RAMDAC

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A random-access memory digital-to-analog converter (RAMDAC) 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 to store the color palette and to generate the analog signals (usually a voltage amplitude) to drive a color monitor. [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 three electron guns (or equivalent in non-CRT displays).

Digital-to-analog converter device that converts a digital signal into an analog signal

In electronics, a digital-to-analog converter is a system that converts a digital signal into an analog signal. An analog-to-digital converter (ADC) performs the reverse function.

Static random-access memory Semiconductor memory

Static random-access memory is a type of semiconductor random-access memory (RAM) that uses bistable latching circuitry (flip-flop) to store each bit. SRAM exhibits data remanence, but it is still volatile in the conventional sense that data is eventually lost when the memory is not powered.

Palette (computing) in computer graphics, a finite set of colors

In computer graphics, a palette is a finite set of colors. Palettes can be optimized to improve image accuracy in the presence of software or hardware constraints.

Contents

As the use of DVI, HDMI, DisplayPort and other digital interface technology becomes increasingly mainstream, the DAC portion of the RAMDAC will likely become obsolete. The video data is transferred digitally via transition-minimized differential signaling (TMDS) or low-voltage differential signaling (LVDS) and no digital-to-analog conversion takes place until the actual display pixels are actuated.

Digital Visual Interface standard for transmitting digital video to a display (such as a monitor) LOCOOO

Digital Visual Interface (DVI) is a video display interface developed by the Digital Display Working Group (DDWG). The digital interface is used to connect a video source, such as a video display controller, to a display device, such as a computer monitor. It was developed with the intention of creating an industry standard for the transfer of digital video content.

DisplayPort digital display interface

DisplayPort (DP) is a digital display interface developed by a consortium of PC and chip manufacturers and standardized by the Video Electronics Standards Association (VESA). The interface is primarily used to connect a video source to a display device such as a computer monitor, and it can also carry audio, USB, and other forms of data.

Transition-minimized differential signaling

Transition-minimized differential signaling (TMDS), a technology for transmitting high-speed serial data, is used by the DVI and HDMI video interfaces, as well as by other digital communication interfaces.

DAC word lengths range usually from 6 to 10 bits. The SRAM's word length is three times the DAC's word length. The SRAM acts as 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 16,777,216 (16.7 million) possible colors for the display. The contents of this SRAM can be altered when no pixel needs to be generated for transmission to the display. A synchronization pulse is required to maintain vertical picture stability. Therefore, a vertical blanking pulse is generated for every frame. This vertical blanking pulse is not visible on the display, nor is any pixel sent. Therefore, the D/A is idle and can allow the user to modify the SRAM color lookup table.

The bit is a basic unit of information in information theory, computing, and digital communications. The name is a portmanteau of binary digit.

In computer architecture, 8-bit integers, memory addresses, or other data units are those that are 8 bits wide. Also, 8-bit CPU and ALU architectures are those that are based on registers, address buses, or data buses of that size. 8-bit is also a generation of microcomputers in which 8-bit microprocessors were the norm.

In computer architecture, 24-bit integers, memory addresses, or other data units are those that are 24 bits wide. Also, 24-bit CPU and ALU architectures are those that are based on registers, address buses, or data buses of that size.

The SRAM can usually be bypassed and the DACs can be fed 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.

For a quick estimation on the pixel clock for a given output, you can do: [2]

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 clock rate of 400  MHz. However, 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.

The clock rate typically refers to the frequency at which a chip like a central processing unit (CPU), one core of a multi-core processor, is running and is used as an indicator of the processor's speed. It is measured in clock cycles per second or its equivalent, the SI unit hertz (Hz). The clock rate of the first generation of computers was measured in hertz or kilohertz (kHz), the first personal computers (PC's) to arrive throughout the 1970s and 1980s had clock rates measured in megahertz (MHz), and in the 21st century the speed of modern CPUs is commonly advertised in gigahertz (GHz). This metric is most useful when comparing processors within the same family, holding constant other features that may affect performance. Video card and CPU manufacturers commonly select their highest performing units from a manufacturing batch and set their maximum clock rate higher, fetching a higher price.

A video card is an expansion card which generates a feed of output images to a display device. Frequently, these are advertised as discrete or dedicated graphics cards, emphasizing the distinction between these and integrated graphics. At the core of both is the graphics processing unit (GPU), which is the main part that does the actual computations, but should not be confused as the video card as a whole, although "GPU" is often used to refer to video cards.

XGI Technology Inc. is based upon the old graphics division of SiS spun off as a separate company, and the graphics assets of Trident Microsystems.

History

IMS G171 RAMDAC on the VGA board IMS G171.jpg
IMS G171 RAMDAC on the VGA board

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 262,144 possible values, and supported pixel-rates up to approximately 30 Mpix/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. 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.

Related Research Articles

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Mode 13h

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Color Graphics Adapter computer display standard

The Color Graphics Adapter (CGA), originally also called the Color/Graphics Adapter or IBM Color/Graphics Monitor Adapter, introduced in 1981, was IBM's first graphics card and first color display card for the IBM PC. For this reason, it also became that computer's first color computer display standard.

The Multi-Color Graphics Array or MCGA is a video subsystem built into the motherboard of the IBM PS/2 Model 30, introduced on April 2, 1987, and Model 25, introduced later on August 11; no standalone MCGA cards were ever made.

Atari Falcon Personal computer

The Atari Falcon030 Computer System is a personal computer released by Atari Corporation in 1992. The machine is based on a Motorola 68030 main CPU, and had a Motorola 56000 digital signal processor, a feature which distinguished it from most other microcomputers of the era.

Professional Graphics Controller

Professional Graphics Controller is a graphics card manufactured by IBM for PCs. It consists of three interconnected PCBs, and contains its own processor and memory. The PGC was, at the time of its release, the most advanced graphics card for the IBM XT and aimed for tasks such as CAD.

Oak Technology was an American supplier of semiconductor chips for sound cards, graphics cards and optical storage devices such as CD-ROM, CD-RW and DVD. It achieved success with optical storage chips and its stock price increased substantially around the time of the tech bubble in 2000. After falling on hard times, in 2003 it was acquired by Zoran Corporation.

Video display controller type of integrated circuit

A video display controller or VDC is an integrated circuit which is the main component in a video signal generator, a device responsible for the production of a TV video signal in a computing or game system. Some VDCs also generate an audio signal, but that is not their main function.

In computing, indexed color is a technique to manage digital images' colors in a limited fashion, in order to save computer memory and file storage, while speeding up display refresh and file transfers. It is a form of vector quantization compression.

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Tandy Graphics Adapter

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References

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.
  1. Shen, John Paul; H. Lipasti, Mikko (2013). "3". Modern processor design : fundamentals of superscalar processors. Long Grove: Waveland Press. p. 154. ISBN   1478607831. OCLC   883168030.
  2. VESA's GTF calculation sheet