Display Technology

Displays are something to give humans a natural method of showing the output of an electronic device. The most common use for display is television technology, or transmitting remote images either live or in a pre-recorded format. However, display technology must be able to provide one thing: to present information in a clear, distinct way that most people can make sense of it. Early display technologies (such as the mechanical television) did show pictures, but the quality was quite bad and images could barely be made out if at all.

History of Displays

There was really only one kind of electronic display technology since it had been around for so long already, the Cathode Ray Tube (CRT). A purely analog device, it works by shooting an electron beam onto a screen coated with phosphors, causing them to glow. However, when it came with broadcasting television, there was a split on how to do it. North America and parts of Asia use 60 Hz AC electricity, so it made sense to match this by making television signals run at this frequency. The National Television System Committee (NTSC) standard was made. The rest of the world however, ran on 50 Hz AC electricity, and came up with their own standard, the Phase Alternating Line (PAL) and SECAM. Quality wise, NTSC is inferior and prone to quality degradation but runs at 60 Hz (less flickering). PAL has better picture quality and higher resolution, but is subject to flicker due to its lower refresh rate of 50 Hz.

In the wake of the new millennium, over-the-air signals started to switch from analog to digital. Digital signals have the advantage of using less bandwidth for the same picture quality. This also allows networks to split their channel into subchannels for more focused content. The only downside is that with poor reception, analog feeds just lose quality, but a digital feed cuts out completely. HDTV standards have also negated the quality differences between NTSC and PAL/SECAM, though the refresh rate remains the same.

The Wackiness of Display Signals

Displays can also carry other kinds of content besides television signals. Commonly they're used to output things to a computer monitor or some kind of electronic device where displaying information would be very important or even necessary. Computer monitor signals have standards, but they're not exactly the same as television signal standards. For one, computer displays encode the picture as intensity values of the primary colors of light (red, green, blue). Televisions carry a brightness and color signal, as to remain backwards compatible with black and white displays. Another is that computer displays tend to use discrete bits for colors while televisions have continuous intensity. But when the digital age broke and HDTVs were becoming more popular, most of the technology for both digital information and HD resolutions was there in computer displays. Thus HDTVs are usually big honkin' computer monitors. In fact, the HDMI cable is based on the digital computer display Digital Video Interface (DVI) standard.

Since much of the world now uses digital displays, numerous display types were developed. The primary purpose of any display at this point is to be able to show colored lights as discrete pixels and there are numerous ways to make pixels. The rage these days for displays is 120 Hz/240 Hz refresh rates and 3D Stereoscopy. The move to 120 Hz/240 Hz is mostly to prevent quality loss while showing movies, which are shot at 24FPS and for being the least common multiple of 24, 30, and 60. Otherwise there is no real gain as everything is still recorded and broadcast at 60 Hz (in NTSC territories anyway). 3D Stereoscopy attempts to try to give an illusion of depth by providing each eye with a slightly altered image, mimicking how humans actually perceive 3D.

Digital Color

Color sent to a digital display (or a computer CRT monitor) carries the intensity of one of the primary colors of light: red, green, or blue. Thus displays can have a setting for all the way on, all the way off, or values in between. One thing to note is that coloring schemes tend to differ in context. 8-bit color could mean the system can display 8-bit colors only, or it may have a color range of 16-bits, but its palette (how many colors it can actually display) is limited to 8-bits. Tricks can also be used to give the perception of more colors than possible using dithering. For example, it's possible to make a shade of peach by alternating red, yellow, and white pixels.

