ORASOFT TRAINING INSTITUTE BCS 6th Semester Subject: Computer Graphics Handout # 7 Video Display Standards: Since there are many different ways to specify a video card's capabilities, and so many potential resolutions, colour modes, etc., video standards were established in the early years of the PC, primarily by IBM. The intention of these video standards is to define agreed upon resolutions, colours, refresh modes, etc., to make it easier for the manufacturers of PCs, monitors, and software to ensure that their products work together. In recent years, IBM's fall from dominance has left the video industry without any clear leader to set standards. This, combined with the desire by various manufacturers to develop newer and faster cards, has left the current market with a plethora of different standards. The Video Electronics Standards Association (VESA) was formed to define new standards and has had some success in creating widely accepted new standards. This section takes a look at standards in use in the video industry. In the beginning, there was one, one model of IBM Personal Computer and one display type. You had one choice of screen colour, green, and no options. Your screen showed text or crude block graphics, but that was all. Monochrome Display Adapter: In that all of the other IBM video standards have become know by their initials, the monochrome Display Adapter has earned the nickname MDA mostly by default even though its official name is the Monochrome Display and Parallel Printer Adapter. As with most lengthy compound names, the MDA’s epithet is quit descriptive. In “monochrome” name mono mean the one and chrome mean colour. The “display adapter” part of the name is a functional description. This board adapts the signal on the bus into a form that can be digested by a video system. Technically, the MDA is a character-mapped system with no provision for graphics other than the IBM and, until recently, it was the best for text processing because it yielded up the sharpest character of any pre-PS/2 display system (Monitor). MDA Dot Box: IBM set the character box for the MDA at 9x14 pixels with a typical character using a 7x9 matrix in the box. The extra dots space individual lines apart for greater readability, something that’s most appreciated when it’s not available. To put this character box on the screen in the default arrangement used by most VDTs (Video Display Terminal), 80 columns and 25 rows, requires 720 pixels horizontally and 350 vertically, a total of 252,000 dots on every screen. Developed & compiled by Abdul Hafeez. Page 1 of 7 6845 Video Controller: The essence of the MDA is the 6845-video controller and four kilobytes of dual ported static RAM used for holding character-mapping video information. That memory is sufficient only for a single video page. The 6845 is a completely programmable device that’s controlled through a series of registers. Cursor: The flashing cursor on the screen is created by the 6845-video controller. Its flashing rate is set by system hardware and cannot be changed. However, flashing can be switched off and size of the cursor can be altered by loading values into the registers of the 6845-video chip. Colour Graphics Adapter (CGA): The first mainstream video card to support colour graphics on the PC was IBM's Colour Graphics Adapter (CGA) standard. The CGA supports several different modes; the highest quality text mode is 80x25 characters in 16 colours. Graphics modes range from monochrome at 640x200 (which is worse than the Hercules card) to 16 colours at 160x200. The card refreshes at 60 Hz. Note that the maximum resolution of CGA is actually significantly lower than MDA 640x200. These dots are accessible individually when in a graphics mode but in text each character was formed from a matrix that is 8x8, instead of the MDA's 9x14, resulting in much poorer text quality. CGA is obsolete, having been replaced by EGA. Flicker and Snow: One of the most notable, and to many people the most obnoxious characteristics of the CGA system is its tendency to flash the text display off and on when the display scrolls high-resolution text mode. This tendency is called flicker, (Different from the flicker of slow frame rate and interlace displays) and Other problem display the some dots on the screen because the frame buffer do not refresh completely and some pixels position update some delay of time. This type of display the pixels on entire screen called the snow, and is a direct result of the sorry, slow processing speed of the PC and XT. The flicker and snow problem reduce by the speed and refresh rate and flicker is reduce by use of Interlace techniques otherwise use high-speed processor which refresh the frame buffer. Hercules Graphics Card (HGC): One weakness of the original MDA display was that it did not support graphics of any kind. A company named Hercules Computer Technology, Inc., headed by Kevin Jenkins created in the early 80s an MDA-compatible video card that supported monochrome graphics in addition to the standard text modes. The Hercules card was actually a very widely accepted standard in the mid-80s; eventually Hercules clones even appeared on the market. Support for the card was included in popular software packages such as Lotus 1-2-3 to allow the display of graphs and charts on the computer screen. It has of course been replaced by later, colour, graphics adapters. In this cards character were formed in the same 9x14 pixels dot box on the screen with full screen resolution of 720x350 pixels, and 50 Hz frame rate. All attributes of the IBM MDA –underline, blink, and high-intensity and inverse video are supported by the HGC. Developed & compiled by Abdul Hafeez. Page 2 of 7 In graphics modes, the HGC slightly alters its on screen resolution to 720 x 348 and 16 colours support but previous card only support the mono colour. Enhanced Graphics Adapter (EGA): By 1984 the shortcomings of the CGA system had become obvious, if just from the skyrocketing white cane sales to the people who were using it regularly. After that new video adapter called the Enhanced Graphics Adapter or EGA popular in market. It increased on screen resolution, it brought the possibility of graphics to monochrome screens such as the venerable green IBM Personal computer display, and it added new BIOS routines that augmented and extended the existing ROM based video support built into the PC and XT. It support 640 x 350 pixels resolution, character were formed in dots boxes measuring 8 x 14 so character were formed from the 7 x 9 matrix. All previous IBM supported graphics modes were also included in the capabilities of the Display Adapter. It support the refresh rate 60 Hz and it is incompatible with the NTSC devices such as Television. (for NTSC term, please check the Class lectures.) and in colours it were gives the 64 colours palette, (mean each pixel have 6 bits for colours) Video Graphics Array (VGA): The VGA name is derived from a VLSI chip used in the implementation of the PS/2 line. Most of the circuitry of the EGA board (including emulation of Motorola’s 6845 video chip) was engineered into this one logical gate-array chip, which IBM dubbed with the “Video Graphics Array” name. The chip name quickly became the label for the entire system, probably because of the resemblance if its abbreviation to those of its predecessors, CGA and EGA. It incorporates all previous video modes and extends them into new, more colourful, higher resolution territory. Of the graphics modes, the sharpest bit-mapped colour images made by the system achieve a resolution of 640 x 480 pixels while displaying 16 simultaneous colours selectable from a palette of 256K. Text resolution under the VGA standard is even sharper than others graphics modes. The spec calls for 720 x 400 pixels in either 16 colours or shades of grey in monochrome. The characters were formed from a 9 x 16 matrix of on screen dots, two dots taller than MDA and a dot wider than EGA. Also it support the other text mode in which allow the 30 rows of text on the screen. In graphics mode it support the 16 colours to 64 colours and after some duration it support the 128 to 262 colours support by increasing the video memory. Memory Controller Gate Array (MCGA): IBM used that card in some models 25 and 30 by the some different problems. For lack of better name, this system has earned the Label Memory Controller Gate Array or MCGA. In NCGA text mode support the 40 rows and 80 columns and characters were formed in a 8 x 16 dot box and graphics mode had the 640 x 480 resolution with the 256 colours. Developed & compiled by Abdul Hafeez. Page 3 of 7 SVGA: Short for Super VGA, a set of graphics standards designed to offer greater resolution than VGA. SVGA supports 800 x 600 resolution, or 480,000 pixels. And now a days its resolution are 800 x 600 and 1024 x 768 and more than. The SVGA standard supports a palette of 16 million colours, but the number of colours that can be displayed simultaneously is limited by the amount of video memory installed in a system. One SVGA system might display only 256 simultaneous colours while another displays the entire palette of 16 million colours. The SVGA standards are developed by a consortium of monitor and graphics manufacturers called VESA.