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ARAMDAC (random-access memory digital-to-analog converter) is a combination of three fastdigital-to-analog converters (DACs) with a smallstatic random-access memory (SRAM) used in computer graphicsdisplay controllers orvideo cards to store thecolor palette and to generate the analog signals (usually a voltage amplitude) to drive a colormonitor.[1] The logical color number from the display memory is fed into the address inputs of the SRAM to select a palette entry to appear on the data output of the SRAM. This entry is composed of three separate values corresponding to the three components (red, green, and blue) of the desired physical color. Each component value is fed to a separate DAC, whose analog output goes to the monitor, and ultimately to one of its threeelectron guns (or equivalent in non-CRT displays).[2]
RAMDACs becameobsolete asDVI,HDMI,DisplayPort and other digital interface technology became mainstream, which transfer video data digitally (viatransition-minimized differential signaling orlow-voltage differential signaling) and defer digital-to-analog conversion until the monitor's pixels are actuated.
The termRAMDAC did not enter into common PC-terminology untilIBM introduced the IBMVGA display adapter in 1987. The IBM VGA adapter used theINMOS G171 RAMDAC. The INMOS VGA RAMDAC was a separate chip, featured a 256-color (8-bit CLUT) display from a palette of262,144 possible values, and supported pixel-rates up to approximately 30 Mpix/s.[3]
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 adddirect-color operation, which is a mode of operation that allows theSVGA-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-assistedspatial 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 aCLUT-based display mode, is rarely used, having been supplanted by True Color display modes. However, manyCAD and video editing applications usehardware overlay, combined with the programmable palette, to ensure the user interface does not disrupt the rendering of editing window.
The size of each DAC of the RAMDAC is 6 to 10bits. The SRAM's word length must be at least three times as large as the size of each DAC. The SRAM acts as acolor lookup table (CLUT). It usually has 256 entries (and thus an8-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 of16,777,216 (16.7 million) possible colors for the display.[2] The contents of this SRAM can be altered when no pixel needs to be generated for transmission to the display, which occurs during thevertical blanking interval between everyframe.
The SRAM can usually be bypassed and the DACs can be fed color directly by display data, forTrue 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:[4]
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 aclock rate of 400 MHz. However,video cards based on theXGI 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 byBarco.[5]