List of colors

• 1-bit color: Black and some kind of color (usually white, amber, or green on monitors). Mostly used in text only terminals.
• 2-bit color: Black, white, and two shades in between. The first Game Boy used this.
• 3-bit color: Each bit is tied to a primary color of light. Possible colors are black, white, red, green, blue, yellow, cyan, and magenta.
• 4-bit color: Same as 3-bit, but the fourth is for intensity. Many 8-bit consoles could only display this many colors at once.
• 6-bit color: The Sega Master System and various 8-bit home computers used this.
• 8-bit color: 256 colors, a common coloring format. Many 16-bit consoles had 8-bit palettes while the last 8-bit consoles had this color range.
• 12-bit color: The amount of colors the Sega-Genesis had total, but it had a 6-bit palette.
• 15-bit color: The color range of the SNES. Also a somewhat common format for computers.
• 16-bit color: A common color range for computers. 5 bits are assigned to red and blue, 6 to green due to the human eye's sensitivity to it.
• 18-bit color: The actual color range of TN panel LCDs.
• 24-bit color: The color range of displays today, which is about 16 million colors. This is the upper limit of human perception.
• 32-bit color: A misnomer, it's really 24-bit color with 8-bits for transparency information.
• 30-bit/48-bit/64-bit color: 30-bits, known as Deep Color, is used in professional displays to ensure color accuracy. 48-bit and 64-bit color is for image editors as to prevent an accumulation of errors while applying color effects.

Display Technologies

Cathode Ray Tube (CRT)

Technology

• CRTs shoot an electron beam at phosphors coated on a screen at a rate of 50 full images (or more) per second, topping out at 160 or 180 Hz for high-end VGA monitors. When an electron beam hits a phosphor, it glows for a moment then fades. This is the only display type primarily used for analog signals.
  • Arcade Light Gun games actually took advantage of the technology, by calculating the difference between the start of a scan and when the gun picked up light.
• A common effect when filming a display using CRT is black bands that slowly scroll down. This is because the filming rate (usually 24FPS) only captures some of what the CRT is showing at the time. Human eyes have what is called persistence of vision, which allows humans to retain the image of something for a brief period of time.

Pros

• Can display any resolution without quality degradation.
• Great color quality with true black
• Practically no input lag, thus highly ideal for Video Games.

Cons

• Contrast isn't as good, as maximum brightness is limited.
• Subject to flickering if the refresh rate is too low.
• Image geometry can be distorted as the phosphor screen is curved (even on so called "flat" displays), often in a way that cannot be corrected by the On-Screen Display (OSD) menu.
• Color convergence can be thrown off, resulting in color fringing.
• Bulky, heavy, runs hot, and power consuming. It's also dangerous to open one up as CRT displays can retain a massive amount of electricity even when turned off.
  • Since CRTs contain what is essentially a linear particle accelerator, the screen can generate a non-trivial amount of X-rays, especially in color sets. CRT glass is leaded to protect against this, but the lead makes disposal a problem and adds to the already-unwieldy bulk of large tubes.
• Subject to image burn-in, where a static image is permanently visible due to certain phosphors wearing out faster than others. Usually seen in airports or arcade cabinets.
• Susceptible to magnetism. Holding a strong magnet against the screen can permanently damage it by twisting or distorting the shadow mask (a piece of metal that keeps the colors separated) behind the screen. Light magnetic distortions are normal due to the Earth's own magnetic field, and are fixable using via degaussing (most CRTs have a built-in degaussing coil, and will automatically use it when powered up).
  • Old monochrome CRTs don't use a shadow mask, making it safer (though still not a good idea) to use strong magnets on their faces.

Plasma Display

Technology

• Made of individual plasma cells. Like CRT, the screen is coated with phosphors that glow, but instead of being excited by electrons, they glow from plasma generated in the plasma cell.
• Based on the same principle as the neon light; early plasma screens were monochrome and glowed with the bright red-orange color of neon.

Pros

• Usually very bright with the highest contrast available.
• Excellent color quality.
• Boasts very high refresh rates (up to 600 Hz).
• Thin profile.
• Very good black levels (Effectively 0); the Pioneer Kuro line even has its main selling point right in the name!

Cons

• Discrete pixels, thus only the native resolution can be displayed clearly. On the flip side, it eliminates geometry issues.
• The front panel is glass (subject to breakage) which makes plasma displays deceptively heavy, runs hot, and is also power consuming.
• Subject to image burn-in, though modern displays aren't as bad as early ones.

