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Monitors

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Monitors

• The monitor is the primary output device for a PC.

• The additional circuitry needed for a computer to interface with a monitor is on the video/display card/adapter.

• There are two basic types of monitor – CRTs (Cathode Ray Tube)– Flat Panels or LCDs (Liquid Crystal Display)

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CRT• A CRT consists of a large vacuum tube. The tube

is narrower in the rear where it houses an “electron gun” and then widens in the front display/screen area. – A color monitor typically has three electron guns.

• Electrons are essentially boiled off of the electron gun’s cathode.

• The electrons are then accelerated toward the electron gun’s anode. They are also collimated.

• The yoke is a set of electromagnets that direct the beam of electrons toward a particular point on the screen.

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Cathode Ray Tube

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Monitor Electron Gun(s)

The device shoots three electron beams toward the screen.

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Mask• A little before the screen is the mask. The mask

absorbs electrons that have been somewhat misdirected. It keeps the “red” electron gun from exciting a blue or green spot.

• Some variations on the idea go by the names– Aperture grill

– Shadow mask

– Slotted mask

– Enhanced dot pitch

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Phosphors• The electron beam is directed toward the screen

which is covered on the inside with phosphors. • A phosphor absorbs the energy from the electron

beam. Then to release that energy and return to its normal state, it emits photons (light “particles”).

• In addition to computer screens, phosphors are used with fluorescent lights and various glow-in-the-dark items. Also some detergents contain phosphors so that clothes will glow under a black light.

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Phosphors (Cont.)• The important characteristics of phosphors are:

– What gets them excited.– What color they emit.– How long they emit (their persistence).

• Color CRTs will use a set of three phosphors corresponding to the colors red, blue and green.

• The persistence is important in monitors – too short and it flickers; while too long and it smears.

• Examples of phosphors include Zinc Sulfide and Strontium Aluminate.

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Dot Pitch

• The screen is covered with interlaced arrays of red, green and blue phosphors.

• A screen characteristic is the dot pitch or phosphor pitch, which is the distance between the centers of two like colored phosphors.

• Computer monitor dot pitches can range from around .18 mm to .39 mm (millimeters).

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Dot pitch

Triad

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Monitor Specs

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Monitor Specs (Cont.)

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Pixels and Resolution • A pixel which is short for picture element, is the smallest

logical unit in a display.

• The smallest possible pixel would consist of a triad – three phosphors, one of each color – but typically pixels consist of more triads.

• A screen is broken into a two-dimensional array of pixels.

• The resolution of the display is given by the number of horizontal elements times the number of vertical elements.

• Because a pixel is just a logical unit, the same display can support a number of various resolutions.

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Resolution Setting

Start/Settings/ Control Panel/ Display/Settings

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Aspect Ratio

• Monitors will support a set of standard resolutions such as 640 480, 800 600, 1024 768, …

• These resolutions have the property that if the horizontal number is divided by 4 and then multiplied by 3, one gets the vertical number.

• The display is said to have a 4:3 aspect ratio.

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Monitor Specs

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Monitor Size

• Given the fixed 4:3 aspect ratio, the size of a monitor can then be given by its diagonal length of the screen.

• The only thing to watch here is the distinction between the display size and the visible display size.

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Monitor Specs

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Painting an image

• The display image is “painted” starting with the electron guns pointing to the upper left hand corner.

• The guns sweep across horizontally. The intensity of their fire (and thus the intensity of the glowing phosphor) depends on the signal they receive.

• The guns return to the left and move one row down without firing.

• With this the guns have completed one raster line.

• The rate at which they can do this is called the horizontal refresh rate (HRR).

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Raster Line/Horizontal Refresh Rate

One horizontal line is known as a raster line.

The number of horizontal lines per second is known as the horizontal refresh rate. It is given in Hertz or kilohertz.

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Vertical Refresh Rate

• The guns sweep through line after line until the lowest line is drawn.

• Then the guns return to the upper left hand corner.

• The number of times the guns return to the upper left hand per second is known as the vertical refresh rate (VRR) or just refresh rate – generally reported in Hertz.

• Too high a VRR can damage the monitor, too low a VRR can cause flickering and user eye strain.

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Refresh Frequency Setting

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Bandwidth

• A monitor’s bandwidth is the maximum number of times the electron gun can be turned on and off per second.

• The gun intensity does not have to change on a triad level but on a pixel level.

• Because the resolution can change, but the bandwidth cannot, the refresh rate can be resolution dependent.

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Refresh Rates and Resolutions

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Color Depth

• Color depth is the number of bits used to specify the color for each pixel.

• The number of possible colors will be 2 raised to the color depth. This will be related to the number of intensity level of the electron guns.

