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In The Lab
HDTV and Monitor basics: A screen odyssey
by chris

HAL 9000
HAL 9000

Dave Bowman: "My God, it's full of stars!" (from 2001, A Space Odyssey)

Whether we are looking at computer monitors or High Definition Television displays, it all comes down to the dots or pixels that flicker there. Each of the different monitor technologies, CRT (Cathode Ray Tube), LCD (Liquid Crystal Display), or Plasma, still generate an image by turning small illuminated points on the screen on and off. By exciting tiny points of red, green, and blue phosphor you will perceive what appears to be a white dot visible on a CRT or Plasma screen. LCD screens display colors by passing or blocking white light through a red, green or blue filtered liquid crystal layer.

All standard displays generate images using three colors of light: red, blue and green. Together, these three points are called a triad, and represent the smallest possible addressable point on the display. When looking at LCD and some other types of displays, you may see a specification for "native mode" or "native resolution." This refers to the screen resolution where one pixel generated by your video source uses only one triad of the display.

click to enlarge
click to enlarge
click to enlarge
click to enlarge
An LCD display at its native resolution uses one triad for each pixel displayed. (NEC LCD400V at 1024x768 native resolution) When the LCD screen resolution is less than the native mode, more than one triad must be used for each pixel. (NEC at 800x600 resolution) A CRT display at its maximum resolution of 1280x1024 showing the triangular array of red, green and blue phosphor dots. The same CRT display at 800x600 resolution. Like the LCD display, multiple triads are used for each pixel.
A black-bordered 800x600 image
A black-bordered 800x600 image

If the native resolution is 1024x768, there should be 1024 red, 1024 green and 1024 blue dots positioned in triad-groups horizontally across the screen. If you changed your screen resolution to something less than 1024x768, and did not "expand" or "stretch" the display, then you will see a black border around the image, with only the central 800x600 dots being used. If the display was set to expand the image, then some pixel information had to be ignored, while other points were represented more than once, just to fill the screen. The result is a fuzzy or ragged looking image, where some lines might appear too wide and others appear thin and sharp. If you are going to use your large screen TV as a computer monitor display, you want to set the video screen size to the best the screen can display, this will usually be identified as the native resolution of the display, even if it is lower than the HDTV resolution. For example, many of our large LCD and Plasma TV screens can display 1920x1080 HDTV resolutions, but only support 1280x720 as the best computer resolution. When working with any displays that have a single native resolution (this includes LCD, Plasma, and DLP), all resolutions except the native mode must be compressed or expanded to fit to display.

"The monolith was 11 feet high, and 11/4 by 5 feet in cross-section. When its dimensions were checked with great care, they were found to be in the exact ratio 1 to 4 to 9 - the square of the first three integers." (Arthur C. Clarke, 2001: A Space Odyssey)

The concept of screen ratio warrants mention as well. You may have noticed that many computer displays are getting wider, and TVs definitely are, with the marketing of HDTV as the must-have next generation standard. The plan is not specifically for HDTV to replace current broadcast TV, but for digital broadcasts to replace the analog radio frequency broadcasts that are still in use now. CRTs may appear to handle different resolutions better than LCD or other display types, mainly because the dot pitch or spacing between the phosphor dots is typically much smaller than with the other technologies. When choosing any display where dot pitch is specified, keep in mind that the small the number, the closer together the screen pixels will be. Pixels that are smaller and closer together can make your display appear sharper and the images clearer and better defined. (I say "can" and not "will" because the sharpest image is also based on the screen resolution you are sending to the display. If you operate your computer display at a screen resolution less than its highest level, you are displaying less information in exchange for making the image appear larger.)

HAL9000: "I'm sorry Dave, I'm afraid I can't do that."


