Click this Image and see the CRT Working principle animation

Displays and Graphic Cards

Display:
A display is a computer output surface and projecting mechanism that shows text and often graphic images to the computer user, using a cathode ray tube (CRT), liquid crystal display (LCD), light-emitting diode, gas plasma, or other image projection technology. The display is usually considered to include the screen or projection surface and the device that produces the information on the screen. In some computers, the display is packaged in a separate unit called a monitor. In other computers, the display is integrated into a unit with the processor and other parts of the computer. (Some sources make the distinction that the monitor includes other signal-handling devices that feed and control the display or projection device. However, this distinction disappears when all these parts become integrated into a total unit, as in the case of notebook computers.) Displays (and monitors) are also sometimes called video display terminals (VDTs). The terms display and monitor are often used interchangably.
Most computer displays use analog signals as input to the display image creation mechanism. This requirement and the need to continually refresh the display image mean that the computer also needs a display or video adapter. The video adapter takes the digital data sent by application programs, stores it in video random access memory (video RAM), and converts it to analog data for the display scanning mechanism using an digital-to-analog converter (DAC).
Displays can be characterized according to:

• Color capability
• Sharpness and viewability
• The size of the screen
• The projection technology

What is CRT?
Stands for "Cathode Ray Tube." CRT is the technology used in traditional computer monitors and televisions. The image on a CRT display is created by firing electrons from the back of the tube to phosphors located towards the front of the display. Once the electrons hit the phosphors, they light up and are projected on the screen. The color you see on the screen is produced by a blend of red, blue, and green light, often referred to as RGB.
The stream of electrons is guiding by magnetic charges, which is why you may get interference with unshielded speakers or other magnetic devices that are placed close to a CRT monitor. Flat screen or LCD displays don't have this problem, since they don't require a magnetic charge. LCD monitors also don't use a tube, which is what enables them to be much thinner than CRT monitors. While CRT displays are still used by graphics professionals because of their vibrant and accurate color, LCD displays now nearly match the quality of CRT monitors. Therefore, flat screen displays are well on their way to replacing CRT monitors in both the consumer and professional markets.

CRT – Anatomy

Anatomy:

A CRT is essentially an oddly-shaped, sealed glass bottle with no air inside. It begins with a slim neck and tapers outward until it forms a large base. The base is the monitor's "screen" and is coated on the inside with a matrix of thousands of tiny phosphor dots. Phosphors are chemicals which emit light when excited by a stream of electrons: different phosphors emit different coloured light. Each dot consists of three blobs of colored phosphor: one red, one green, one blue. These groups of three phosphors make up what is known as a single pixel.

Click this Image and see the CRT Working principle animation

• In the "bottle neck" of the CRT is the electron gun, which is composed of a cathode, heat source and focusing elements. Color monitors have three separate electron guns, one for each phosphor colour. Images are created when electrons, fired from the electron guns, converge to strike their respective phosphor blobs.
• Convergence is the ability of the three electron beams to come together at a single spot on the surface of the CRT. Precise convergence is necessary as CRT displays work on the principal of additive coloration, whereby combinations of different intensities of red green and blue phosphors create the illusion of millions of colours.
• When each of the primary colors are added in equal amounts they will form a white spot, while the absence of any colour creates a black spot. Misconvergence shows up as shadows which appear around text and graphic images.
• The electron gun radiates electrons when the heater is hot enough to liberate electrons (negatively charged) from the cathode. In order for the electrons to reach the phosphor, they have first to pass through the monitor's focusing elements.
• While the radiated electron beam will be circular in the middle of the screen, it has a tendency to become elliptical as it spreads its outer areas, creating a distorted image in a process referred to as astigmatism. The focusing elements are set up in such a way as to initially focus the electron flow into a very thin beam and then - having corrected for astigmatism - in a specific direction.
• This is how the electron beam lights up a specific phosphor dot, the electrons being drawn toward the phosphor dots by a powerful, positively charged anode, located near the screen.

• The deflection yoke around the neck of the CRT creates a magnetic field which controls the direction of the electron beams, guiding them to strike the proper position on the screen.
• This starts in the top left corner (as viewed from the front) and flashes on and off as it moves across the row, or "raster", from left to right.
• When it reaches the edge of the screen, it stops and moves down to the next line. Its motion from right to left is called horizontal retrace and is timed to coincide with the horizontal blanking interval so that the retrace lines will be invisible.
• The beam repeats this process until all lines on the screen are traced, at which point it moves from the bottom to the top of the screen - during the vertical retrace interval - ready to display the next screen image.
• Since the surface of a CRT is not truly spherical, the beams which have to travel to the centre of the display are foreshortened, while those that travel to the corners of the display are comparatively longer. This means that the period of time beams are subjected to magnetic deflection varies, according to their direction.
• To compensate, CRT's have a deflection circuit which dynamically varies the deflection current depending on the position that the electron beam should strike the CRT surface.
• Before the electron beam strikes the phosphor dots, it travels thorough a perforated sheet located directly in front of the phosphor.
• Originally known as a "shadow mask", these sheets are now available in a number of forms, designed to suit the various CRT tube technologies that have emerged over the years.

Number of important functions:

• They "mask" the electron beam, forming a smaller, more rounded point that can strike individual phosphor dots cleanly
• They filter out stray electrons, thereby minimizing "overspill" and ensuring that only the intended phosphors are hit
• By guiding the electrons to the correct phosphor colors, they permit independent control of brightness of the monitor's three primary colours.

When the beam impinges on the front of the screen, the energetic electrons collide with the phosphors that correlate to the pixels of the image that's to be created on the screen. When this happens each is illuminated, to a greater or lesser extent, and light is emitted in the color of the individual phosphor blobs. Their proximity causes the human eye to perceive the combination as a single colored pixel.

Дата добавления: 2015-10-28