Useful Notes: Display Technology
Displays are something to give humans a natural method of showing the output of an electronic device. The most common example that everyone can relate to is the television. The requirement though, is that it has to show a recognizable image. Early display technologies, like mechanical television had poor quality and images could barely be made out
History of Displays (Or the Days of Analog Displays)For the longest time, there was only one kind of display technology: the cathode ray tube (CRT). 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 rest of the world however, ran on 50Hz AC electricity, and came up with their own standard. Since both used similar tranmissions techniques, the only thing that needed to happen was that 60 Hz broadcasts repeated every fifth frame from 50 Hz broadcasts, while 50Hz broadcasts dropped every sixth frame from 60 Hz broadcasts. Then in the The Fifties, color TV standards were being developed. To remain backwards compatible with the old monochrome TV, the same signal was sent with an embedded color signal. Monochrome televisions lacked the circuitry to decode this portion. However, the way to do was problematic standards wise. The US National Television System Committee was the first to propose a color TV standard which got called NTSC after it. In the Europe the situation was even more complex, with two competing standards being proposed — Germany promoted the Telefunken-developed Phase Alternating Line system (PAL), while in France the "Séquentiel couleur à mémoire" (SECAM) standard was created by Thomson, which was later also adopted by the Soviet Union. Quality wise, NTSC is inferior (it has a lower resolution and is notoriously poor at reproducing the colornote ) and prone to quality degradation, but runs at 60Hz (less flickering). PAL has better picture quality and higher resolution, but is subject to flicker due to its lower refresh rate of 50Hz. SECAM, despite being chronologically first, was even better, but more complex and expensive to produce. When computers first started to appear, they borrowed from televisions and adopted the same technology. Well mostly. When they were powerful enough to display colors, instead of using a brightness and color, the displays were simply fed red, green, and blue signals that varied in intensity. Also by then, monitors were starting to use standard connectors and signals (before then, each manufacturer's computer only worked with its own monitor, or used a TV), so there was no need for backwards compatibility.
The Transition From Analog to DigitalComputers were first to adopt a digital signal standard, mostly because computers were digital and needed an interface that didn't convert digital to analog and back to digital. While analog signals used discrete levels, these were still, well, analog interfaces in nature. The first true digital interface (DVI and LVDS) came about the same time as LCD monitors were starting to become mass produced. Later after 2000, television broadcasts were going into digital broadcasting, as it was a more efficient use of airwaves with digital compression. Displays were also transitioning from the bulky CRT to the more space efficient plasma and LCD, which were more or less purely digital divices. When coming up with a cable to transmit these new HD digital signals, the computer's DVI standard was used to form HDMI. Today there's a push for the DisplayPort standard for computer monitors, which uses packets of information, rather than a stream of data as DVI more or less mimics the signaling of CRTs. Digital signals have since negated any quality difference between regions. The only thing that's different is the 50 Hz/60 Hz refresh rate. Today, the high end features of display technology are very high refresh rates, sterescopic 3D, and well... higher resolutions.
Digital ColorA special side topic that affected both analog and digital displays. In early computerized systems, memory was a precious resource. Thus only a certain amount of bits could be reserved for colors when saving a display buffer. These bits mostly held the intensity for red, green, and blue signals. For many systems that outputted to analog displays, there were tricks to support a lot of colors without eating so much memory. The frame could be rendered with so many colors (called a pallete) out of more range of colors. For instance, the Nintendo Entertainment System had a color range of 8-bits, but it could only display 4-bits of color for each frame. It was up to the system's GPU and RAMDAC to mix these colors in the end. Today practically all modern systems that support color stop at 24-bits, which is about 16 million colors total. The reason is that this is on the upper edge of human perception. Some displays do offer more, but this is more to confer confidence in artists who need the color accuracy. Image formats may also support higher bits for color, but this is to prevent errors from accumulating while applying effects.
Issues common to all displaysScreen burn-in/Image Persistence When the image of what's being displayed doesn't change much, it wears those pixels out faster than the others. And when it shows something else, a faint ghost of that image appears. This is called burn-in. You can commonly see this in digital signages (like the timetables at airports) or on video arcade cabinents. Only displays that emit light are suspectible to this and is a permanent condition. Displays that block or reflect light may have an issue called image persistence, where if the pixels are left in a high voltage state for long, they have a tendency to stick that way. Unlike burn-in, image persistence usually goes away on its own. Input Lag This is the time it takes for the screen to show response from an input, such as moving a cursor. This mostly concerns games and applications where tight timing is important. However, significant input lag is unpleasant regardless. Dead/stuck pixels Dead pixels are pixels which will not respond to any input and usually remain either black, white, or some color. These are defective and cannot be repaired. Stuck pixels on the other hand, are defective, but may resolve itself over time. A popular method to resolve a stuck pixel is to flash it several colors rapidly for a few minutes. Contrast Ratio This is more of a problem with marketing than the display itself. Contrast ratio is the measurement of brightness from the display's pure white against pure black. The rating is under ideal conditions, i.e., a pitch black room. For displays that need another source of light to work (LCD for instance), they cannot completely block light, so the manufacturer may fudge with the environment to get some ideal number. The worst is the term "dynamic contrast" on LCD type displays, which is actually a marketing fluff term. Basically, the display may adjust its backlight and amplifies the color input, giving an illusion the contrast is higher than what it normally is.
