RAMDAC

A RAMDAC (random-access memory digital-to-analog converter) is a combination of three fast

CRT

displays).

RAMDACs became

HDMI, DisplayPort and other digital interface technology became mainstream, which transfer video data digitally (via transition-minimized differential signaling or low-voltage differential signaling

) and defer digital-to-analog conversion until the monitor's pixels are actuated.

History

The term RAMDAC did not enter into common PC-terminology until

INMOS G171 RAMDAC. The INMOS VGA RAMDAC was a separate chip, featured a 256-color (8-bit CLUT) display from a palette of 262,144 possible values, and supported pixel-rates up to approximately 30 Mpix/s.^[2]

As clone manufacturers copied IBM VGA hardware, they also copied the INMOS VGA RAMDAC. Advances in semiconductor manufacturing and PC processing power allowed RAMDACs to add direct-color operation, which is a mode of operation that allows the

SVGA-controller to pass a pixel's color value directly to the DAC-inputs, thereby bypassing the RAM lookup-table. Another innovation was Edsun's CEGDAC, which featured hardware-assisted spatial anti-aliasing

for line/vector draw operations.

By the early 1990s, the PC chip industry had advanced to the point where RAMDACs were integrated into the display controller chip, thus reducing the number of discrete chips and the cost of video cards. Consequently, the market for standalone RAMDACs disappeared. Today, RAMDACs are still manufactured and sold for niche applications, but in obviously limited quantity.

In modern PCs, the RAMDAC(s) are integrated into the display controller chip, which itself may be mounted on an add-in-board or integrated into the motherboard core-logic chipset. The original purpose of the RAMDAC, to provide a

CLUT-based display mode, is rarely used, having been supplanted by True Color display modes. However, many CAD and video editing applications use hardware overlay

, combined with the programmable palette, to ensure the user interface does not disrupt the rendering of editing window.

Design

The size of each DAC of the RAMDAC is 6 to 10

frame

.

The SRAM can usually be bypassed and the DACs can be fed color directly by display data, for True color modes. In fact this has become very much the normal mode of operation of a RAMDAC since the mid-1990s, so the programmable palette is mostly retained only as a legacy feature to ensure compatibility with old software. In many newer graphics cards, the RAMDAC can be clocked much faster in true color modes, when only the DAC part without the SRAM is used.

A quick estimation on the pixel clock for a given output can be found with:^[3]

Pixels, horizontally, per line × lines, vertically, per display × 1.4 (factor in any blanking) × rate of display updates (refresh rate)

The ability to drive transitions for sharp edges usually incurs, for the RAMDAC, a significant requirement in excess of the pixel clock.

As of 2006, the DAC of a modern graphics card runs at a

video cards based on the XGI Volari XP10 run at 420 MHz DAC. The highest documented DAC frequency ever achieved on a production video card for the PC platform is 550 MHz, set by BarcoMed 5MP2 Aura 76Hz by Barco.^[4]

References

OCLC 883168030
.

^ "Famous Graphics Chips: IBM's VGA". Retrieved 2024-04-13.

^ VESA's GTF calculation sheet

^ https://web.archive.org/web/20070527140116/http://www.barco.com/corporate/en/products/product_specs.asp?element=2600

External links

RAMDAC (random access memory digital-to-analog converter) / TechTarget, 2005

The RAMDAC / The PC Guide, 2001

Retrieved from "https://en.wikipedia.org/w/index.php?title=RAMDAC&oldid=1218732513"

[shen-1] OCLC 883168030
.

[2] "Famous Graphics Chips: IBM's VGA". Retrieved 2024-04-13.

[vesa-3] VESA's GTF calculation sheet

[barco-4] ttps://web.archive.org/web/20070527140116/http://www.barco.com/corporate/en/products/product_specs.asp?element=2600

[2]

[3]

[4]