Chapter 1 Digital Display Devices

Chapter 1 Digital Display Devices

1. Types of Electronic displays2. Types of LCD displays3. Basic operating principles of LCD display4. DSTN display5. TFT display6. Comparison between CRT display and LCD display7. Review Questions

1. Types of Electronic Displays

Emissive Non-emissive (based on light control)

1.1 Types of Emissive Display

1.1.1 Neon light

Neon light is used in advertising and commercial signage. These signs are made of long, narrow glass tubes, and these tubes are often bent into all sorts of shapes. The tube of a neon light can spell out a word, for example. These tubes emit light in different colours.

Inside the glass tube there is a gas like neon, argon or krypton at low pressure. At both ends of the tube there are metal electrodes. When you apply a high voltage to the electrodes, the neon gas ionizes, and electrons flow through the gas. These electrons excite the neon atoms and cause them to emit light that we can see. Neon emits red light when energized in this way. Other gases emit other colours.

1.1.2 Cathode ray tube (CRT) monitor

A CRT monitor contains millions of tiny red, green, and blue phosphor dots that glow when struck by an electron beam that travels across the screen to create a visible image. Fig 1 shows how this works inside a CRT.

The terms anode and cathode are used in electronics as synonyms for positive and negative terminals.

1 EC5103PA

Chapter 1 Digital Display Devices

A: Cathode D: Phosphor-coated screen B: Conductive coating E: Electron beam C: Anode F: Shadow mask

Fig 1 Inside a CRT

In a cathode ray tube, the "cathode" is a heated filament. The heated filament is in a vacuum created inside a glass "tube." The "ray" is a stream of electrons generated by an electron gun that naturally pour off a heated cathode into the vacuum. Electrons are negative. The anode is positive, so it attracts the electrons pouring off the cathode. This screen is coated with millions of tiny red, green, and blue phosphor dots that glow when struck by an electron beam that travels across the screen to create a visible image.

1.1.3 Light emitting diode (LED)

A light-emitting diode is a semiconductor diode that emits incoherent narrow-spectrum light when electrically biased in the forward direction of the p-n junction, as in the common LED circuit. This effect is a form of electroluminescence.

LEDs are often used as small indicator lights on electronic devices and increasingly in higher power applications such as flashlights and area lighting. The colour of the emitted light depends on the composition and condition of the semiconducting material used, and can be infrared, visible, or ultraviolet. LEDs can also be used as a regular household light source. Besides lighting, interesting applications include sterilization of water and disinfection of devices.

1.1.4 Electro-luminescent display (ELD)

The technology used to produce a very thin display screen, called a flat-panel display, used in some portable computers. An ELD works by sandwiching a thin film of phosphorescent substance between two plates. One plate is coated with vertical wires and the other with horizontal wires, forming a grid. When an electrical current is passed through a horizontal and vertical wire, the phosphorescent film at the intersection glows, creating a point of light, or pixel.1.1.5 Plasma display panel (PDP)

2 EC5103PA

Chapter 1 Digital Display Devices

A type of flat-panel display that commonly used for large TV displays (typically above 940 mm). It works by sandwiching neon gas between two plates. Each plate is coated with a conductive print. The print on one plate contains vertical conductive lines and the other plate has horizontal lines. Together, the two plates form a grid. When the electric current pass through a horizontal and a vertical line, the gas at the intersection glows, creating a point of light, or pixel. You can think of a gas-plasma display as a collection of very small neon bulbs. Images on gas-plasma displays generally appear as orange objects on top of a black background.

1.1.6 Vacuum fluorescent display (VFD)

A vacuum fluorescent display (VFD) is a display device used commonly on consumer-electronics equipment such as video cassette recorders, car radios, and microwave ovens. Unlike liquid crystal displays, a VFD emits a very bright light with clear contrast and can easily support display elements of various colours. The technology is related to both the cathode ray tube and the nixie tube.

