There are many types of output devices. In the sections below, we explore two common output devices: the laser printer and the video display.
LASER PRINTERS
A laser printer consists of a charged drum in which a laser discharges selected areas according to a bit mapped representation of a page to be printed. As the drum advances for each scan line, the charged areas pick up electrostatically sensitive toner powder. The drum continues to advance, and the toner is transferred to the paper, which is heated to fix the toner on the page. The drum is cleaned of any residual toner and the process repeats for the next page. A schematic diagram of the process is shown in Figure 8-30.
Since the toner is a form of plastic, rather than ink, it is not absorbed into the page but is melted onto the surface. For this reason, a folded sheet of laser printed paper will display cracks in the toner along the fold, and the toner is sometimes unintentionally transferred to other materials if exposed to heat or pressure (as from a stack of books).
Whereas older printers could print only ASCII characters, or occasionally crude graphics, the laser printer is capable of printing arbitrary graphical information. Several languages have been developed for communicating information from computer to printer. One of the most common is the Adobe PostScript language. PostScript is a stack-based language that is capable of describing objects as diverse as ASCII characters, high level shapes such as circles and rectangles, and low-level bit maps. It can be used to describe foreground and background colors, and colors with which to fill objects.
VIDEO DISPLAYS
A video display, or monitor, consists of a luminescent display device such as a cathode ray tube (CRT) or a liquid crystal panel, and controlling circuitry. In a CRT, vertical and horizontal deflection plates steer an electron beam that sweeps the display screen in raster fashion (one line at a time, from left to right, starting at the top).
A configuration for a CRT is shown in Figure 8-31. An electron gun generates a
stream of electrons that is imaged onto a phosphor coated screen at positions controlled by voltages on the vertical and horizontal deflection plates. Electrons are negatively charged, and so a positive voltage on the grid accelerates electrons toward the screen and a negative voltage repels electrons away from the screen. The color produced on the screen is determined by the characteristics of the phosphor. For a color CRT, there are usually three different phosphor types (red, green, and blue) that are interleaved in a regular pattern, and three guns, which produce three beams that are simultaneously deflected on the screen.
A simple display controller for a CRT is shown in Figure 8-32. The writing of
information on the screen is controlled by the “dot clock,” which generates a continuous stream of alternating 1’s and 0’s at a rate that corresponds to the update time for a single spot on the screen. A single spot is called a picture element, or pixel. The display controller in Figure 8-32 is for a screen that is 640 pixels wide by 480 pixels high. A column counter is incremented from 0 to 639 for each row, then repeats, and a row counter is incremented from 0 to 479, which then repeats. The row and column addresses index into the frame buffer, or “display RAM” that holds the bit patterns corresponding to the image that is to be displayed on the screen. The contents of the frame buffer are transferred to the screen from 30 to 100 times per second. This technique of mapping a RAM area to the screen is referred to as memory-mapped video. Each pixel on the screen may be represented by from 1 to 12 or more bits in the frame buffer. When there is only a single bit per pixel, the pixel can only be on or off, black or white; multiple bits per pixel allow a pixel to have varying colors, or shades of gray.
Each pixel in the display controller of Figure 8-32 is represented by eight bits in the frame buffer, which means that one out of 28 = 256 different intensities can be used for each pixel. In a simple configuration the eight bits can be partitioned for the red, green, and blue (R, G, and B) inputs to the CRT as three bits for red, three bits for green, and two bits for blue. An alternative is to pass the eight-bit pixel value to a color lookup table (LUT) in which the eight-bit value is translated into 256 different 24-bit colors. Eight bits of the 24-bit output are then used for each of the red, green, and blue guns. A total of 224 different colors can be displayed, but only 28 of the 224 can be displayed at any one time since the LUT has only 28 entries. The LUT can be reloaded as necessary to select differ- ent subsets of the 224 colors. For example, in order to display a gray scale image (no color), we must have R=G=B and so a ramp from 0 to 255 is stored for each of the red, green, and blue guns.
The human eye is relatively slow when compared with the speed of an electronic device, and cannot perceive a break in motion that happens at a rate of about 25 Hz or greater. A computer screen only needs to be updated 25 or 30 times a second in order for an observer to perceive a continuous image. Whereas video monitors for computer applications can have any scan rate that the designer of the monitor and video interface card wish, in television applications the scan rate must be standardized. In Europe, a rate of 25 Hz is used for standard television, and a rate of 30 Hz is used in North America. The phosphor types used in the screens do not have a long persistence, and so scan lines are updated alternately in order to reduce flicker. The screen is thus updated at a 50 Hz rate in Europe and at a 60 Hz rate in North America, but only alternating lines are updated on each sweep. For high resolution graphics, the entire screen may be updated at a 50 Hz or 60 Hz rate, rather than just the alternating lines. Many observers believe that the European choice of 50 Hz was a bad one, because many viewers can detect the 50 Hz as a flicker in dim lighting or when viewed at the periphery of vision.
On the other hand, the Europeans point to the United States NTSC video trans- mission standard as being inferior to their PAL system, referring to the NTSC system as standing for “Never The Same Color,” because of its poorer ability to maintain consistent color from frame to frame.
The data rates between computer and video monitor can be quite high. Consider that a 24-bit per pixel monitor with 1024 ´ 768 pixel resolution and a refresh rate of 60 Hz will requires a bandwidth (that is, the amount of information that can be carried over a given period of time) of 3 bytes/pixel ´ (1024 ´ 768) pixels ´ 60 Hz, or roughly 140 MB per second. Fortunately, the hardware described above maps the frame buffer onto the screen without CPU intervention, but it is still up to the CPU to output pixels to the frame buffer when the image on the screen changes.