Vacuum Fluorescent Display (VFD)

Technology

• Similar to plasma, but built on vacuum tube technology rather than the neon light. Phosphors painted in patterns on the back of the tube are excited by electrons emitted from a filament just behind the screen. A set of grids between the back and the filaments allow control over various areas of the screen; this allows for LCD/LED-style multiplexing.
• Usually monochrome, specifically a pleasing turquoise color (though Sony has sometimes used a more whitish phosphor on their VFDs).

Pros

• Very bright and readable, even in bright light.
• Will run well in cold temperatures, making them popular for automotive use (digital dashboards, radios, and such).

Cons

• Uses more power than LCD or LED-matrix displays, but not quite as much as plasma.
• Limited range of colors available; almost all VFDs use the same few shades of turquoise, white, red and orange.
• Not as thin as LCD or LED-matrix displays.
• Like all vacuum tubes, they're fragile and the filaments have a limited life. However, since the filaments don't need to be run hot, their lifetimes can be very long (decades).

Proto-plasma -- Nixie and Panaplex tubes

Technology

• They work just like plasma screens, but instead of individual pixels, fully-formed numbers (Nixie) or seven-segmented digits (Panaplex) are used.
• Very common in older digital equipment from the 1960s and early 1970s.

Pros

• Largely the same as modern plasma screens; these were the brightest small displays around until VFDs and LED-matrix screens became popular in the late 1970s.

Cons

• Require high voltages to work, making them impractical for battery-powered equipment (though some people have done it anyway).

Liquid Crystal Display (LCD)

Technology

• The display has a material called liquid crystal, which depending on the voltage applied to it can either block light or let light through.
  • In reflective LCDs, ambient light is reflected back to the user while the liquid crystal blocks or allows some of that light back. Used in very low power consuming devices but has the problem of requiring a good light source to see well. Front lights can either be embedded or bought separately. The Game Boy line all used reflective LCDs.
  • In backlit LCDs, an always on backlight shines behind the liquid crystal. While still lower power consuming, it also works poorly in bright light. Practically every LCD made today for consumer devices uses a backlit model.
  • There are also transreflective LCDs, which are a combination of the two. Development started in 2010. It's mostly targeted for portable electronics due to its power saving potential.
• The amount of energy consumed for what the LCD is displaying is actually inverted than what one would expect. Showing white consumes the least amount of energy, while showing black consumes the most.

Pros

• Thin and light profile. Suitable for portable displays.
• Consumes the least amount of energy for an active display.

Cons

• Because LCDs have discrete pixels, it cannot display a resolution clearly except for the "native resolution." On the other hand, geometry is always perfect.
• Many LCDs use TN panels. While TN panels are cheap, they also are limited to 18-bit color (using tricks to simulate other colors) and have poor viewing angles. Off-center viewing angles can lead to color shifts and even inversion.
• While much better today, LCDs suffer from slow response times (how fast a pixel can change), which may induce the effect known as ghosting. There's also the problem that there is no industry standard of measuring response times. A 2ms response time is only such in a special case, but it may be as bad as 8ms.
• High end panels such as PVA suffer from increased input lag due to image processing. It can be bad enough to be noticeable in a side-by-side comparison while running a GUI.
  • Other high end panels such as IPS, are a happy compromise between PVA and TN.
• Backlit LCDs have poor sunlit outdoor readability, requiring a VERY bright backlight to be readable.

LED Display

Technology

• What most people think is an LED display are actually an LCD using light emitting diodes as the lighting element, as opposed to cold cathode fluorescent lights (CCFLs).
• Displays made out of colored LEDs for each pixel (bypassing the LCD entirely) are real, but are used in very large displays like those seen for outside displays.
  • Older handheld devices (such as Mattel's Pocket Sports games and many early Texas Instruments calculators), and things that require a low-cost screen that's also bright and easy to read screen (alarm clocks, VCRs, etc.) use LED-matrix displays as well. They went out of style in calculators and games in the early 1980s due to their high power consumption; since alarm clocks and VCRs typically run on mains power, battery life isn't an issue there.

