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How do projectors work?

Discussion in 'Projectors, Screens & Video Processors' started by Captain chaos, Jun 8, 2003.

  1. Captain chaos

    Captain chaos
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    Can someone give me a brief overview of how LCD, DLP and CRT projectors actually work, (ie the components involved creating the image)

    What are the drawbacks/benefits of each format?

    Thanks for your time and trouble!

    CC
     
  2. Mr Bump

    Mr Bump
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    Very briefly:

    LCD. Image is formed by shining light (through/off? not too sure) a LCD panel. Pros: Very good refresh. Reasonably cheap for a good picture. Cons: Distance between pixels is quite far, causing visible horizontal and vertical lines on the screen - the 'chicken wire effect'

    DLP. Image formed on a chip consisting of thousands or tiny mirrors that can each tip plus or minus 10 deg (12 on newer models) to choose whether to reflect light or not - the more a mirror is turned 'on' in a phase, the brighter the dot on the screen. 1054 greyscale is formed. Light is then passed through a three coloured wheel before hitting the chip, giving different scales of red green and blue hitting the screen in very quick succesion, which your brain interprets as an addition of the three, or a colour. Pros: Well defined picture - pixels are very close together. Good contrast. Cons: 'Rainbow effect' - some people's brains can 'break down' the three colours back to their RGB components, giving a very annoying three coloured flash on the screen, usually when you refocus on a different area. Cheaper pjs are more susceptible, although some people see them even on £10,000 machines.

    CRT. Image created by three CRT tubes (like in a 'normal' TV), that are red green and blue. The three images are overlaid over each other to form a multicoloured image. Pros: Best image - very smooth and great definition. Cons: Expensive, bulky and difficult to set up.
     
  3. Captain chaos

    Captain chaos
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    What about things like inputs / outputs, is DVI a digital connection?

    I take it that S-Video, RGB SCART and component are all analogue forms of signal transmittion?
     
  4. Mr Bump

    Mr Bump
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    Most all use the three standards (composite, s-video and component). Some LCD and DLP projectors have started using DVI as a connection, as they are often used with PCs/Laptops. DVI can be analogue as well as digital, seemingly. All others are analogue. I can't see any reason why you would use RGB scart with a pj......
     
  5. Captain chaos

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    Thanks, im just brushing up on my knowledge, I am starting work in John Lewis' Home Cinema dept next Friday.

    Is there anything else you can tell me about home cinema in general.

    I know we stock a lot of Bose, and Teac AV hardware, are they good quality?

    I just want to be as clued up as I can be, I don't want to be one of those brainless PC World moron assistants who don't know the first thing about anything.

    Thanks for your help!
     
  6. theritz

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    EDIT: Beaten to it by Mr. Bump (interrupted while typing). Leaving this intact anyway........SG.


    Some searching on the net (Google) would probably tell you lots, but (briefly)

    LCD:

    LCD projectors work by light being broken into three beams by a prism, passing through polarising filters and then three LCD panels (Red Green and Blue filtered), then focused by a lens onto a screen. TheLCD panels them selves work through the combination of polarized panels and twisted nematics liquid crystals to control the light that is able to pass through the display. The liquid crystals are naturally twisted allowing free flow of light through both the crystals and the polarized plates. However, when an electric current is passed through the crystals they straighten out, changing the angle of the passing light. Liquid crystals behave very predictably to an electric charge, so the voltage can be adjusted to determine the exact angle the light will be directed. When the angle of the light differs from the angle of the polarizing filter, the light is blocked creating a dark area on that particular pixel on the screen. Color is created using red green and blue color filtering. By adjusting the voltage, each color can be adjusted in intensity.




    DLP:

    DLP is somewhat simpler (!!) - it uses a Digital Mirror Device (DMD) - effectively a chip whose surface is covered with tiny mirrors - on an XGA resolution chip, there would be 1024x768 (786,432) tiny mirrors which can be tilted when a voltage is applied to each. In simple terms, light shines off the surface of the mirrors and then through a rotating colour wheel which has red green and blue segments - for each frame, the mirror reflects th three colour components which are focussed in quick sucession on the screen - models vary as to the number of colour segments,and the speed of the colour wheel, which affects the "refresh rate" (not quite the right term, I guess) of the picture.

    CRT:

    CRT works by using separate Red Blu and Green Cathode Ray Tubes (CRT) with high light output to focus separate red blue and green components of a picture on the screen. In simple terms, CRT tubes work by focussing a beam of electrons against a glass surface coated with a phosphor which glows where the electron beam hits it. The CRT "draws" the image by varying the strength of the electron beam, creating variations in the brightness of the phosphor at particular points in it's path. Doing this for each of the Reg Green and Blue CRT, and each being focussed separately by it's own lens, creates the image on the screen.

    The primary difference between CRT and the others is that is a direct display device - when something from a CRT projector is supposed to be black, the CRT puts out no light, and the same goes for greys etc. - it outputs the appropriate amount of light; consequently it has super contrast. The other two use a light source shing through or reflected off, digital devices to create their image - inevitably when something is supposed to be black in the image some light still gets through, conseqently complaints about poor blacks with digital projectors.