Display TechnologiesFor a list of display connectors, see the page on That Other Wiki.
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Obsoleted display technologies note
Cathode Ray Tube (CRT)
- 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.
- Can display any resolution without quality degradation.
- Great color quality with true black
- Practically no input lag, thus perfect for Video Games.
- Contrast isn't as good, as maximum brightness is limited.
- Subject to flickering if the refresh rate is too low.
- This is why most people who claim to get headaches or eye problems from CRTs have them, especially if the room was lit with fluorescents, which have a 120hz flicker (in the US) from the AC power. 60hz would interact with the fluorescent lights and make the flickering even worse. Setting the refresh to 72hz or more made the display clear and stable. It's even worse in Europe, Asia and the Pacific, where the AC cycle is 50hz, which makes the flickering more pronounced.
- 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. Also, the lead protection can not eliminate it completely, but only reduce it to miniscule amounts.
- 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.
Vacuum Fluorescent Display (VFD)
- 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).
- However, since it's based on phosphor technology, it is possible to have a VFD with multiple albeit segmented colors, as seen in various electronics from the 80s (i.e. the spectrum analyzer on some stereos, or the record icon on some VCRs). Read: a VFD can show multiple colors, but the color of a segment on the VFD cannot be changed once it's manufactured.
- 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).
- 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. Blue VFDs are usually achieved by passing White through a color filter, true blue VFDs are rare and expensive.
- 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).
- Due to limitations of the manufacturing process of VFDs, it's not possible to have a full color dot matrix VFD, let alone one capable of going beyond low-resolution graphics.
Proto-plasma — Nixie and Panaplex tubes
- 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.
- 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.
- Require high voltages to work, making them impractical for battery-powered equipment (though some people have done it anyway).
Current display technologies
- 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.
- Usually very bright with the highest contrast available.
- Excellent color quality.
- Boasts very high refresh rates (up to 600Hz).
- Thin profile.
- Very good black levels (Effectively 0); the Pioneer Kuro line even has its main selling point right in the name!
- Discrete pixels, thus only the native resolution can be displayed clearly. The issue with this is that they tend to produce blurry images if a non-native resolution is fed to the display. 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. Modern displays though are approaching similar power consumption as early LCD sets.
- Subject to image burn-in, though modern displays aren't as bad as early ones.
- As of 2014, pixel density appears limited. One needs to buy a 50" TV before reaching 1080p.
Liquid Crystal Display (LCD)
- 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. It can be thought of like a shutter, where the off state is open and the on state is closed.
- Thin and light profile. Suitable for portable displays.
- Consumes the least amount of energy for acceptable results, though it depends on the content.
- 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 modern LCD displays come with a scaler built in, however using a non-native resolution will result in blurry images and perhaps a condition known as video lag due to the scaler resizing the received image to the native resolution (you may have noticed that some TVs have a delay issue with non-native resolution inputs. That's video lag).
- Pixel response time is often higher than competing technologies (though not as bad as some). Most panels average 8ms-16ms using the ISO standard test of full black to full white, with 2ms-6ms from gray-to-gray. Poorer response times often result in a smearing effect known as ghosting.
- Some panels (usually the older Passive Twisted Nematic displays) have narrow viewing angles which cause color shifting if not viewed head on. This is a problem if the screen size is too large or if the viewing angle isn't ideal.
- Backlit LCDs have poor sunlit outdoor readability, requiring a VERY bright backlight to be readable.
- The light source in older backlit displays are constantly being turned on and off. If the frequency is off, it may cause headaches due to flickering. However, newer active matrix TFT displays (ie IPS and PLS) do not do this and keep the backlight constantly on, alleviating the issue. Since the resason for the flickering was to reduce ghosting in the older Passive TN displays and is largely irrelevant as active matrix displays are fast enough to not result in ghosting. Some companies like Samsung, however, are trying to reintroduce flickering backlights, claiming that it helps in producing crisper 3D images where active 3D glasses are used.