1.1.7 Field emission display (FED)

A field emission display (FED) is a type of flat panel display using field emitting cathodes to bombard phosphor coatings as the light emissive medium.

Field emission displays are very similar to cathode ray tubes, however they are only a few millimeters thick. Instead of a single electron gun, a field emission display (FED) uses a large array of fine metal tips or carbon nanotubes (which are the most efficient electron emitters known), with many positioned behind each phosphor dot, to emit electrons through a process known as field emission.

FEDs are energy efficient and could provide a flat panel technology that features less power consumption than existing LCD and plasma display technologies. They can also be cheaper to make, as they have fewer total components. Nano-emissive display is the name given by Motorola for field emission display.

1.1.8 Organic LED (OLED)

OLEDs are solid-state devices composed of thin films of organic molecules that create light with the application of electricity. OLEDs can provide brighter, crisper displays on electronic devices and use less power than conventional light-emitting diodes (LEDs) or liquid crystal displays (LCDs) used today.

Such systems can be used in television screens, computer displays, portable system screens, advertising, information and indication. OLEDs can also be used in light sources for general space illumination, and large-area light-emitting elements. OLEDs typically

3 EC5103PA

Chapter 1 Digital Display Devices

emit less light per area than inorganic solid-state based LEDs which are usually designed for use as point-light sources.

It is beginning to replace LCD technology in handheld devices such as PDAs and cellular phones because the technology is brighter, thinner, faster and lighter than LCDs, use less power, offer higher contrast and are cheaper to manufacture.

Photo courtesy Sony Corporation

Fig 2 (a) OLED display for Sony Clie (b) Sony 11-inch OLED, released in Japan at the end of 2007

The biggest technical problem for OLEDs is the limited lifetime of the organic materials. In particular, blue OLEDs historically have had a lifetime of around 14,000 hours when used for flat-panel displays, which is lower than typical lifetime of LCD, LED or Plasma display technology – each currently rated for about 60,000 hours, depending on manufacturer and model.

The intrusion of water into displays can damage or destroy the organic materials. Therefore, improved sealing processes are important for practical manufacturing and may limit the longevity of more flexible displays.

1.2 Types of Non-Emissive Display

1.2.1 Liquid crystal display (LCD)Refer to page 5.

1.2.2 Digital micromirror device (DMD)

Digital Light Processing (DLP), a new technology developed by Texas Instruments used for projecting images from a monitor onto a large screen for presentations. DLP uses tiny mirrors housed on a special kind of microchip called a Digital Micromirror Device (DMD). The result is sharp images that can be clearly seen even in a normally lit room.

The number of mirrors corresponds to the resolution of the screen. DLP 1080p technology delivers more than 2 million pixels for true 1920x1080p resolution, the highest available.

4 EC5103PA

Chapter 1 Digital Display Devices

1.2.3 Electrophoretic ink (e-ink)

An electronic paper display (EPD) is a specialized type of electronic paper (e-paper) that combines the uses and advantages of a computer display and paper. Electronic paper displays are extremely thin, use minimal amounts of power and provide a high-contrast viewing surface like paper, but can be easily updated like a monitor.

EPDs are a technology enabled by electronic ink (e-ink) that carries a charge enabling it to be updated through electronics. Electronic ink is ideally suited for EPDs as it is a reflective technology which requires no front or backlight, is viewable under a wide range of lighting conditions, including direct sunlight, and requires no power to maintain an image.

There are several different technologies to build e-paper, some of which can use plastic substrate and electronics, so that the display is flexible. It is considered more comfortable to read than conventional displays. This is due to the stable image which does not need to be constantly refreshed, the large viewing angle, and the fact that it uses reflected ambient light. It has a similar contrast ratio to that of a newspaper and is lightweight and durable, however it still lacks good color reproduction.

Applications include e-book readers capable of displaying digital versions of books and e-paper magazines, electronic pricing labels in retail shops, time tables at bus stations, and electronic billboards.

