How it works
Everyone knows that the data processed in the computer is to change everything into two numbers, 0 and 1, and then continue to accumulate and calculate, the graphics The accelerator card also relies on these 0s and 1s to perform various processing such as color, depth, and brightness for each pixel. But it is more difficult to display it, because all the monitors people use can work in analog mode, so this requires a D/A converter. The role of RAMDAC is exactly this, it will be processed by the graphics acceleration chip. The data is converted from digital to analog, so that the monitor can display the image.
Unique value
The role of RAMDAC is obvious. In actual use, is the performance of RAMDAC important? Some friends may think of these problems, because today's graphics accelerator cards are marked with the number of HZ RAMDAC. RAMDAC's fast clock (HZ) indicates that it can handle a higher refresh rate. The refresh rate refers to the number of times the screen is redrawn per second. Human eyes will feel flickering when the refresh rate is lower than 75, which affects normal use and vision. In order to protect our priceless eyes, a good graphics accelerator card is not only To ensure that the number of colors is extremely high, it is also necessary to ensure a high refresh rate. Therefore, the better (faster) RAMDAC is used, the graphics accelerator card can provide higher resolution, color depth and refresh rate, and the performance of the graphics accelerator card will be better. This is why the RAMDAM clock of today's graphics accelerator card is getting faster and faster. Friends who read the above content may think what RAMDAC is like? Can you find it on the graphics accelerator card? I am afraid that this problem will disappoint my friends. Today's graphics acceleration chip production process is getting better and better, RAMDAC has been integrated into the graphics acceleration chip, so most of the graphics acceleration cards we see only have video memory and a main chip, but can not see the RAMDAC chip. However, there are some exceptions, such as professional graphics cards. Because of its design, RAMDAC is still an independent chip.
Judging Criteria
The conversion rate of RAMDAC is expressed in MHz, which determines the refresh rate (similar to the meaning of the "bandwidth" of the display). The higher the working speed, the wider the frequency band, and the better the picture quality at high resolution. This value determines the maximum resolution and refresh rate supported by the graphics card with sufficient video memory. If you want to achieve a refresh rate of 85Hz at a resolution of 1024×768, the rate of RAMDAC is at least 1024×768×85×1.344÷1.06≈90MHz.
The RAMDAC of early graphics cards was generally 300MHz, and quickly developed to 350MHz. The RAMDAC of mainstream graphics cards can reach 400MHz, which is enough to meet and exceed the resolution and refresh rate that most displays can provide.
Simple classification
R7500, GF4600 level and before
The RAMDAC of the graphics card is 8bit. In an 8-bit RAMDAC, each color channel of red, green and blue (hereinafter referred to as RGB) in the VGA signal occupies 8 bits. For example, in the red color channel, the color is divided into 2 to the 8th power from dark to light, that is, 256 levels . The 8bit RAMDAC graphics card supports up to 24-bit true color, and the number of colors that can be displayed is 16.7M. Our common 32-bit color is 24-bit true color plus 8-bit alpha value. The alpha value represents the transparency of the color. For example, the color of the object behind colored glass, water, and fog must be loaded into the alpha value.
R8500,MATROX Parhelia-512, NV30 level
R8500,MATROX Parhelia-512, NV30 level and later graphics card RAMDAC is 10bit, RGB each color channel occupies 10 bits, For example, in the red color channel, the color from dark to light is divided into 2 to the 10th power, that is, 1024 levels, and the number of colors that can be displayed in 30-bit color is 1024 to the 3rd power, that is, more than 1 billion. As long as the operating system supports it, future graphics cards equipped with 16-bit RAMDAC can display 64-bit colors.
Imaging problems
In RAMDAC, the digitally expressed RGB color channel is converted by a digital-to-analog converter into a VGA driving voltage, and the VGA driving voltage is transmitted to the CRT and then amplified and converted into an electron gun. The electron flow, the electron flow bombards the screen phosphor to emit light and display the image. Because RAMDAC is integrated inside the display chip, the display chip design of each manufacturer is different, so the display effect is also different. A very important part of the display effect is the color-voltage correspondence table of RAMDAC. For example, the thickest color in the red color channel corresponds to the highest voltage of the channel, and the lightest color represents the lowest voltage of the channel, and the difference between the highest voltage and the lowest voltage Determine the color range, I think the difference between the highest/lowest voltage of ATI RAMDAC is greater than NV, and its differential filter circuit smoothes the voltage difference very well, so that the color contrast is large (clear) and the color transition is smooth (image color block Less).
and low-pass filter circuit
The low-pass filter circuit is a circuit that only allows signals below a certain frequency to pass. Usually, the bandwidth of the video signal is about several megahertz to ten megahertz. Generally, we only need to pass useful signals. Higher or lower frequency signals are not what we want, but can be understood as noise. The low-pass filter circuit is to remove these useless signals, otherwise it will cause 2D display. Quality declines. The low-pass filter circuit is usually located on the circuit board near the VGA plug and consists of a triode amplifier circuit and a capacitor/resistance filter circuit. The function of the triode amplifier circuit is to amplify the VGA signal to offset the signal attenuation caused by the capacitor/resistance filter circuit.
Notes on low-pass filter circuits
Low-pass filter circuits do not have as many components as possible. Many people think that the more components, the better the workmanship is. That is a wrong idea and constitutes a low-pass filter. The quality of the transistors, capacitors, and resistances of the circuit are different. For example, the chip capacitors, even if they are of the same factory and the same batch, the capacitance value between individuals will have a difference of less than 0.2%. Too many capacitors in series will cause the error to be superimposed. , Its error will cause distortion of the VGA signal. In the same way, a filter circuit composed of too many capacitors in series in the sound card will reduce the resolution of the sound and cause the sound to be ambiguous. After solving the trade-off of filtering clutter and avoiding distortion-fish and bear's paw can not be achieved, ATI built the filter circuit in the display chip to solve the problem of external low-pass filter circuit. So see that some A cards omit the external low-pass filter circuit, don’t make a fuss, only practical applications can comment on the 2D/3D image quality of the graphics card.
In the public board design, the combination of the triode amplifier circuit with the external low-pass filter circuit and the capacitor/resistance filter circuit is strictly calculated, which does not affect the integrity of the VGA signal, but also effectively filters out impurities. However, in actual production, even if it is manufactured according to the public board, its transistors, capacitors, and resistors may not be the parts of the public board designed by the chip manufacturer, which may cause the VGA signal to be distorted. Therefore, well-known manufacturers will use the best accessories to ensure the display quality of their graphics cards, so it is good to buy graphics cards from major manufacturers.