75 Glossary Terms
Description
This article is from the comp.sys.ibm.pc.hardware.video Frequently Asked Questions, by Michael Scott with numerous contributions by others. (v1.0).
75 Glossary Terms
8514/a IBM video graphics standard. Supports pixel addressabilities up to 1024x768 and 256 colours. It is _not_ a superset of VGA.
addressability (pixel addressability)
This refers to the number of pixels that a video controller can display. It is quoted as the (# horizontal pixels) by the (# vertical pixels). Common PC pixel addressabilities include: 320x200, 640x480, 800x600, 1024x768, 1280x1024 & 1600x1200
aperture grille
An array of vertical wires which act in a similar manner as a shadow mask. Their basic purpose is to permit the correct electron beam to strike its corresponding colour phosphor only. This results in crisp pixel definition, and superior colour brightness than is realized with more traditional designs. The aperture grille was first used by Sony in their Trinitron design.
AT bus
Advanced Technology (IBM) bus. The standard PC compatible peripheral bus to which add-in cards like video, i/o, internal modems, sound are added. Also called the ISA bus, it runs at a maximum of 8.33 MHz and has a 16-bit wide data path.
bandwidth
Also called video bandwidth. This is a measure of how much gross throughput a monitor can handle (in MHz). Bandwidth at a given pixel addressability is a function of the vertical refresh rate and monitor timing. see 'How do I calculate the minimum bandwidth required for a monitor?"
BIOS
Basic Input Output System. The video BIOS basically tells the computer how to talk to the video subsystem at boot time. The video BIOS calls are used by DOS for VGA (and SVGA) modes.
BITBLT
A VGA video operation which copies an array of values to a rectangular region in video RAM.
bit planes
This is the number of bits which are available to store colour information for each pixel displayed. The number of colours which can be displayed is calculated as two to the exponent 'n', where n is the number of bit planes. i.e. 4 bit equals 16 colours, 8 bit equals 256 colours and 24 bit equals 16.7 million colours. see "How does colour depth (bit planes) relate to the number of colours?"
colour depth
Refers to the amount of memory (and therefore number of simultaneously displayable colours) available to store colour information for each pixel. see 'bit planes'.
CPU
Central Processing Unit. This is the heart and brains of your computer. It is responsible for executing code, moving data, calculations, etc. For PC's, this chip is a member of the X86 family including 8088 through 80486, Pentium and Nextgen.
CRT
Cathode Ray Tube. Basically the same technology as is in modern television sets. One or more beams of electrons are focused onto phosphor, causing it to glow. The phosphor is arranged into an array (usually close to rectilinear), and the electron beam scans the phosphor on the screen (similar to how you read text - left to right and top to bottom), usually 60+ times per second.
degauss
Magnetic interference caused by a change in the position of a monitor in relation to the earth's magnetic field or the presence of an artificial magnetic field can cause discolour- ation. To correct this, all colour monitors automatically degauss at power-on and some also have a manual degaussing button. This allows the monitor to compensate for the change in the magnetic field by realigning the electron guns. In some low cost monitors without degauss buttons it is necessary to leave the power turned off for at least 20 minutes in order to get maximum degaussing.
display
Usually used to indicate the monitor or flat-panel device used as the primary visual interface.
display adapter
Usually this is the same as the video card, but some mother- boards have built-in video, and so don't require an additional card. The display adapter contains video memory which stores what is displayed on the computer's monitor. They have a wide range of features, from a basic frame buffer, to advanced 3D geometric rendering engines.
dot clock
Technically, this refers to the digital clock signal that transfers data into the video card's digital to analog converter. However, it has also become a measure of the maximum gross data throughput of a monitor. It is measured in MHz, and indirectly determines the maximum pixel addressability and vertical refresh rate that a monitor can handle. See "What do those monitor specifications mean?"
dot pitch
The distance between a phosphor dot of one phosphor triad to its closest diagonal neighbour of the same colour on a monitor. Expressed in mm - i.e. .28 dot pitch means .28 mm between triads. A smaller value indicates that the phosphor dots are more closely spaced, and that the resulting image displayed will be crisper.
dot stripe
see 'stripe pitch'
DRAM
Dynamic Random Access Memory. The vast majority of system RAM in modern computers is of this type because of it's low cost. It is also the most common type of RAM used for video cards. A specialized type of DRAM called VRAM is also used in higher end video cards. see "What is the difference between VRAM and DRAM?"
EGA
Enhanced Graphics Adapter (IBM). Precursor to VGA, all EGA video modes are supported in VGA, though register compatibility is not 100%. EGA cards generate a digital signal, and thus will not drive a modern, analog monitor.
