Video Display Devices

Video Display Devices• A Video display device is an output device for presentation

of information in visual way . When the input information is supplied as anelectrical signal, the display is called an electronic display.

• Common applications for electronic visual displays are televisions or computer monitors.

Different types of Video display Devices

• Cathode Ray Tube

• Raster Scan displays

• Random Scan displays

• Color CRT-monitors

• Direct View Storage Tube

• Flat-Panel Displays

• Light-emitting Diode (LED)

• Liquid-crystal Displays (LCDs)

CRT(Cathode Ray Tube)

CRT• Classical output device is a monitor.

• Cathode-Ray Tube (CRT)

• Invented by Karl Ferdinand Braun (1897)

• Beam of electrons directed from cathode (-)tophosphor-coated (fluorescent) screen (anode (+))

• Directed by magnetic focusing and deflection coils(anodes) in vacuum filled tube

• Phosphor emits photon of light, when hit by anelectron, of varied persistence (long 15-20 ms fortexts / short < 1ms for animation)

• Refresh rate (50-60 Hz / 72-76 Hz) to avoid flicker /trail

• Phosphors are organic compounds characterized bytheir persistence and their color (blue, red, green).

Cathode-Ray Tube (CRT) Horizontal deflection and vertical deflection direct

the electron beam to any point on the screen Intensity knob: regulates the flow of electrons by

controlling the voltage at the control grid (highvoltage reduces the electron density and thusbrightness) Accelerating voltage from positive coating inside

screen (anode screen) or an accelerating anode

Image maintenance Charge distribution to store picture information

OR Refresh CRT: refreshes the display constantly to

maintain phosphor glow.

• Characteristics of Cathode-Ray Tube (CRT)• Intensity is proportional to the number of electrons

repelled in beam per second (brightness)

• Resolution is the maximum number of points thatcan be displayed without overlap; is expressed asnumber of horizontal points by number of verticalpoints; points are called pixels (picture elements);example: resolution 1024 x 768 pixels. Typicalresolution is 1280 x 1024 pixels.

• High-definition systems: high resolutionsystems.

Categories of CRT

CRT

Based on Technology Based on Color

Raster Scan System Random Scan System Beam Penetration Method Shadow Mask Method

Random Scan Display/Calligraphic displays Random scan systems are also called vector, stroke-writing,

or calligraphic displays.

Random scan system uses an electron beam which operateslike a pencil to create a line image on the CRT. The image isconstructed out of a sequence of straight line segments.

Each line segment is drawn on the screen by directing the

beam to move from one point on screen to the next, whereeach point is defined by its x and y coordinates.

After drawing the picture, the system cycles back to the first

line and design all the lines of the picture 30 to 60 time eachsecond.

When operated as a random-scan display unit, aCRT has the electron beam directed only to theparts of the screen where a picture is to bedrawn.

Random-scan monitors draw a picture one line at

a time and for this reason are also referred to asvector displays (or stroke-writing orcalligraphic displays

• Refresh rate on a random-scan system dependson the number of lines to be displayed.

• Picture definition is now stored as a set of line-drawing commands in an area of memoryreferred to as the refresh display file.

• Random-scan systems are designed for line-drawing applications and can-not displayrealistic shaded scenes.

• Since picture definition is stored as a set of line-drawing instructions and not as a set ofintensity values for all screen points, vectordisplays generally have higher resolution thanraster systems.

• Advantages:

• High resolution

• Easy animation

• Requires little memory

• Disadvantages:

• Requires intelligent electron beam (processor controlled)

• Limited screen density, limited to simple, line-based images

• Limited color capability.

• Improved in the 1960’s by the Direct View Storage Tube (DVST) from Tektronix.

Raster-Scan Displays

• In raster scan approach, the viewing screen isdivided into a large number of discrete phosphorpicture elements, called pixels.

• Each pixel on the screen can be made to glow witha different brightness. Color screen provide for thepixels to have different colors as well as brightness.

• In a raster-scan system, the electron beam is sweptacross the screen, one row at a time from top tobottom. As the electron beam moves across eachrow, the beam intensity is turned on and off tocreate a pattern of illuminated spots.

• Picture definition is stored in a memory area calledthe refresh buffer or frame buffer.

• This memory area holds the set of intensity valuesfor all the screen points. Stored intensity values arethen retrieved from the refresh buffer and "painted"on the screen one row (scan line) at a time (Fig. 2.6).