- LCDs may come in glossy or matte finish. Glossy finishes offer a clearer picture, but there is bad glare in bright light. Matte finishes have excellent glare reduction in all lighting conditions, but may appear fuzzy.
There's a few methods of how the liquid crystal element interacts with light, each with pros and cons.
- Passive Twisted Nematic (TN) - Offers the best response time (although it has ghosting issues, necessitating the flickering backlight) and is the cheapest to make, but cannot product 24-bit color (most panels are 18-bit color that employs some trick to make it look like 24-bit color) and the viewing angles are narrow. If you view a TN panel off center, colors and contrast start shifting.
- Thin Film Transistors - Sometimes called Active Matrix, Slightly newer than Passive Twisted Nematic. Offers fast response time and lacking the ghosting issues and flickering backlight of Twisted Nematic. It's advertised to produce richer colors than Passive twisted Nematic, but it's viewing angles is only slightly better than Passive Twisted Nematic. This is, technically, the granddaddy of all modern LCD panels.
- Vertical Alignment (VA) - An older panel type that offers better viewing angles and color reproduction than TN, but is mostly falling out of favor because of its high input lag.
- In-Plane Switching (IPS) - Offers so-so response time, but offers great viewing angles. Cheaper panels are 18-bit color with tricks while higher end ones can go up to 30-bit color. Though some report an issue with "IPS glow" where the backlight has too much bleed when showing a dark image.
- Plane Line Switching (PLS) - Very similar to IPS.
- 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).
- There are two types for LEDs to distribute light: edge lit and array. Edge lit displays have the LEDs on the edge and the light is as uniformly distributed as possible across the display. Array based has the LEDs behind the display itself.
- 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. Work is in progress to shrink the technology so that it's possible to have such displays for the living room, but apart from a few prototypes presented at electronic shows and aside from a couple of very expensive professional-grade displays (and even then they're still combined with LCD technology), there are no affordable full LED-based displays for the average consumer yet.
- 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.) actually use VFDs, although there are a handful out there based on true LED technology.
- Very thin profile. LED-backlit displays have been as thin as less than half an inch.
- Array based LED displays can use local dimming to increase contrast ratios.
- 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.
- They are more reliable, robust, and energy efficient than CCFL based LCDs.
- Traditionally were more expensive, but processes today have made them as expensive, if not cheaper, than CCFL types.
Digital Light Processing (DLP)
- 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.
- 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.
- 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. Likewise, the filtering has been known to cause headaches in some people, akin to CRT flickering.
- 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 of a neutral color, ideally white.
- 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.
- "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.
- 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.
- DLP-based projectors also borrow a lot of pros and cons from DLP display panels mentioned above.
- 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.
- 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
- 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.
- The display has to black itself out first before displaying new content. This also has the issue of sometimes having image persistence.
Emerging display technologies
Organic Light Emitting Diode (OLED) Display
- 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.
- A cousin of this called OLET, for transistor instead of diode, is practically the same with one difference. OLED still needs a thin-film-transistor to control each diode, whereas OLET is both the controlling transistor and lighting element.
- 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.
- Not as energy efficient as LCDs note
- Still very expensive for displays larger than small consumer electronics. Although some companies are releasing large television sets in the 50"+ range by 2013-2014
- 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.
- A lot of consumer OLEDs use a PenTile Matrix to arrange the subpixels so that there are more (smaller) green and red subpixels and fewer (but larger) blue subpixels. This may give the display a subtly odd hue compared to an LCD.
Quantum Dot Display
- A particle called a quantum dot which is more easily excitednote is used to produce light. The technology is very similar to OLED, but differs on the finer details.
- Even weirder is the color each quantum dot emits is based on its size, while everything else is based on the voltage passing through the element.
- Since they are more inorganic, they can last longer than OLED.
- Given that color is generated based on size of the dot, not the voltage passing through, this may lead to much greater power savings (less circuitry for various voltages needed, etc.)
- Blues are less saturated, but it's being worked upon.
Interferometric Modulation (IMOD) Display
- Similar to DLP, it uses a microelectromechanical-system (MEMS) chip to reflect light. Unlike DLP, it works just fine in ambient light and doesn't require a color filter, as each MEMS element is designed to reflect a specific wavelength of light (which is where that Techno Babble name comes from).
- Very good visibility in bright light.
- Can retain state without power, or at least, using very little power.
- Small enough that the current major manufacturer of this technology is using it for a smart watch.
- Refresh rate is a little poor, recent models have just gotten up to 30Hz
- Doesn't work in darkness, but a backlight can be used.