EISA
Extended Industry Standard Architecture. This 32-bit bus standard was created primarily to compete with IBM's MCA bus. It runs at speeds of up to 8.33 MHz. EISA is a dying standard.
graphics controller
This is a generic term to describe the video hardware in a computer. Sometimes it is built onto the motherboard, but usually it is a separate daughter card that fits into one of the expansion bus slots. The interface between the graphics controller and the main processor is one of the ISA, EISA, MCA, VLB or PCI buses. The graphics controller is responsible for generating the video signal that is sent to the monitor. Typically a graphics controller contains a graphics coprocessor which may be a graphics accelerator, video RAM and a RAMDAC.
graphics coprocessor
A secondary processor dedicated to performing video display tasks.
graphics accelerator
This is a highly misused and now almost meaningless term. For the purposes of this FAQ, a graphics accelerator is a coprocessor which is capable of specific graphics operation, independent of the main system CPU. See the section "How does a video accelerator work, and will one help me?"
GUI
Graphical User Interface. In contrast to text-based interfaces like DOS or UNIX, GUI's provide more flexibility in terms of colour, pixel addressability and types of objects that can be displayed. Examples of GUI's include X-Windows, Microsoft Windows 3.1, OS/2.
Hercules
A monochrome display adapter which is MDA compatible and provides graphics modes up to 720x348
horizontal refresh
see horizontal scan rate
horizontal scan rate (horizontal frequency)
The frequency, expressed in kHz (thousands of times per second), at which the horizontal deflection circuit operates. This roughly translates to the number of scanlines displayed on a monitor in one second.
interlaced
Standard NTSC television signals are interlaced, meaning that each video frame is divided into two separate fields of alternating scanlines. The resulting fields are displayed sequentially, such that what was originally a 30 frame per second (fps) refresh becomes 60 Hz at half the vertical pixel addressability. Thin horizontal lines will appear to flicker on an interlaced display since their effective refresh rate is only 30 Hz.
ISA Industry Standard Architecture. This is a 16-bit bus standard which runs at speeds of up to 8.33 MHz. The vast majority of peripheral add-in cards like modems, sound cards, cdrom interfaces and other low-bandwidth applications are still ISA based. VLB and PCI provide higher bandwidth for video and disk I/O operations.
Look-up Table (LUT)
At higher pixel addressabilites, most graphics controllers can not simultaneously display as many colours as they are capable of generating. Because of video card memory limitations, only a subset of all possible colours can be displayed at one time. A look-up table stores the mapping information which determines which subset of all possible colours are available at any given time.
MDA
Monochrome Display Adapter (IBM)
monitor
Usually a CRT-based device which directs an electron beam onto coloured phosphor, causing it to glow. Monitors use the same basic technology as televisions, but are capable of much higher pixel addressabilities and resolutions.
motherboard
The main component of the computer, which contains the CPU (brain), main memory slots, keyboard connector and expansion bus slots, among other possible components.
non-interlaced
This means that an entire frame is displayed with each screen refresh. Non-interlaced displays produce a more pleasing screen image since thin horizontal lines don't flicker with each screen refresh.
OEM
Original Equipment Manufacturer. Often manufacturers will produce versions of their products in large quantities for other companies who either stick their name on them or use them as components for their systems. OEM products often make it to the retail sales arena where they are sold at lower prices. An OEM version of a card _may not_ be equivalent to the retail version.
PCI
Peripheral Components Interconnect. This is basically the Pentium equivalent to the VLB, but with improvements. It is a 64-bit standard, but is currently only implemented as 32 bits - look for 64 bit PCI in the future. It performs asynchronously to the main CPU, meaning that the PCI bus operates at 33 MHz regardless of the CPU clock. It also allows more than two devices on the bus, unlike VLB.
phosphor triad (dot triad)
This is the smallest dot that can theoretically be resolved on a colour monitor and consists of three phosphor dots - one each of red, green and blue. When struck with the electron beam, these dots glow producing a bright spot on the screen. Practically, 1.2 or more dot triads comprise each pixel on the screen, although the pixel addressability of some monitors is greater than their resolution, and in this case a pixel can be smaller than a dot triad. The result in this case is that small objects may not be resolvable.
pixel
This is the smallest addressable display unit available at a given video addressability. There is no physical thing on a display that can be called a pixel. Pixels exist only in the graphics controller bitmap. The screen image in the bitmap is composed of an array of pixels, arranged in a rectilinear fashion, with the X axis running horizontally, perpendicular to the Y axis. A pixel consists of intensity only (in grayscale monitors) or colour and intensity information (red, green & blue in colour). While a pixel usually corresponds to a square or rectangular area, it is displayed as a number of spots on a CRT. One pixel usually consists of 1.2 or more dot triads. Flat panel displays are a special case where individual pixels correspond directly to a picture element on the display.
pixel addressability
see addressability
RAM
Random Access Memory. RAM comes in different types, including DRAM (Dynamic RAM) and VRAM (Video RAM) among others. DRAM is used as main system memory, while both DRAM and VRAM can be used on graphics cards.