Electronic paper should not be confused with digital paper, which is a pad to create handwritten digital documents with a digital pen.

Fig 3 (a) Sony reader (b) Seiko watch (c) 2mm thick flexible active-matrix display

5 EC5103PA

Chapter 1 Digital Display Devices

2. Types of LCD Displays

2.1 LCD displays is the most common type of flat panel display. It is commonly used in digital watches, notebook computers, handheld computers, calculators, miniature televisions, digital still and video cameras and monitors.

Fig 4 LCD digital flat panel display

2.2 LCD displays utilise two sheets of polarising material with a liquid crystal solution between them. An electric current passed through the liquid causes the liquid crystals (rod-shaped molecules) to align so that light cannot pass through them. Each crystal, therefore, is like a shutter, either allowing light to pass through or blocking the light.

2.3 LCD displays produce colour using either passive matrix or active matrix technology.

Passive matrix LCD displays consist of a grid of horizontal and vertical wires. At the intersection of each grid is an LCD element that constitutes a single pixel. The displays consume less power, less expensive and have a narrower viewing angle than active matrix LCDs. TN (twisted nematic), STN (supertwisted nematic) and DSTN (dual scan twisted nematic) are different types of passive matrix technology.

Active matrix LCD (AMLCD) displays, sometimes also called TFT (thin film transistor) displays, are a higher quality and more expensive type of display in which transistor are built into each pixel to switch each one on or off within the screen. It produces a sharper display with a broader viewing angle than passive matrix.

3 Basic Operating Principles of LCD Display

3.1 LCD molecules are arranged in a loosely ordered fashion with their long axes parallel in the natural state. When they come into contact with a grooved surface in a fixed direction, they line up in parallel along the grooves.

3.2 The first principle of a LCD consists of sandwiching liquid crystals between two finely grooved surfaces, where the grooves on one surface are perpendicular (at 90 degrees) to the grooves on the other, as shown in Fig 5.

6 EC5103PA

Chapter 1 Digital Display Devices

If the molecules at one surface are aligned north to south, and the molecules on the other are aligned east to west, then those in-between are forced into a twisted state of 90 degrees.

Light follows the alignment of the molecules, and therefore is also twisted through 90 degrees as it passes through the liquid crystals.

When a voltage is applied to the liquid crystal, the molecules rearrange themselves vertically, allowing light to pass through untwisted.

Fig 5 Basic operating principle of LCD

3.3 The second principle of an LCD relies on the properties of polarising filters and light

itself. Natural light waves are orientated at random angles.

A polarising filter is simply a set of incredibly fine parallel lines. These lines act like a net, blocking all light waves apart from those (coincidentally) orientated parallel to the lines.

A second polarising filter with lines arranged perpendicular (at 90 degrees) to the first would therefore totally block this already polarised light. Light would only pass through the second polariser if its lines were exactly parallel with the first, or if the light itself had been twisted to match the second polariser.

3.4 A typical twisted nematic (TN) liquid crystal display as shown in Fig 6 consists of two polarising filters with their lines arranged perpendicular to each other.

7 EC5103PA

Light

Polarisingfilter

Polarisingfilter

Liquid crystal(rod-like molecules)

Chapter 1 Digital Display Devices

In-between these polarisers are the twisted liquid crystals. Therefore light is polarised by the first filter, twisted through 90 degrees by the liquid crystals, finally allowing it to completely pass through the second polarising filter.

When voltage is applied across the liquid crystal, the molecules realign vertically, allowing the light to pass through untwisted but to be blocked by the second polariser.

Consequently, no voltage equals light passing through, while applied voltage equals no light emerging at the other end – normally white mode. The crystals in an LCD could be alternatively arranged so that light passed when there was a voltage, and not passed when there was no voltage - normally black mode.

Since computer screens with graphical interfaces are almost always lit up, power is saved by arranging the crystals in the normally white mode.

