LUT chips
Henry Sour Katzenstein was living in Los Angeles in 1981 when he developed the idea for an entirely new way to produce a DAC. The limitation on existing systems was the switching speed, which was a function of the need to switch from 1 to 0 or back while producing enough voltage to drive the DAC to the levels needed by the display. This was normally accomplished using a color mapping chip and then separate amplifiers. Given the existing semiconductor fabrication (fab) systems of the era, this limited most systems to about 25 MHz or less, which was enough for displays running around 640 pixels horizontally. Moreover, the rapid switching of the relatively high-power outputs, produces generated noise on the power supply that often came through in the signal and on the motherboard.
Katzenstein's concept was to produce output based on current levels, not voltage, and to control that current by mixing two current sources, positive and negative. This meant that the incoming power was always "full on" and only the outputs varied, eliminating the switching noise. Current-controlled circuitry was well developed in the analog market, and high-speed current switches were easy to fab. The downside to this approach was that the resulting IC was always drawing full power, which dissipated as heat, and the ICs ran extremely hot.
The basic concept was developed enough by 1983 that he formed Brooktree, named after a street he formerly lived on in San Diego, using venture capital funding. The videoDAC was put on the market in 1985. Running at 75 MHz, about four times the speed of most contemporary systems, it could drive a display up to 2,048 pixels wide at 60 fps. Although mass production did not begin until 1988, when computer monitors with this sort of resolution began to appear, the design quickly began to steal market share from companies like AMD, Analog Devices and Texas Instruments. Large customers included Apple Computer, Sun Microsystems, Toshiba and IBM.
The company's next product was a similar design with one key difference, the mapping from the input values to output was no longer through a fixed table, but one that could be written to. This meant that "color 100" could be green for one program, and blue for another, with the output RGB levels stored as an 18- or 24-bit value in RAM. The result, known to the company as a RAMDAC but more commonly known today generically as a LUT-DAC, gave the system much greater flexibility. While successful for a time, it was not long before the relentless improvements in fabrication allowed companies to incorporate the LUT-DAC into the same IC as the rest of the controller logic, and Brooktree's line of separate ICs began to wane.
