Naked Science Forum
Non Life Sciences => Technology => Topic started by: taregg on 19/02/2014 18:54:37
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what is difference pixel shape between digtal and analog screen..?
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See the pixel aspect ration column in the table on http://en.wikipedia.org/wiki/Standard-definition_television (http://en.wikipedia.org/wiki/Standard-definition_television) - the pixels are not square.
On digital screens they are probably always square, but correct me if I'm wrong - I haven't searched for info on this but am merely judging by machines I've programmed on (which are not televisions). A lot of the screen modes you can set on a computer will give you "pixels" that aren't square, but they are not the same as the pixels of the actual screen - the only modes where they match up will be the ones that match the resolution of the physical screen, and then they will usually (or maybe always) be square.
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CRTs are typically considered analog with a sweeping beam activating different points along the screen. Would even a CRT computer monitor be considered analog?
LCDs, LEDs, and related technologies are considered digital with each spot on the screen having a specific address/activation.
I'm not sure about grids. Sony had a wire mesh that they put behind their CRT screens to help divide the pixels. I believe it was a mesh of vertical lines, with 1 or 2 horizontal lines. (and horizontal sweep of the electron beam).
If the horizontal sweep gave a fairly square bottom, and the vertical wire grid gave a square vertical edge, then one might end up with fairly square or rectangular pixels.
Squares or rectangles would be dependent on the aspect ratio. Since the number of pixels sometimes varies for a specific screen size such as a laptop screen, I'd imagine that the pixels are not 100% square.
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Dots and rectangles are available in both CRT and LCD formats ...
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2Fthumb%2F4%2F4d%2FPixel_geometry_01_Pengo.jpg%2F240px-Pixel_geometry_01_Pengo.jpg&hash=4c45eca677d4b4b74e457c2f12bf1f26)
http://en.wikipedia.org/wiki/File:Pixel_geometry_01_Pengo.jpg
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Analogue TV isn't transmitted as pixels, but as lines.
- US (NTSC) TV has about 480 lines, depending on exactly how you count them
- European (PAL) TV has about 576 lines
- Each line is the full width of the screen.
- So you could say that each line is 100% of the picture width, but only about 0.2% of the picture height.
JPEG and MPEG images are not transmitted as pixels either. They are initially grouped into Macroblocks consisting of a square of 8x8 or 16x16 pixels, and compressed using a Discrete Cosine Transform (http://en.wikipedia.org/wiki/JPEG#Block_splitting).
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Since the number of pixels sometimes varies for a specific screen size such as a laptop screen, I'd imagine that the pixels are not 100% square.
If you set up a screen mode which matches up with the horizontal and vertical numbers of pixels, you can usually then plot a square on the assumption that the pixels are square and have it actually come out looking square rather than as a rectangle. It saves a lot of processing time if you don't have to adjust everything to add distortions that cancel out those caused by rectangular pixels. Check the specification of your machines and see if you can find any laptop or tablet that doesn't have square pixels - I can't find one that doesn't, and I doubt any screen manufacturer would want to make such an awkward screen (unless the aim is to make an old television).
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All computer graphics/video formats assume square pixels; 1920x1200 24" computers monitors are 16x10 aspect ratio (divide the two and they'll match - both give 1.6 horizontal:vertical) so the aspect ration of the screen and aspect ratio of pixels are the same and hence the pixels are considered square (both physically if you get your microscope out and practically if you plot a circle - it's square).
19" monitors are 1280x1024 but are 4x3 aspect - same applies.
For TV resolutions we have 1920x1080 (16x9 - same calculation as above) for current HD (don't get started on UHD-TV!) which are square pixels, but for SD television we have 720x576. The same resolution applies for both 4x3 and 16x9 widescreen SD TV (which became popular in the mid-90s). Interestingly the US never had a tradition of widescreen SD tele which is why they bought into HD quicker than us; wider pictures that looked better; it was much more differentiated than we had in Europe.
Now then, given that when we go between 4x3 and 16x9 widescreen at SD we maintain the resolution clearly the aspect ratio of the pixel changes; and in both cases SD pixels are NEVER square. At 4x3 they nearly are (do the calculation) and that's where the confusion arises. Digital TV largely developed away from computer graphics with different resolutions, frame rates, colour space, interlaced etc.
Finally (as has been pointed out) it is the case that analogue TV (which can be 4x3 or 16x9) there are no pixels; only lines and fields (two fields make a frame) and 5.5Mhz of analogue bandwidth over a 52uSec line. Dr. Philip Willis's research into frame-less and pixel-less video will probably dictate to us how we "do" UHD-TV if it *ever* takes off!
http://www.bath.ac.uk/news/2012/12/11/pixel-die/