, sharper resolution (more lines and faster refresh times) are popping up on store shelves all the time.

Also, the DVDs you play on your TV and computer monitor are produced differently, according to the whim of the manufacturer. Some produce interlaced DVDs, other manufacturers go from film to DVD directly–not interlaced. In any case, the DVD player software on your computer and the DVD player hardware under your TV both must adapt the input to be compatible with the output screen. These tools do this without much help from you.

And it’s a good thing, too. If your computer were to play an interlaced image on your computer monitor today, you would notice horizontal streaking whenever the camera pans. Sometimes, the conversion process requires that an interlaced DVD be de–interlaced so that it can be viewed on your monitor. Your DVD player software de–interlaces the picture continuously when you view your DVD on your computer monitor. How?

Not every de–interlacing process is the same; there are various methods. One method is called Weave. This method combines the two fields into one progressive frame. The Bob method takes one scan line and electronically mixes it with the next scan; odd numbered lines are mixed together and the even numbered lines are discarded. Your DVD player software will switch between these and other de–interlacing methods as needed throughout movie playback.

Although all DVD player software comes with de–interlacing ability, not all DVD player software is created equal. Each one uses different combinations of de–interlace tools to produce the proper picture, some methods superior to others. The closer the DVD player software comes to backtracking to the way the video was interlaced in the first place, the better the final video version will be.