ORASOFT TRAINING INSTITUTE

BCS 6th Semester Subject: Computer Graphics Handout # 7

Video Display Standards: Since there are many different ways to specify a '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.

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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 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.

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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 “” 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.

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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.

Super VGA (SVGA) and Other Standards Beyond VGA VGA was the last well-defined and universally accepted standard for video. After IBM faded from leading the PC world many companies came into the market and created new cards with more resolution and colour depths than standard VGA (but almost always, backwards compatible with VGA). Most video cards (and monitors for that matter) today advertise themselves as being Super VGA (SVGA). What does a card saying it is SVGA really mean? Unfortunately, it doesn't mean much of anything. SVGA refers collectively to any and all of a host of resolutions, colour modes and poorly accepted pseudo-standards that have been created to expand on the capabilities of VGA. Therefore, knowing that a card that supports "Super VGA" really tells you nothing at all. In the current world of multiple video standards you have to find out specifically what resolutions, colour depths and refresh rates each card supports. You must also make sure that the monitor you are using supports the modes your video card produces; here too "Super VGA compatible" on the monitor doesn't help you. To make matters more confusing, another term is sometimes used: Ultra VGA or UVGA. Like SVGA, this term really means nothing also. Some people like to refer to VGA as 640x480 resolution, SVGA as 800x600, and UVGA as 1024x768. This is overly simplistic however, and really is not something that you can rely upon. The proliferation of video chipsets and standards has created the reliance on software drivers that PC users have come to know so well. While Windows, for example, has a generic VGA driver that will work with almost every video card out there, using the higher resolution capabilities of your video card requires a specific driver written to work with your card. (The VESA standards have changed this somewhat, but not entirely). IBM did create several new video standards after VGA that expanded on its capabilities. Compared to VGA, these have received very limited acceptance in the market, mainly because they were implemented on cards that used IBM's proprietary Micro Channel Architecture (which received no acceptance in the market). You may hear these acronyms bandied about from time to time: 8514/A: This standard was actually introduced at the same time as standard VGA, and provides both higher resolution/colour modes and limited hardware acceleration capabilities as well. By modern standards 8514/A is still rather primitive: it supports 1024x768 graphics in 256 colours but only at 43.5 Hz (interlaced), or 640x480 at 60 Hz (non-interlaced).

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XGA: This acronym stands for Extended Graphics Array. XGA cards were used in later PS/2 models (compatible IBM Computers); they can do bus mastering on the MCA bus and use either 512 KB or 1 MB of VRAM (Video RAM). In the 1 MB configuration XGA supports 1,024 x 768 graphics in 256 colours, or 640x480 at high colour (16 bits per pixel). XGA-2: This graphics mode improves on XGA by extending 1,024x768 support to high colour, and also supporting higher refresh rates than XGA or 8514/A. The closest thing to a true SVGA standard is the set of standards created by VESA.

VESA Super VGA Standards: (This paragraph is optional please ignore for Shah Abdul Latif Exam) In an attempt to bring some order to the chaos of competing and incompatible Super VGA standards on the market, the Video Electronics Standards Association (VESA) has worked to establish new video interface standards. The intention of these standards is to once again provide a standardized application program interface between video hardware and application software. This would allow software developers to write their code to work with a single standard video model instead of having to write custom code to support the many different cards in use in the market today. Originally ignored by many vendors, VESA support is now becoming generally accepted as beneficial, and something that buyers look for when shopping for a video card. This is in part due to the growing number of programs (especially games) that require VESA SVGA compatibility in order to function at peak performance.

Accelerated Graphics Port (AGP): The need for increased bandwidth between the main processor and the video subsystem originally lead to the development of the local I/O bus on the PCs, starting with the VESA local bus and eventually leading to the popular PCI bus. Much as was the case with the ISA bus before it, traffic on the PCI bus is starting to become heavy on high-end PCs, with video, hard disk and peripheral data all competing for the same I/O bandwidth. To combat the eventual saturation of the PCI bus with video information, a new interface has been pioneered by Intel, designed specifically for the video subsystem. It is called the Accelerated Graphics Port or AGP. AGP was developed in response to the trend towards greater and greater performance requirements for video. As software evolves and computer use continues into previously unexplored areas such as 3D acceleration and full-motion video playback, both the processor and the video chipset need to process more and more information. The PCI bus is reaching its performance limits in these applications, especially with hard disks and other peripherals also in there fighting for the same bandwidth. Another issue has been the increasing demands for video memory. As 3D computing becomes more mainstream, much larger amounts of memory become required, not just for the screen image but also for doing the 3D calculations. This traditionally has meant putting more memory on the video card for doing this work. There are two problems with this: Cost: Video card memory is very expensive compared to regular system RAM.