(a) 0v transmitting state (b) >5v non-transmitting state Fig 6 A typical twisted nematic (TN) LCD at normally white mode

8 EC5103PA

Chapter 1 Digital Display Devices

4. DSTN Displays

Fig 7 DSTN display at normally white mode

4.1 With DSTN, or dual scan twisted nematic screens, the orientation of alignment layers varies between 90 degrees and 270 degrees, depending on the total rotation of the liquid crystals between them.

4.2 A backlight is added, typically in the form of cold-cathode fluorescent tubes mounted along the top and bottom edges of the panel, the light from these being distributed across the panel using a plastic light guide or prism.

The image which appears on the screen is created by this light as it passes through the layers of the panel.

As shown in Fig 7, with no power applied across the LCD panel, light from the backlight is vertically polarised by the rear filter and refracted by the molecular chains in the liquid crystal so that it emerges from the horizontally polarised filter at the front – normally white mode.

Applying a voltage realigns the crystals so that light can't pass, producing a dark pixel.

4.3 In a colour monitor, each pixel is made out of 3 subpixels that have either red, green, or blue colour filters. Each subpixel is energized with different intensities, creating a range of colours perceived as the mixture of these dots.

Fig 8 shows that additional red, green and blue coloured filters are used to create a single multi-coloured pixel.

9 EC5103PA

No power is applied

Chapter 1 Digital Display Devices

(a)

(b)

10 EC5103PA

Chapter 1 Digital Display Devices

Fig 8 Structure of a LCD monitor

4.4 Passive matrix LCD has slow response time. With rapidly changing screen content such as video or fast mouse movements, smearing often occurs because the display can't keep up with the changes of content.

4.5 In addition, passive matrix driving causes ghosting, an effect whereby an area of "on" pixels causes a shadow on "off" pixels in the same rows and columns.

5. TFT displays

5.1 In a TFT (Thin Film Transistor) screen, also known as active matrix, an extra matrix of transistors is connected to the LCD panel - one transistor for each colour (RGB) of each pixel. These transistors drive the pixels, eliminating the problems of ghosting and slow response speed.

5.2 The active matrix is a method of addressing an array of simple LC cells--one cell per monochrome pixel. In its simplest form there is one thin-film transistor for each cell. This arrangement is shown in Fig 9.

Fig 9 Simple TFT Active Matrix Array 5.3 TFT screens can be made thinner, lighter, and refresh rates now approach those of CRTs

as the current runs about ten times faster than on a DSTN screen.

5.4 VGA screens need 921,000 transistors (640x480x3), while a resolution of 1024x768 needs 2,359,296 and each has to be perfect.

5.5 The complete matrix of transistors has to be produced on a single, expensive silicon wafer and the presence of more than a couple of impurities means that the whole wafer must be discarded.

11 EC5103PA

Row select(Gate)

Column select (Data source)

Chapter 1 Digital Display Devices

This leads to a high wastage rate and is the main reason for the high price of TFT displays. It's also the reason why in any TFT display, there are liable to be a couple of defective pixels where the transistors have failed.

5.6 There are two phenomenon which define a defective LCD pixel: a "lit" pixel, which appears as one or several randomly-placed red, blue and/or green

pixel elements on an all-black background, or a "missing" or "dead" pixel, which appears as a black dot on all-white backgrounds. The former is the more common and is the result of a transistor occasionally shorting on, resulting in a permanently "turned-on" (red, green or blue) pixel.

Unfortunately, fixing the transistor itself is not possible after assembly. It is possible to disable an offending transistor using a laser. However, this just creates black dots which would appear on a white background.

5.7 Permanently turned on pixels are common in LCD manufacturing. LCD manufacturers set limits - based on user feedback and manufacturing cost data - as to how many defective pixels are acceptable for a given LCD panel.

The goal in setting these limits is to maintain reasonable product pricing while minimising the degree of user distraction from defective pixels.

For example, a 1024x768 native resolution panel - containing a total of 2,359,296 (1024x768x3) pixels - which has 20 defective pixels, would have a pixel defect rate of (20/2,359,296)*100 = 0.0008%.

