thumb|A [[Brooktree RAMDAC]]

A RAMDAC (random-access memory digital-to-analog converter) is a combination of three fast digital-to-analog converters (DACs) with a small static random-access memory (SRAM) used in computer graphics display controllers or video cards to store the color palette and to generate the analog signals (usually a voltage amplitude) to drive a color monitor.

RAMDACs became obsolete as DVI, 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

thumb|IMS G171 RAMDAC on a [[VGA adapter]]

The term RAMDAC did not enter into common PC-terminology until IBM introduced the IBM VGA display adapter in 1987. The IBM VGA adapter used the INMOS G171 RAMDAC. The INMOS VGA RAMDAC was a separate chip, featured a 256-color (8-bit CLUT) display from a palette of possible values, and supported pixel-rates up to approximately 30Mpix/s.

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. The display controller chip 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, was supplanted by True Color display modes. Eventually, as digital display interfaces became mainstream, the RAMDAC became obsolete.

Design

The size of each DAC of the RAMDAC is 6 to 10 bits. The SRAM's word length must be at least three times as large as the size of each DAC. The SRAM holds a color lookup table (CLUT). It usually has 256 entries (and thus an 8-bit address). If the DAC's word length is also 8 bits, we have a 256× 24-bit SRAM which allows a selection of 256 out of (16.7 million) possible colors for the display.

  • RAMDAC (random access memory digital-to-analog converter) / TechTarget, 2005
  • The RAMDAC / The PC Guide, 2001