• For example “true color” uses 32 bits and thus has 232 = 4,294,967,296 possible colors.

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Color depth setting

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RAMDAC

• The CRT uses an analog signal, so part of the video card is a RAMDAC.

• RAMDAC stands for Random Access Memory Digital-to-Analog Converter.

• The RAM part uses SRAM to hold the color table that is used to convert a logical color into three separate digital signals. (It’s in memory because one can change the color depth.)

• Those three digital signals are sent to three DACs (digital-to-analog converters).

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LCD: Pros and Cons

• Pros:– Thinner

– Lighter

– Less power

– No flicker

– No radiation

– Brighter?

– Less problem with reflection

• Cons– More expensive (but

they have come down in price a lot recently)

– More limited viewing angle

– Fixed resolution

– Poor with fast changing video

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Light

• Varying electric fields produce magnetic fields.

• Varying magnetic fields produce electric fields.

• The combined result is a propagating electromagnetic disturbance that we like to call light.

• I.e., light is an electromagnetic wave

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Directions

• Light is made up of electric fields and magnetic fields.

• Electric fields and magnetic fields points in particular directions.

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Radiation

• With the light that comes from a heated object, such as a light bulb filament or the sun, there is no preferred direction for the electric field.

• The electric field is in a random direction that changes from instant to instant.

• (Laser light is different in this respect.)

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Interacting with a material

• When light impinges on a material, the electric and magnetic fields of the light interact with the charged electrons and protons making up the material.

• This can result in reflection or absorption or transmission.

• What if the material has a different behaviors in different directions?

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Huh?

• Consider a set of metal bars, they could conduct electricity along the bars but not perpendicular to them.

• Then the part of the light with the electric field in one direction would respond differently from the part of the light with the electric field in some other direction.

• Some light may be absorbed and some light may be transmitted.

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Polarizing

• There’s no “maybe” about it. These directionally prejudiced materials exist.

• A polarizing film or filter is used to make sunglasses. Light is reduced by transmitting only light with a particular direction.

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Effect of a Polarizer

Before: random red direction can be thought of as part vertical (blue) and part horizontal (green).

After: after polarizing the only “component” left is aligned with the polarizer (in this case horizontal).

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Cross polarizers

• An electric field in a generic direction (i.e. light having a generic polarization) can be thought of as being made up some part vertical and some part horizontal.

• A vertical filter would take out the vertical part, a horizontal filter would take out the horizontal part.

• If we had a vertical filter followed by a horizontal filter (so-called cross polarizers), there would be nothing left.

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An intermediate• If there’s an intermediate polarizer that is neither

horizontal nor vertical some light gets through. • The vertical leads to vertical pointing electric

fields. • But vertical can be thought of as made of part

parallel to the intermediate direction and part perpendicular to it. The parallel part gets through.

• That part can be thought of as part vertical and part horizontal and the horizontal part gets through.

• Really.

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A funny intermediate

• This is where the liquid crystals come in. • The liquid crystal is going to serve as this

intermediate between two crossed polarizers.

• It can help twist the polarization of light so that it is not blocked out by the crossed polarizers or in other conditions it does not twist the light and the light is blocked out.

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Liquid Crystal

• In crystals molecules tend to stay in one place and to be lined up.

• In liquids molecules tend to flow around and to lie in random directions.

• Liquid crystals are an intermediate phase associated with certain elongated molecules. Liquid crystals tend to flow but also tend to be aligned.

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Here’s the twist

• The aligned, elongated molecules mean that these materials have a selected direction – like the metal bars, they can serve as a polarizer.

• The liquid crystal is arranged so that direction of the molecules slowly twist from vertically aligned to horizontally aligned.

• This twisted liquid crystal slowly guides the polarization of the light from vertical or horizontal so that some light gets through the cross polarizers.

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Going straight

• Applying an electric field to the twisted liquid crystal can cause it to straighten up.

• Without the twist, the polarization of the light stays vertical and is blocked out when it gets to the horizontal polarizer.

• Varying the field, varies twist, varies the intensity if light that passes through.

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Source of light• One approach to LCDs is to use the ambient light

from the room. The outside light passes through the liquid crystals and polarizers, is reflected by a mirror and returns unless the cross polarizing effect takes place. – Note many digital watches are also LCD

• A more dependable approach is to use backlighting. A light source is placed behind the polarizers and liquid crystal.

• The brightness of a display is measured in units called nits. – Low end: 100 nits – high end: 300 or more nits

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References

• PC Hardware in a Nutshell, Thomson and Thomson

• All in One A+ Certification, Michael Myers

• http://computer.howstuffworks.com/monitor.htm/printable

• http://plc.cwru.edu/tutorial/enhanced/files/lcd/tn/tn.HTM