Side note: What's the difference between HDTV and HDTV-Ready? A display that is HDTV-Ready does not have an ATSC tuner or decoder, although it must support display modes of at least 720p or 1080i screen resolution. Without an ATSC tuner, a display cannot receive any over-the-air (antenna) HD broadcasts. They also cannot do anything directly with cable or satellite without going through some sort of HD signal converter such as an HD satellite receiver or HD Cable box. Digital cable is not the same as HD, it only means that the standard 480i resolution is transmitted as a digital signal not analog; it must still be converted to analog before your standard TV can display it.

ATSC: Advanced Television Standards Committee; see http://www.atsc.org for more information on Digital TV standards.
For as long as there have been methods of either producing a composite video signal from your computer, or connecting devices other than antennas to your TV, I have had people ask if they could use their TV as a computer display. A standard (NTSC) TV image is generated by scanning 525 lines at 30 frames per second, although the actual working image uses only about 480 lines (interlaced). Since a standard TV and computer image is usually based on a 4:3 aspect ratio, this gives you a horizontal resolution of right around 640 pixels. HDTV images are generated at a 16:9 widescreen ratio that is usually identified by the number of vertical lines, typically 720 or 1080. This means that a widescreen display with a 16:9 ratio should have a maximum video resolution of 1920x1080 pixels. The image on HDTV screens may be an interlaced (alternating scans of odd and even numbered lines) or progressive (single pass) one, which is indicated with an "i" or "p" following the scan line value. When shopping for HDTV capable displays, you will see the specifications for the resolutions supported to be something like 480p, 720p, 1080i or 1080p. This is the main reason why the "High Definition" level of detail is so impressive when you compare it to a standard 640x480 TV image, even without blowing it up to 50 or 60 diagonal inches.

HAL9000: "It's puzzling. I don't think I've ever seen anything quite like this before."

Many of the large format LCD, Plasma, or DLP (Digital Light Processing) displays have multiple connections for the video source. The type of connection you use may determine the quality of the image and the maximum resolution that is displayed. It seems that one of the common complaints of people buying a new High Definition TV is that the images they see at home don't look as good as the ones in the store. The obvious reason for this is that they are probably not using a high-definition video source. If you send a standard satellite, cable TV, or DVD signal to your display, the resolution is based on that NTSC standard of roughly 640x480. All you will get is a really big view of the same image you sent to your small screen. Even with a special (ATSC) HDTV tuner in the display, or when using an external HDTV decoder from your satellite or cable company, you need to select a connection method that will transmit the sharpest, highest-resolution signal to your display.

HDMI HDMI: High Definition Multimedia Interface provides for uncompressed digital video and audio over a single connection. HDMI includes support for DRM* More information at http://www.hdmi.org

DVI: Digital Video or Digital Visual Interface. DVI-D (digital only) and DVI-I (digital/analog) connectors. With a DVI-I connector, you can use an adapter to connect a standard VGA display to the video card. More information can be found at http://www.ddwg.org
Component Video

Component Video splits analog video into two or more signals. RGB (Red Green and Blue) is one common method used on some large-screen TVs.

The video synchronization signal must be sent on a fourth cable or can be combined with one of the color signal cables.
S-Video S-Video: Separate-Video transmits an analog signal that carries video as separate brightness and color signals at 480i resolution. S-Video is a 4-pin connector found on some video cards, TVs, and other video equipment.
RF RF: Radio frequency over cable or antenna can carry a combined analog video and audio signal. Pictured is a common 75-ohm Coax with an "F" connector.
Composite Composite Video: Composite video carries analog video signals (picture only.) On most equipment, composite video uses a yellow RCA connector. The red and white connectors are used for analog audio signals; left and right channels of stereo sound. Like s-video, composite video is based on 480i resolution.
This particular analog video card has a composite video connection and a standard VGA connection.
VGA: Video Graphics Array was an analog video standard introduced by IBM around 1987. The maximum resolution for VGA is 640x480, although multiple variations evolved since its introduction, including the SVGA and XGA "standards".

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* DRM: Digital Rights Management. Already some HD resolution video can be blocked from being displayed on any device unless it can be transmitted in a digital format that supports anti-piracy features. This is supposed to prevent anyone from making exact digital copies of the media.

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