Phil Storrs PC Hardware book

The PC Video Acronyms

Scan Frequencies

PC history spans an alphabet soup of Video Standards with different resolutions, colours, and capabilities. They're less important now that VGA has become the lowest common denominator, but we should all understand what has gone before.

The original IBM PC's Monochrome Display Adapter provided an 80-column, 25-line, character only display. The Hercules Graphics Card (HGC or Mono Graphics Adapter, MGA )added support for 720 by 350-pixel graphics to the original IBM MDA standards.

The first Colour Graphics Adapter standard, limited to two colour Graphics at a crude 640 by 200 resolution and four colour graphics at 320 by 200. In Character Modes, CGA could provide 16 foreground colours and eight background colours in Character Modes.

IBM's first attempt at better graphics. The Enhanced Graphics Adapter standard in it's earlyest form, provided 16 colours at 640 by 350 resolution. The EGA Standard was enhanced with up to 256 Kbytes of Video RAM and could provide up to of 64 colours, depending on the amount of Video RAM and the resolution required.

Introduced on the first IBM PS/2 computers in 1987, the Video Graphics Array in it's Generic form, offers a 720 by 400 pixel character mode and two graphics modes, 16 colours at 640 by 480 resolution and 256 colours at 320 by 200. This resolution is known as MCGA or Multi-Colour Graphics Array mode and was popular for game software for some time. The Generic VGA refresh rate is 60Hz, but many monitors and Video Interfaces support 70Hz or 72Hz refresh rates.

VGA Video Interfaces can run software designed for CGA or EGA by emulation techniques. VGA monitors use Analog Video Signals provided via a 15-pin connector, rather than the 9-pin (TTL) connectors used with the older Digital (TTL) Video Standards. This is called a Miniature (or compact) DB15S connector).

The advantage of an Analog Video Signal is that the Video Display Monitor can show, not just 16 or 64 colours, but an almost unlimited range of colours, depending on the abilities of the Video Interface Card it is plugged into. With Digital Video Signals, the number of colours is limited to the number of Binary States, the number of Digital Video Signals represent. CGA has four Video Signals, 2 4 provides 16 colours, and EGA has six Video Signals, 2 6 provides 64 colours.

Generic VGA provided 16 or 256 colors. The Introduction of 15 or 16-bit High Colour (32,768 or 65,536 colours, respectively) and, then, 24-bit True Colour, brought about the need for a larger Video RAM and much faster Video Interface Cards. True Colour provides 16.7 million colours, as many as the human eye can see.

The phrase "Super VGA" technically means 800 by 600 resolution, at 16 colours. This standard was set by the VESA group in the early days of VGA. Today, modern VGA Video Interface Cards offers at least 1,024 by 768 resolution, and some go to 1,280 by 1,024 or even higher. The main limiting factor is the amount of Video RAM available to the Video System and the number of colours required at these extended resolutions.

This was IBM's attempt to regain the initiative in setting video standards. This standard is defined as 1,024 by 768 resolution by 256 colours. Like IBM's older 8514/A standard, XGA allowed interlacing at this resolution. IBM later adopted a non-interlaced version called XGA-2.

The characteristics of the common DOS Video Systems

Video stan Horiz Freq Vertical Freq Video RAM Starting Add Connector Vid. Signals
CGA 15,750 60 16K B8000 DB9 4 TTL
MDA 18,432 50 4K B0000 DB9 2 TTL
Hercules 18,432 50 64K B0000 DB9 2 TTL
EGA 21,800 60 128-256K A0000 DB9 6 TTL
VGA (Generic) 31,500 50-70 256-512K A0000 Min DB15 3 Analog
Super VGA (VESA standard) 35,160 56 512K-1Meg A0000 Min DB15 3 Analog


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