Discussion in 'General TV Discussions Forum' started by pixel, Apr 29, 2003.
Why is it that TV pictures always have jagged lines? Are there some TV's that don't have it?
Jagged lines arise because a TV picture consists of a sequence of horizontal stripes (scan lines). When you try to show a shallow diagonal edge to something, the only way it can be 'drawn' by the horizontal lines on the TV is with steps in it.
Imagine trying to sketch a picture of the slightly sloping edge of something, but the only tools you are allowed to use are a pencil, paper, and a ruler which MUST be exactly horizontal across the page. You are only allowed to draw horizontal lines, and each must be a precise distance down the page from the last. You'd have to draw a sequence of horizontal lines, each one a bit shorter or longer than the last to get your diagonal edge. So you'd get stair-steps.
They CAN be overcome, but not with regular TVs. Upmarket high-resolution LCD devices, DLP devices, and CRT devices with inbuilt 'scalers' use sophisticated algorithms and processing to interpolate between the scan lines in the video signal and produce the appearance of a smooth edge - by artificially generating additional 'lines'. Obviously, the more lines, the closer they are together, and the less visible the stair-stepping effect is.
Great description Nigel,you should be a school teacher!
Agreed - good description Nigel. Although I thought that progressive scan also removed this problem.
Progressive scan does not remove the problem. It lessens it. Although this isn't an accurate description, it's similar in effect to having twice as many scanlines (each half the distance from the next) - so jaggies are still potentially there, just much smaller so less visible. And the scalers that are used in all LCD and DLP devices often are better able to cope with a progrssive signal, so they make a better job of interpolating between the lines.
Thanks. So I guess that means only projectors can give a picture without the problem.
Not even them, really. Projectors come in two major groups - CRT ones (which use scan lines to create the image) and fixed panels ones (DLP, LCD and some other cutting-edge technologies) which use a matrix of pixels to produce the image.
So, in that respect they are the same as TVs and in either case, the picture is always made up of some sort of structure, whether it be lines or a pixel grid. Given a structure, you will always get some stair-stepping.
But, as I said before, the finer the structure, the less visible stair-stepping is (provided that the initial signal is scaled in some way to reduce it).
You CAN externally scale an image before sending it to a CRT TV - provided the TV has the right inputs and abilities to reproduce such a signal. Few do.
You CAN externally scale an image before sending it to a CRT projector - and projectors are more likely to have the right inputs and abilities.
You CAN externally scale an image before sending it to a fixed panel device. However, by definition, a fixed panel device MUST have a scaler/deinterlacer inside. You get a benefit if your external scaler is better than the internal one. For example, on the SonyVW12HT it is hard to improve upon the internal scaler.
There are LCD and Plasma TVs also. These also must have internal scalers and, again, can vary in standard.
I have a 22inch Samsung LCD TV and it makes a pretty good job of removing stair-steps from regular (RGB, SVideo, Analog broadcast) signals.
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