5.8 One of the largest cost elements in a standard TFT panel is the external driver circuitry, requiring a large number of external connections from the glass panel, because each pixel has its own connection to the driver circuitry.

This requires discrete logic chips arranged on PCBs around the periphery of the display, as shown in Fig 10, a-Si (amorphous silicon ) TFT LCD module; thus limiting the size of the surrounding casing.

5.9 The p-Si (polysilicon) technology allows the driver circuitry and peripheral electronics to be made an integral part of the display.

The technology will yield thinner, brighter panels with better contrast ratios, and tougher than a-si panels. It allows larger panels to be fitted into existing casings.

12 EC5103PA

Chapter 1 Digital Display Devices

Fig 10 a-si and p-si TFT LCD panels6. Comparison Between CRT Display And LCD Display

6.1 CRT monitors have two specifications for screen size: the CRT size (the actual size of the picture tube) and the viewable screen size (the usable screen area). Because the CRT picture tube is enclosed in the plastic casing, the viewable screen size is smaller than the overall CRT size.

Unlike CRTs, the diagonal measurement of an LCD is the same as its viewable area.

Fig 11 (a) CRT screen size (b) LCD screen size

As shown in Table 1, the diagonal screen size of a LCD of 19 inches, is equivalent to a 17-inch CRT display's viewing.

Flat Panel size CRT size Typical resolution

13.5in 15in 800x600

14.5in to 15in 17in 1024x768

18in 21in 1280x1024  or1600x1200

Table 1

Popular screen sizes are 15, 17, 19 and 21 inches. Notebook screen sizes are smaller, typically ranging from 12 to 17 inches. As technologies improve in both desktop and notebook displays, even larger screen sizes are becoming available. For professional

13 EC5103PA

Chapter 1 Digital Display Devices

applications, such as medical imaging or public information displays, some LCD monitors are 40 inches or larger!

Unlike CRT monitors, LCD monitors display information well at only the resolution they are designed for, which is known as the native resolution (see Table 1). Digital displays address each individual pixel using a fixed matrix of horizontal and vertical dots. If you change the resolution settings, the LCD scales the image and the quality suffers.

6.2 The LCD monitor is thinner and lighter than CRT monitor.6.3 Unlike CRT display, the LCD monitors have no convergence problems, because each cell

is switched on and off individually.

The LCD monitors are also called "soft" screens, since their images seems to have a "softer" quality than those from traditional CRT monitors. The image does not flicker thus causing less eye strain.

6.4 It is possible for one or more cells on the LCD panel to be flawed. On a 1024x768 monitor, there are three cells for each pixel - one each for red, green, and blue - which amounts to nearly 2.4 million cells (1024x768x 3 = 2,359,296). There's only a slim chance that all of these will be perfect; more likely, some will be stuck on (creating a "bright" defect) or off (resulting in a "dark" defect).

6.5 LCD panels are lit by fluorescent tubes through the back of the unit; sometimes, a display will exhibit brighter lines in some parts of the screen than in others. Ghosting may occur where a particularly light or dark image can affect adjacent portions of the screen.

6.6 Viewing angle problems on LCDs occur because the technology using transmissive method, which works by modulating the light that passes through the display. While CRTs are emissive displays, the light is emitted at the front of the display, which is easily viewed from greater angles. The viewing angle on LCD monitors is getting wider and wider and current models offer over 160 degrees.

6.7 An important difference between CRT monitors and LCD panels is that the former require an analogue signal to produce a picture and the later require a digital signal.

The problem occurs because most panels are designed for use with current graphics cards, which have analogue outputs. In this situation the graphics signal is generated digitally inside the PC, converted by the graphics card to an analogue signal, then fed to the LCD panel where it has to be converted back into a digital signal. This process limits the display's performance and compromise the image quality.