Tidbits

• Very thin profile. LED-backlit displays have been as thin as less than half an inch.
• Localized LED lighting can produce very high contrast ratios, allowing for true-black.
• Some LCD displays do use RGB LED backlighting for an increased color gamut, rather than the usual white LED or CCFL. These are all invariably upmarket professional-grade displays where color accuracy is critical.

Organic Light Emitting Diode (OLED) Display

Technology

• This display consists of organic compounds that can produce bright light when given a voltage. Because the pixels themselves emit light, no always-on backlight is needed, increasing image quality.

Pros

• Very thin profile. Thickness in the couple of millimeters are possible. In fact, it's possible to build flexible OLED displays.
• Very good brightness and contrast ratios. Comparable to plasma.
• It's possible to actually print an OLED display, which when perfected, can lead to drastically lower costs.

Cons

• Not as energy efficient as LCDs.
• Very expensive. The first commercial display was an 11" TV, that sold for $1000. It's mostly kept to small consumer electronic such as smart phones and MP3 players.
• Blue OLEDs have an issue with both lifespan (16,000 to half-brightness vs. 60,000 of other technologies) and efficiency at high brightness.
• Will be damaged upon contact with water, thus the display has to be sealed.
• Subject to burn-in.

Digital Light Processing (DLP)

Technology

• A light source (usually a white light lamp/bulb, but more recently, RGB lasers) is shined upon a chip with microscopic mirrors that represent a pixel. Each mirror can tilt to adjust the amount of light that hits the screen. Color filters are used for color in single-chip designs. Three-chip designs can bypass the color wheel entirely, as do laser-lit designs.
• Can be used either as an actual projector, or a self-contained rear projector TV.

Pros

• Very good image quality reproduction, especially laser DLP models that allow for incredible color gamut.
• Tends to be more user-serviceable. Lamps used in self-contained TVs can be replaced. Laser-based sets should not need regular replacement.

Cons

• If used as a TV, it's bulky due to the throwing distance needed for the projector. Inherently not flat-panel. On the flip side, they're deceptively light.
• "Rainbow effect"-this can be an issue for some users in cheaper single-chip DLP sets. Scanning your vision quickly across cheaper DLP sets will allow you to briefly see the red, green, and blue filtered images.

Other Projectors

Technology

• Basically, shoot bright light onto a screen. This makes projectors flexible as to where you can put them and as long as the surface is flat and white, where you can show them.
• Comes in LCD (uses LCDs as the color filter), DLP (uses a static color filter with a DLP chip), or laser (a red, green, and blue laser rapidly shine the image on screen) varieties. Very old models may be CRT-based.

Pros

• "Screen size" is as good as how bright the image is.
• Portable, due to their inherently small size, in fact some cellphones have projectors in them.

Cons

• Except the laser type, projectors tend to run very hot and are loud as a result (there's usually a fan to keep the lamp cool).
• Requires cooling down after use. Suddenly disconnecting the power can damage the lamp.
• The room used for a front projector must be as dark as possible, or the contrast of the projected image will severely suffer.

Electronic Ink

Technology

• Similar to reflective LCD. The ink is electromagnetically sensitive and suspended in a fluid. When given a voltage, the ink moves from one side of the cell to another, which changes how much light is reflected back. The principle is old though, Magna Doodles used a similar technique.

Pros

• Lowest power consumption. E-Ink retains its state even without power so it only needs power to change something.
• Very thin profile. Typically used in portable E-Book readers

Cons

• Image quality is acceptable, but currently available in Black and White only. Color based displays are in development.
• Only useful for showing static images as response time is very poor.

