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Progressive vs. interlaced

interlaced video has been around as long as there have been televisions with cathode ray tubes (CRTs). All analog television standards are based on interlaced video, and we're so used to it that it's not something that we typically think about. High definition video can be either interlaced or progressively scanned. It's important to understand these terms, and it's really not too difficult.


Progressive scan video is scanned from side to side, top to bottom: line 1, line 2, line 3 etc, up to the end of the frame. It's as simple as that. In fact, it's exactly how you'd expect video to be scanned if you didn't know any better!


interlaced video is scanned from side to side, top to bottom, as well. The difference is that every 60th of a second, every other line making up the complete frame is scanned. Then, a 60th of a second later, the lines in-between the lines that have already appeared are scanned. Effectively, half the picture's vertical resolution is sent in the first 60th of a second, and the second half is sent in the second 60th of a second. When the video is played back, the whole thing happens in reverse, giving the appearance of a complete frame. Each of these "halves" of a frame is called a "field."

The effect to the viewer is quite distinct. First, the image doesn't flicker as much as it would if it was a simple 30 fps Progressive scan. This is because, to the viewer, it looks as if they are seeing 60 frames per second. Of course, what they are actually seeing is 60 fields per second; but for flicker reduction, the effect is the same as seeing 60 frames.

If you were to look at an interlaced picture on a screen for a 60th of a second, you'd only see half the vertical resolution ? every other line. But, because our eyes and brains conveniently offer a "persistence" effect, when you look at the screen "normally," what you actually see is something approaching full resolution. Persistence allows us to accumulate visual data from the two distinct fields, making them seem like one complete image.


720p and 1080i

Progressively scanned high definition video tends to have a resolution of 1280 by 720 pixels, normally referred to as "720p", where the "p" stands for Progressive. Likewise, ?1080i? is interlaced video with a frame size of 1920 by 1080 pixels. Interlacing is actually a form of compression. Offsetting the two fields making up a frame by half a frame's duration, halves the total amount of information needed to transmit or store the video. Uncompressed interlaced high definition video generates around a gigabit per second. Without interlacing, the rate would be twice that.
quote]That's the technobabble.

Long story short, look at text or wall corners in your games, in 720p you'll see letters and edges jumpy, like they dont stay in the same area, in 1080i they dont.