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Limited Size: The amount of memory on the video card is limited: if you decide to put 6 MB on the card and you need 4 MB for the frame buffer, you have 2 MB left over for processing work and that's it (unless you do a hardware upgrade). It's not easy to expand this memory, and you can't use it for anything else if you don't need it for video processing. AGP gets around these problems by allowing the video processor to access the main system memory for doing its calculations. This is more efficient because this memory can be shared dynamically between the system processor and the video processor, depending on the needs of the system. The idea behind AGP is simple: create a faster, dedicated interface between the video chipset and the system processor. The interface is only between these two devices; this has three major advantages: it makes it easier to implement the port, makes it easier to increase AGP in speed, and makes it possible to put enhancements into the design that are specific to video. AGP is considered a port, and not a bus, because it only involves two devices (the processor and video card) and is not expandable. One of the great advantages of AGP is that it isolates the video subsystem from the rest of the PC so there isn't nearly as much contention over I/O bandwidth as there is with PCI. With the video card removed from the PCI bus, other PCI devices will also benefit from improved bandwidth. AGP is a new technology and was just introduced to the market in the third quarter of 1997. The first support for this new technology will be from Intel's 440LX Pentium II chipset. More information on AGP will be forthcoming as it becomes more mainstream and is seen more in the general computing market. Interestingly, one of Intel's goals with AGP was supposed to be to make high-end video more affordable without requiring sophisticated 3D video cards. If this is the case, it really makes me wonder why they are only making AGP available for their high-end, very expensive Pentium II processor line. Originally, AGP was rumoured to be a feature on the 430TX Pentium socket 7 chipset, but it did not materialize. Via and other companies are carrying the flag for future socket 7 chipset development now that Intel has dropped it, and several non-Intel AGP-capable chipsets will be entering the market in 1998. For still more information, please check out class lectures. Just a minute, its text and graphics support same as SVGA but also its support the high resolution, more than 2048 x 1537 and 32 bits colours information (that are maximum colours now a days).

3D Video Cards The new market for 3D accelerators and 3D acceleration features has spawned a large crop of 3D video cards with varying capabilities. There are several different approaches that are taken to providing a system with 3D capabilities. While the available cards and technologies are changing rapidly, you will generally find that the cards on the market break out as follows: 2D Only (Conventional) Cards: These are regular video cards that do not incorporate any special 3D acceleration functions. Usually these are either older cards, or newer cards that are optimised for 2D performance. When using a card of this type, it is necessary to pair it with a 3D card to obtain 3D acceleration functions.

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Dedicated 3D Cards: These are accelerators that are designed only for 3D hardware functions. Since they do not do conventional 2D acceleration, they need to work with a 2D card in most cases to deliver good 2D+3D performance. Most of the higher-quality 3D cards are of this variety. They typically use a feature connector to connect directly to the 2D card. This lets the 3D card perform its acceleration functions to provide a video stream without requiring its own RAMDAC or bus control logic. Combination 2D+3D Cards: In an effort to tackle the cost problem of using an additional, separate card for 3D acceleration, many companies are developing cards that perform both 2D and 3D functions. For many users, this is a good, cost-effective compromise. Most of these cards provide from moderate to good 2D performance, and support for some to most of the 3D acceleration features. However, like most compromises, these cards typically don't provide the level of performance or feature support that dedicated 3D cards do. It is important to research these cards well, since many of them support only a small subset of the 3D acceleration features found on 3D cards.

3D Video Acceleration: Because the computer screen is two-dimensional, everything that the PC displays must be two-dimensional as well. In years past, this has meant that programmers and users have not generally tried to work with three-dimensional objects on PCs. In order to work with 3D objects, it is necessary for them to be converted to 2D images. This requires special processing and a lot of computation power that until recently was not available in the PC world. It is not as simple as say, a television camera that converts a 3D image to 2D automatically. These cards are process in experiment and use for some special type of task like some games but do not support each computer applications for 3D but you can use as 2D.

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