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What do line doublers actually do?

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Grand Dizzy

Joined: 11 Dec 2004
Posts: 2

PostPosted: Sat 11 Dec 2004, 1:00    Post subject: What do line doublers actually do? Reply with quote

Hi, I'm buying a new TV and I'd like to know what line doublers actually do with interlaced signals. I thought this might be a good place to ask.

I've read that line doubling is just doubling the lines of an interlaced signal to fill the gaps. That seems like a reasonable explanation, and it seems to fit the name "line doubler".

But if you double the lines, don't you half the resolution? It would turn a 576-line picture into a 288-line picture (whilst keeping the framerate of 50Hz).

To add to my confusion, the FAQ here says that a line doubler blends the two fields together. I take this to mean that both fields are shown at once. This would maintain the higher resolution, but surely it would half the frame rate and the animation would not look smooth?

This whole thing confuses me! Can anyone help?
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Site Admin

Joined: 04 Feb 2003
Posts: 587
Location: Lisboa, Portugal

PostPosted: Sun 12 Dec 2004, 3:53    Post subject: Reply with quote

The FAQ here doesn't mention line doublers at all.

There's a lot of confusion between line doublers, scan rate doublers and deinterlacers.

"Line doubling" is the term normally used to describe the conversion between SD (standard definition - 480 or 576 lines tall) to HD (high definition - 720 or 1080 lines tall). It doesn't necessarily double the number of lines, it simply "stretches" the image to fill the screen, creating new lines by interpolation.

A scan rate doubler doubles the number of screen updates per second. For example, instead of 50 updates per second (the normal rate for PAL), you get 100. This gives you a much more stable image, but is not as simple as it seems, due to interlacing. If you simply scanned each frame twice, the image would keep shaking back and forth. If you scanned each field twice, the image would appear to bob up and down by 1 pixel. To look good, it needs to interpolate every other field. To look very good, it needs motion prediction (to simulate 100 real updates per second, with fluid motion). Chips that do this tend to freak out when you have conflicting motion on the screen (ex., vertically scrolling text over a horizontally scrolling background). Turning off motion prediction eliminates this problem, but makes motion a bit less fluid, so I'd say it's an acceptable price to pay.

A deinterlacer is just something that takes an interlaced image or a sequence of two fields and converts it to one (or two) non-interlaced images. The basic method is to eliminate one of the fields and duplicate the other one's lines, a slightly better method is to interpolate the lines, and really good deinterlacers also use motion prediction, and double the frame rate as well.

Confused enough? If not, you must try harder. Wink

Most modern 100 Hz TVs have pretty decent motion prediction algorithms. The output is still interlaced, but it's interlaced at 100 fields per second, instead of 50. Each field is 288 (visible) lines tall, as in normal video, although plasma / LCD / TFT panels may use more lines (by resampling the original image), since they have a fixed physical resolution, and manufacturers like to use the same panel for PAL, NTSC and VGA (CRTs are more flexible in that respect - the same CRT can simulate different resolutions natively).

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