Display Connectors

Analog

• Radio Frequency (RF): A single wire carries broadcast frequency video and audio signals. Typically acceptable quality, but still rather poor.
• Composite: A single wire carries baseband video signal. Typically identified by its yellow connector.
• S-Video: Four connectors carry the luminance (brightness) and chrominance (color) signals. Image quality is a bit superior to composite. Identified by its black, circular connector with four pins.
• Component: Three wires carry the Y (brightness), Pr, and Pb signals. Pr and Pb carry red and blue, with green capable of being derived from a subtraction. The only analog carrier capable of HD quality signals as well as progressive scan SD quality signals. Identified by its red, green, and blue connectors. Contrary to popular belief, these don't carry discrete red, green, blue values, but they can in a pinch. Y signals can also be used in a composite input, but no color will show.
• VGA: A computer centric connector. Identified by its normally blue connector with 15 pins. Typically carries red, green, and blue intensity signals as well as syncing signals.
  • Analog RGB: Similar to VGA, this format was used by some personal computers of the late 1980s (specifically the Amiga and the Apple IIgs), and supported by many monitors that also supported 9-pin digital video. Runs at the NTSC or PAL sync rate, so isn't directly compatible with VGA without a line doubler.
• SCART/Péritel: 21-pin, bidirectional connector. Carries a variety of signals and is usually encouraged if available for its quality. Fairly rare outside Europe, but does have limited use elsewhere.

Digital

• 9-pin digital:
  • MDA: A specification developed at IBM for the IBM Personal Computer, this provided monochrome video with an intensity line (2-bit gray) and ran at a fixed resolution of 720x350. Later cards like the Hercules Display Card and IBM's own EGA could use this interface as well.
  • RGBI: Red-Green-Blue-Intensity. Now obsolete, it provided up to 16 colors (8 regular and 8 bright), and was used by the CGA on the IBM Personal Computer, as well as on RGB cards for the Apple II. The NEC PC-9800 used a version without the intensity line. IBM's EGA extended the interface to 6 bits (RGBrgb), but only in EGA's high-resolution (350-line) mode.
• DVI: A computer centric connector. Identified by its normally white connector with 30 some pins. DVI can display beyond HD signals (up to 2560x1600), but cannot display more than 24-bit color.
• HDMI: A TV centric connector, directly compatible with DVI. Also supports 5.1 surround sound audio through the same cable, something DVI does not do.
• DisplayPort: Set to supersede HDMI. Not compatible with either HDMI or DVI. In addition to audio and video, DisplayPort can carry data, such as for USB devices.

Issues common to all displays

• Screen burn-in/Image Persistence: When an image is shown for a long time, it wears out on the pixels faster than the others. But once something else is shown, the uneven wear will rear its ugly head. It's possible to see an "afterimage" in front of what should be there. This plagues many digital signs (especially schedules at the airport) as well as arcade cabinets. Technologies that emit light are prone to screen burn in while technologies that block/reflect light are prone to image persistence (image persistence however isn't usually permanent). It's uncertain whether or not DLP or laser based technology is subject to either.
• Input lag: The time it takes for the screen to show the response of an input. Typically a concern in games that require tight timing. However, significant input lag can cause an unpleasant experience outside of tight timing requirements.
• Dead/Stuck pixels: Applicable only to displays with discrete pixels (i.e., anything other than CRT and E-ink). Dead pixels are that, they are permanently disabled in some form or fashion. Usually seen in LCDs as a black pixel that won't go away and was a common issue until the mid 2000s when LCDs were becoming popular. A stuck pixel however, is a pixel that could show other colors, but it appears to be stuck showing color. Stuck pixels can be fixed by having it rapidly flash colors for a few minutes.
• Contrast Ratio: This is the difference between the brightest pixel on the TV versus the darkest pixel. The problem is that this is rated under ideal conditions, usually in a pitch black room. For LCDs this is a major problem, as while brightness can help increase contrast ratio, since LCDs can't completely block light, increasing the brightness may decrease the contrast ratio. Some displays attempt to change the dynamic range by adjusting how bright the backlight (if used) is automatically.
  • An even worse problem is the idea "dynamic contrast ratio". The dynamic part is that the contrast ratio is either shifting from one brightness range to another (so the display is only capable of 1000:1 say, but it can slide around a range of 1,000,000:1), or basically the contrast ratio can expand to the marketed rating depending on, again, the lighting conditions.