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DISPLAY DEVICES

Display devices are the crts and other displays that are part of computer terminals, computer consoles, and microcomputers. They are designed to project, show, exhibit, or display softcopy information (alphanumerics or graphic symbology).

The information displayed on a display device screen is not permanent. That is where the term soft-copy comes from. The information is available for viewing only as long as it is on the display screen. Two types of display devices used with personal/microcomputers are the raster scan crt's and the flat panel displays.

Raster Scan Crts

Raster scan crts (tv scan video monitors or display monitors) are used extensively in the display of alphanumeric data and graphics. They are used primarily in nontactical display applications such as SNAP II user terminals and desktop computers.

The raster is a series of horizontal lines crossing the face of the crt screen (fig. 2-28). Each horizontal line is made up of one trace of the electron beam from left to right. The raster starts at the top left corner of the crt screen. As each horizontal line is completed, the blanked electron beam is rapidly returned or retraced to the left of the screen.

Figure 2-28. - Raster or TV scan.

Vertical deflection moves the beam down, and the horizontal sweep repeats. When the vertical sweep reaches the bottom line of the raster, a vertical blanked retrace returns the sweep to the starting position of the raster, and the process is repeated.

Each completed raster scan is referred to as a field; two fields make up a frame. The display rate of fields and frames determines the amount of flicker in the display that is perceived by the human eye. Each field is made up of approximately 525 horizontal lines. The actual number of horizontal lines varies from device to device. A frame consists of the interlaced lines of two fields. The horizontal lines of the two fields are interlaced to smooth out the display. A display rate of 30 frames per second produces a smooth, flicker-free raster and corresponding display on the screen.

PICTURE ELEMENTS. - The actual display of data results from the use of picture elements. A picture element is a variable dot of light derived from video signals input to the display monitor. The picture elements, often called pixels or pels, are contained in the horizontal scan lines crossing the face of the crt screen. The horizontal and vertical sweeps are continuous and repetitive in nature.

Pictures with alphanumeric characters and graphics can be created and displayed by varying the intensity or brightness of the picture element dots. This is done in conjunction with the phosphor coating on the face of the crt.

The number of picture elements in each horizontal line varies from device to device. The actual number of picture elements is dependent on the frequency bandwidth of the video monitor, the number of characters to be displayed on a line, and the physical size of the screen.

Each picture element is addressable by a row and column address. Picture elements are numbered from left to right on each horizontal line (column number). Each horizontal line has a row number. Picture elements, at a minimum, will have off (blanked) or on (full intensity) states. Many display devices have the capability to display picture elements at varying degrees of intensity for the display of graphics.

Characters are assembled on the screen in much the same way as a dot-matrix print head prints a character. It takes several horizontal lines and picture elements on each line to create a character. Figure 2-29 shows the generation of the character A, 7 picture elements wide and 9 horizontal lines high. The character is built using what is, in effect, a 7 by 9 dot matrix. The picture elements used to build the character would be at full intensity; the remaining picture elements in the matrix would be blanked. If dark characters on a lighted screen were desired, then the character picture elements would be blanked and the remainder displayed at full intensity.

Figure 2-29. - A 7 by 9 picture element character.

Approximately 640 picture elements per horizontal line are required for the display of an 80 character line. Therefore, you can expect 140,000 picture elements on a raster scan display screen (80 alphanumeric characters per line and 25 lines).

HORIZONTAL AND VERTICAL RESOLUTION. - Horizontal resolution is defined in terms of the number of picture elements that can be displayed on the horizontal line without overlapping or running into each other. It is often stated in terms of lines of resolution. In other words, a monitor with a horizontal resolution of 1,000 lines can display 1,000 vertical lines using 1,000 picture elements per line.

Vertical resolution depends on the number of horizontal scan lines used by the particular display raster. Generally, the greater the number of scan lines, the easier it is to resolve a horizontal line of display. This characteristic remains true up to a point, called the merge point, where the variation between the lines cannot be detected by the human eye.

DISPLAYING DATA ON RASTER SCAN SCREENS. - Raster scan displays are repetitive in nature. The raster frame is displayed approximately 30 times a second.

The basic video monitor does nothing more than display the video signals it receives. If no video signals are received, then all the picture elements remain blanked, and the screen is blank in each frame. For data to be displayed accurately, each and every frame must blank and unblank the same picture elements.

The digital logic that drives video monitors is designed to take advantage of the repetitive nature of frames. There can only be a fixed number of picture elements on the screen of a display; therefore, the contents of the display screen are organized into a data unit called a page.

