Date: Sun, 02 Apr 2000 16:39:27 -0400 From: "Louis A. Mamakos" <louie@TransSys.COM> To: Nick Sayer <nsayer@quack.kfu.com> Cc: freebsd-questions@FreeBSD.ORG, freebsd-hardware@FreeBSD.ORG Subject: Re: FreeBSD / XFree86 and "digital" VGA-LCD cards? Message-ID: <200004022039.QAA57923@whizzo.transsys.com> In-Reply-To: Your message of "Sun, 02 Apr 2000 12:49:43 PDT." <38E7A457.919DD71E@quack.kfu.com> References: <38E7A457.919DD71E@quack.kfu.com>
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There are a couple of FreeBSD'ers with digital panels, mostly the SGI 1600SW 1600x1024 display. This has a digital interface, and is a big win. Here's why: In a typical video card, you've got a RAMDAC being fed 24 bit RGB samples from the frame buffer (8 bits each red, green and blue). The RAMDAC converts these sample to analog voltages for each pixel; it needs to do this really damn fast to keep up with the frame rate of the monitor. So, now you've got these 3 analog voltages zinging their way down the wire into your monitor. On a "real" CRT, each of these voltages drives and amplifier which modulates the electron beam current which excites the phosphers on the screen of your display, which then produces the image of pornography on your screen :-) This is done for each of red, green and blue, otherwise the skin tones are not accurately reproduced. On an LCD monitor, there is no electron death beam exciting the phosphers; instead there is an analog-to-digital converter sampling each of the R,G and B voltages. This is then used to drive the LCD shutter on each pixel. There's two problems here: first, you're trying to recover the value of the pixel through a D/A and then A/D conversion process which is going to lose precision. Further, you're only estimating where the pixel is on the screen; e.g., the dot pitch of the actual display and how that's mapped to the varying voltage of the analog signal. The digital flat panel displays fix both of these problems. First, there is no tedious conversion of the pixel value to and from an analog voltage; you simply convey that to the display as bits, eliminating the need for the high-speed RAMDAC on the video display card and the high-speed ADC in the LCD display. Further, you can precisely address each pixel on the display since you're explicitly clocking out the RGB values for each pixel. This gives you an extraordinarily sharp display, (even compared to "analog" LCD panels) resulting in unparalleled high quality display of Internet pornography :-) That's background on why you want to do it. The how is harder to explain. The SGI 1600SW is usually sold paired with a Number 9 Revolution IV graphics board which has the digital display interface (as well as analog VGA output). When this display was first produced, there were no standards for the nature of the digital display interface, so your choices of display adapters was limited. Since then, a display interface was "standardized", but it seems that only recently have these things hit the market. So you need to be careful to investigate these interoperability issues before purchasing. If I had to guess, the Viewsonic monitor probably sports the newer standard interface; it would be interesting to see if anyone is shipping video cards which support it yet. So most of the experience so far has been with the proprietary digital interface on the SGI display. I don't know what the issues might be with the ViewSonic specifically. From the web page, the VP-181 looks very, very nice. If you can get a video card with the digital interface, life only gets that much better.. louie To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hardware" in the body of the message
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