RAMDAC
Random Access Memory Digital-to-Analog Converter. This is part of the graphics card which converts the digital intensity values for each of the red, green and blue guns (usually an 8-bit number) to analog voltages which are sent to the monitor. A RAMDAC can use its RAM to store look-up table (LUT) information.
refresh rate
When referring to monitors, the number of times that the video card refreshes the entire screen in one second. Expressed in Hz (Hertz).
resolution
The most common misinterpretation of this term is that it is the same as pixel addressability. In fact, resolution is more closely related to dot pitch, since it is a limitation of the monitor rather than of the graphics controller. The resolution limits how small an object a monitor is able to display.
RGB
Red, Green and Blue. By varying the intensity of each of these colours in a single pixel, the human eye can be fooled into seeing a wide range of colours. For example, a combination of red and green appears as yellow, even though no light with a yellow wavelength is emanating from the screen. This works because the optical system integrates the photons striking a region on the retina, and the combined impulses from green and red sensitive cones are seen as yellow.
scanline
The movement of a monitor's electron gun from one side of the screen to the other results in the appearance of a horizontal line of varying intensity and colour. Typically, 200 to 1200 horizontal scan lines (lined-up vertically on top of each other) make-up the image you see on your display.
shadow mask
This is usually an invar mask which acts to block the electron beam from striking the wrong phosphors in a CRT. The beam passes through holes in the mask to strike the correct phosphor while shadowing neighbouring phosphor. i.e. it prevents a beam intended to strike a red phosphor from striking a neighbouring green phosphor by causing an electron shadow over the green dot.
stripe pitch
This is similar to dot pitch, but applicable to Sony Trinitron and similar tubes which use fine vertical wires (aperture grille) to separate phosphors. Dot stripe is measured as the distance between the vertical stripes that result. Measures of dot pitch and dot stripe are _not_ directly comparable.
Trinitron
A common but proprietary picture tube design developed by Sony. Uses fine vertical wires instead of the more traditional shadow mask. see "Why does my monitor have 1/2/3 faint horizontal lines on it?"
vertical refresh rate (vertical scan rate)
The number of fields (on an interlaced display) or frames (on a non-interlaced display) that are displayed in one second. A field or frame covers the entire screen area. This is measured in Hz (cycles per second). It is limited by the monitor and video card (pixel addressabilities and colour depths). Modern monitors and video cards provide refresh rates of 60Hz+.
VESA
Video Electronics Standards Association. This group has produced standards for the VLB (Vesa Local Bus), VESA SVGA video modes and standards for minimum screen refresh rates at various pixel addressabilities.
VGA
Video Graphics Array (IBM). Supports pixel addressabilities of up to 640x480x16. This is the de facto video standard and consists of a number of video modes. It is still heavily supported by DOS-based applications and games. see "What is VGA, and how does it work?"
video card
A dedicated piece of hardware which performs graphics operations. Also called a display adapter. Consists of microchips and other electronic components mounted on a pc-board which connects into a slot (ISA, EISA, MCA, VLB or PCI) on the motherboard.
viewable area
Typically monitors are advertised by the diagonal size of the picture tube in inches. Common sizes are 14", 15", 17", 20"+. However, the amount of the screen that can be seen is usually less. For example, most 17" monitors have only a 15.5" diagonal area used for display, in part because the actual phosphor area is only about 16" due to the glass thickness. This is partially due to the fact that the monitor's case covers the edge of the tube, and partially because monitor manufacturers want to make you think you're getting a larger display than you are. see "Size" under "What do those monitor specifications mean?"
VLB
VESA Local Bus. This 32 bit bus was originally designed to provide higher bandwidth for video cards than is available with the ISA bus. It is optimized for the 486 CPU and can run at speeds up to 40 MHz with one card on the bus, or up to 33 MHz with two cards on the bus. The speed of the VLB is dependent, and runs synchronously with, the main system CPU. Some VLB cards are not designed to run faster than 33 MHz, though some mother- boards will clock the bus at up to 50 MHz! VLB 2.0 has been written, but has not been implemented on many 486 motherboards. VRAM
Video Random Access Memory. A specialized type of DRAM, VRAM is dual-ported, meaning it can be read from and written to at the same time. see "What is the difference between VRAM and DRAM?"
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