The page contains the status of every picture element on the display screen. The page is usually stored in some form of random-access memory, RAM chips being the most common. The contents of page memory, or, as it is sometimes called, video memory, are continually scanned by the video generation logic and used to develop the video signals for the picture element display. The picture element locations in page memory are read in time to develop the video signals for the picture element display on the horizontal lines.

If the display is to be changed, the contents of page memory must be changed. The display on the screen changes as new data is stored in page memory. Two addressing methods are used with page memory.

Unformatted Displays. - Displays that reference page memory by picture element address are called unformatted or fully populated displays. These displays are more commonly used for graphics rather than alphanumeric characters.

Formatted Displays. - Often displays are organized by character position and line number. These displays are known as formatted displays. This display method is used with devices displaying alphanumeric characters only or those with an alternate graphic capability.

The video generation logic of these types of displays scans the entire page memory, as before, to generate the display picture elements. The difference is in the way the new data is written into the page memory. Individual picture element addresses are not used. Character addresses are used to reference page memory.

The screen is organized into character lines. Each line is made up of a fixed number of character positions or columns. A fixed number of character lines can be displayed. A common arrangement found on display screens is twenty five 80-character lines, or 2,000 characters.

The character set that can be displayed on a device's formatted screen is stored in ROMs or PROMS. That is, the dot-matrix (picture element) patterns for each individual character to be displayed are stored. Different character sets may be displayed by simply replacing the appropriate ROM or PROM chips with new chips containing different character patterns.

Upon receipt of a character code and a row and column address, the device logic reads the picture element pattern (dot matrix) from the ROM and writes the pattern into the appropriate character position in the page memory. The desired character is then displayed at the correct position. Other display devices store the codes in page memory and convert the codes to picture element dots when scanning memory to refresh or redisplay the characters on the screen. The use of formatted displays greatly simplifies the programming requirements for the display of alphanumeric data.

Flat Panel Displays

A number of display methods are in use that are designed to reduce the depth of the crt display caused by the length of the tube. These devices are collectively known as flat panel displays. Three types of flat panel displays commonly in use with computer systems are liquid crystal displays (LCDs), gas plasma displays (GPDs), and electroluminescent displays (ELDs).

The screens of these flat panel displays are made up of pairs of electrodes. Each pair of electrodes is used to generate one picture element.

The liquid crystal display differs from the gas plasma and electroluminescent displays in that it does not generate its own light for the picture elements. The LCD requires an external light source, often called a backlight, for computer applications. The liquid crystal material between the charged electrodes becomes translucent when voltage is applied and allows the backlight to shine through as a picture element.

In the gas plasma and electroluminescent displays, the picture element light is generated by ionizing a gas (neon or neon argon) between the charged electrodes (gas plasma display) or by stimulating a luminescent material in the same manner (electroluminescent display). In either case, the picture element only emits light when the electrodes have voltage applied to them.

One of the advantages of flat panel displays is that smaller voltages are required for their operation than for a crt. Gas plasma displays use approximately 200 volts to charge the electrodes, and electroluminescent displays require only 20 volts.

The picture elements in these displays are addressed by the row and column method. Displays with as many as 737,280 picture elements (960 rows by 768 columns) have been developed.

The picture elements on flat panel displays are not lighted continually. This would require a large amount of power and generate excessive heat. A sequential scan similar to a crt raster is used. Once again a page memory is required. The picture element electrodes are on and off as the scan sequentially addresses page memory.

Those picture elements that are to display a dot are momentarily turned on and off starting with the first picture element in the top row, or line, and ending with the last picture element on the bottom row. The picture elements are turned on and off at a high enough frequency that the human eye cannot detect the flicker of the off-on-off cycle.

The sequential scan used to light the picture elements is continuous and repetitive. Once again, the page memory must be changed to change the display. Flat panel displays may be formatted or unformatted in the same manner as crt displays.

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Q.42 What is the primary purpose of a keyboard? answer.gif (214 bytes)
Q.43 Raster scan or tv scan video monitors are used extensively for what purpose? answer.gif (214 bytes)
Q.44 How many fields make up a frame? answer.gif (214 bytes)
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Q.46 What are picture elements often called? answer.gif (214 bytes)
Q.47 Vertical resolution depends on what? answer.gif (214 bytes)
Q.48 Flat panel displays are designed to reduce what problem of a crt display? answer.gif (214 bytes)
Q.49 What does the liquid crystal display require for computer applications?answer.gif (214 bytes)




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