    There's probably a million other differences too, but that one is the most obvious technical one, the other being the fundamentally different method of creating the image - pixels vs. CRT - the CRT is credited with creating a filmic image without ant of the digital "structure" that comes from LCD or DLP.

    CRT's are also th size of a small car, weight 10 tons, and have enough high voltage running around inside them to kill your whole street. Hopefully (!) Chris Frost will be back to fix any glaring errors in this post.

    Accusations of CRT/LCD bias will not be entertained.


    Sean G.EDIT: Beaten to it by Mr. Bump (interrupted while typing). Leaving this intact anyway........SG.
     
  7. Mr Bump

    Mr Bump
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    Far better run down than my poor little effort anyway... :smashin:
     
  8. theritz

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    Mr. Bump,

    No way, m8, Your post was on the button - I was typing up my stuff (including checking out some stuff on the net.. !!) and had to go and sort out something on my daughters pc.... came back and continued typing, while the world went on merrily without me !!!

    Sigh...... such is life !!


    See ya,

    Sean G.
     
  9. Gordon @ Convergent AV

    Gordon @ Convergent AV
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    There is an article on DLP on my website. It is also on this one I believe.

    Who wants to tacle D'ila then.

    I also suggest you read the sticky by NicRhodes all about composite, s-video etc.

    Then go read all about what progressive scan is.

    Good luck in your new job. By being here and asking questions I already think you are going to go far.

    Gordon
     
  10. theritz

    theritz
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    Gordon,



    Hmmm, give it a go, any way.....


    D-ILA is JVC's LCOS (Liquid Crystal on Silicon) tecnique, D-ILA stands for Direct drive Image Light Amplifier. This is a reflective LCD technology - in effect, the controlling chip, liquid crystal layer and a glass seal are fabricated as a complete unit. The unit is not transmissive like a normal LCD panel, light is pointed into the device ( by a device like a prism) and is reflected out again, with the pixel layer controlling the output. LCOS has a high aperture ratio, which significantly reduces the appearance of panel structure (screendoor) in the image.


    See ya,

    Sean G.
     
  11. Captain chaos

    Captain chaos
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    Thanks for the responses guys, much appreciated, any more info would be gratefully received.
     
  12. Kramer

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    More info :confused: Gees, I think the guys have said all there is about the various technologies.

    :D
     
  13. Captain chaos

    Captain chaos
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    Well off the top of my head (no looking at facts and figures) I know that the Panny 300 has 960 lines of vertical resolution, the HS2 has about 860, the HS10 has about 1,350, the Z1 has 964.

    Not bad for about 4 weeks research!

    John Lewis only keep stock of the HS2 and the AE300, any others they have to order in.
     
  14. LV426

    LV426
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    One major(?) factor that distinguishes 3-tube CRT projectors from D-ILA, DLP, and LCD (ie. single lens) projectors, which may be of relevance to your prospective customers is....

    In a single lens projector, all of the alignment of the three primary colours is internal. This effectively makes the projector as portable/movable as a slide projector. All you have to do to get a reasonable image is to point it, fairly square on, to a screen, make focus, zoom (usually manual) and keystone (usually via a menu) adjustments if necessary, and away you go. DIY installation is therefore relatively easy, and, as an alternative to a fixed (maybe ceiling) installation, such a PJ can easily be brought out and fired up when needed, and put away again afterwards.

    With a 3-tube CRT - the three separate images produced by the three tubes have to be converged (aligned) after installation. This process needs repeating if ever the projector (or screen) is moved. It is normally a job for an installation engineer and can take half a day to do it properly. So, they really need a permanent installation.
     
  15. theritz

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    Captain Chaos,

    I'm assuming the above is supposed to be humourous....... you've got two of the resolution figures wrong, and if it took you 4 weeks to discover that much...............:eek: :eek: (HS2, 858x484, HS10 1366x768), "about" is not really a relevant descriptor when dealing with resolution of projectors - if you said it to me in a shop, I'd assume you either didn't know or were too feckless to bother finding out. The following springs to mind......

    If you want facts and figures about projectors and some reviews, projectorcentral.com is your friend.


    Sean G.
     
  16. calscot

    calscot
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    I think you mean HORIZONTAL resolution. Here in Milton Keynes we're very pedantic about H's and V's, otherwise we would get lost all the time as that's how the roads are numbered... :D

    Cheers,
    Cal.
     
  17. Captain chaos

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    OK, im sorry theritz, I'll remember that the HS10 has 1366 and the HS2 has 858.
     
  18. Gordon @ Convergent AV

    Gordon @ Convergent AV
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    Captain. You need to concentrate harder. CALSCOT has pointed out a major flaw in your figures....

    Vertical resolution is measurement of HORIZONTAL LINES.

    So HS10 is NOT 1366 and HS2 is not 858.....
     

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