14 EC5103PA

Chapter 1 Digital Display Devices

Fig 12 Interface between LCD monitor and PC using (a) digital interface (b) analogue interface

Currently, most LCD monitors plug into a computer's familiar 15-pin analogue VGA port and use an analogue-to-digital converter to convert the signal into a form the panel can use.

DVI : Digital Visual Interface VGA : Video Graphics Adapter

Fig 13 A graphics adapter with both analogue (VGA)and digital (DVI) connectors

There are two main types of DVI connections:

DVI-digital (DVI-D) is a digital-only format. It requires a video adapter with a DVI-D connection and a monitor with a DVI-D input. The connector contains 24 pins/receptacles in 3 rows of 8 plus a grounding slot for dual-link support. For single-link support, the connector contains 18 pins/receptacles.

15 EC5103PA

(a)

(b)

Chapter 1 Digital Display Devices

DVI-integrated (DVI-I) supports both digital and analog transmissions. This gives you the option to connect a monitor that accepts digital input or analog input. In addition to the pins/receptacles found on the DVI-D connector for digital support, a DVI-I connector has 4 additional pins/receptacles to carry an analog signal.

Fig 14 Types of DVI connectors

If you buy a monitor with only a DVI (digital) connection, make sure that you have a video adapter with a DVI-D or DVI-I connection. If your video adapter has only an analog (VGA) connection, look for a monitor that supports the analog format.

6.8 Table 2 provides a feature comparison between a 13.5in passive matrix LCD (PMLCD) and active matrix LCD (AMLCD) and a 15in CRT monitor:

Display

TypeViewing Angle

Contrast Ratio

Response Speed Brightness Power

Consumption Life

PMLCD 49-100 degrees 40:1 300ms 70 - 90 45 watts 60K

hours

AMLCD > 140 degrees 450:1 25ms 70 - 90 50 watts 60K

hours

CRT > 190 degrees 300:1 n/a 220 - 270 180 watts Years

Table 2

Contrast ratio is a measure of how much brighter a pure white output is compared to a pure black output. The higher the contrast the sharper the image and the more pure the white will be. Traditionally the CRT monitors always had better contrast and LCD monitors were lagging behind. Recently some of the best LCD monitors have come very close and according to some they match what the CRT is capable of in terms of contrast. High contrast is most important for gaming and movie playback.

16 EC5103PA

Chapter 1 Digital Display Devices

Response time is measured in milliseconds and indicates how fast the monitor's pixels can change colours. An AMLCD has a much better response time than a PMLCD. Ghosting or trailing effects start to become evident when the response time slows down to 20 ms. One should definitely look for monitors offering 16ms or even 12 ms response time. Conversely, response time doesn't apply to CRTs because of the way they handle the display of information (an electron beam exciting phosphors).

The higher the level of brightness represented in the table as a higher number, the brighter the white displays.

When it comes to the life span of an LCD, the figure is referenced as the mean time between failures for the flat panel. This means that if it is runs continuously it will have an average life of 60,000 hours (about 6.8 years) before the light burns out. The CRTs can last much longer than that. However, while LCDs simply burn out, CRT's get dimmer as they age, and in practice don't have the ability to produce an ISO compliant luminance after around 40,000 hours of use.

7. Review Questions

1. Name two types of emissive displays.

2. Write down two advantages of OLED as compared to LCD display.

3. What are LCD monitors and how do they work?

4. Give the full name for the following:i) AMLCDii) PMLCDiii) TFTiv) DSTNv) DVI

5. An ____________________ is a liquid crystal display structure in which switching transistors are attached to each pixel to switch each one on or off.

6. Give two advantages of an active matrix LCD as compared to passive matrix LCD.

7. State two types of passive matrix LCD technology.

17 EC5103PA

Chapter 1 Digital Display Devices

8. What is the function of the backlight of a LCD panel?

9. What are the two phenomenon which define a defective LCD pixel?

10. State four shortcomings of CRT monitor as compared to LCD monitor.

18 EC5103PA