• Each screen point is referred to as a pixel or pel(shortened forms of picture element

Color CRT Monitors

• 1. Beam Penetration Method

• 2. Shadow Mask Method

Beam Penetration Method• The beam-penetration method for displaying color

pictures has been used with random-scan monitors.

• Two layers of phosphor, usually red and green, are

coated onto the inside of the CRT screen, and the

displayed color depends on how far the electron beam

penetrates into the phosphor layers.

• A beam of slow electrons excites only the outer red

layer. A beam of very fast electrons penetrates through

the red layer and excites the inner green layer.

• At intermediate beam speeds, combinations of red and

green light are emitted to show two additional colors,

orange and yellow.

• The speed of the electrons, and hence the screen color

at any point, is controlled by the beam-acceleration

voltage.

• Beam penetration has been an inexpensive way to

produce color in random-scan monitors, but only four

colors are possible, and the quality of pictures is not as

good as with other methods.

Shadow Mask Method

• Shadow-mask methods are commonly used inraster-scan systems (including color TV) becausethey produce a much wider range of colors thanthe beam-penetration method.

• A shadow-mask CRT has three phosphor color dotsat each pixel position. One phosphor dot emits ared light, another emits a green light, and the thirdemits a blue light.

• This type of CRT has three electron guns, one foreach color dot, and a shadow-mask grid justbehind the phosphor-coated screen.

• The three electron beams are deflected andfocused as a group onto the shadow mask,which contains a series of holes aligned withthe phosphor-dot patterns.

• When the three beams pass through a hole 'inthe shadow mask, they activate a dot triangle,which appears as a small color spot on thescreen.

• The phosphor dots in the triangles arearranged so that each electron beam canactivate only its corresponding color dot whenit passes through the shadow mask.

.

• Another configuration for the three electron guns is an in-line arrangement in which the three electron guns, and the.

• Corresponding red-green-blue color dots on the screen, are aligned along one scan line instead of in a triangular pattern.

• This in-line arrangement of electron guns is easier to keep in alignment and is commonly used in high-resolution color CRTs.

• It provides the variety of color range

DVST• This is an alternative method to monitor a screen

image, as it sores the picture information inside theCRT instead of refreshing the screen.

• A direct-view storage tube (DVST) stores thepicture information as a charge distribution justbehind the phosphor-coated screen.

• Two electron guns are used in a DVST. One, theprimary gun, is used to store the picture pattern;the second, the flood gun, maintains the picturedisplay.

• A DVST monitor has both disadvantages andadvantages compared to the refresh CRT. Becauseno refreshing is needed, very complex pictures canbe displayed at very high resolutions without flicker.

http://ecomputernotes.com/images/DVST.jpg

Disadvantages of DVST systems

• do not display color and that selected parts of apicture cannot be erased.

• To eliminate a picture section, the entire screenmust be erased and the modified pictureredrawn.

• The erasing and redrawing process can takeseveral seconds for a complex picture.

Flat-Panel Displays

• The term flat panel display refers to a class of video devicethat have reduced volume , weight and power requirementcompared to a CRT.

• A significant feature of flat-panel displays is that they arethinner than CRTs, and we can hang them on walls or wearthem on our wrists.

Categories of flat-panel displays:

• emissive displays: The emissive displays (or emitters) aredevices that convert electrical energy into light.

• non emissive displays: Non emmissive displays (or nonemitters) use optical effects to convert sunlight or light fromsome other source into graphics patterns. The mostimportant example of a non emissive flat-panel display is aliquid-crystal device.

Light-emitting Diode (LED)

• In LED, a matrix of diodes is arranged to formthe pixel positions in the display and picturedefinition is stored in a refresh buffer.Information is read from the refresh bufferand converted to voltage levels that areapplied to the diodes to produce the lightpatterns in the display

Liquid-crystal Displays (LCDs) • Liquid crystal displays are the devices that produce a

picture by passing polarized light from thesurroundings or from an internal light source througha liquid crystal material that transmit the light.

• Liquid-crystal displays (LCDs) are commonly used insmall systems, such ' as calculators and portable,laptop computers.

• These non-emissive devices produce a picture bypassing polarized light from the surroundings or froman internal light source through a liquid-crystalmaterial that can be aligned to either block ortransmit the light