On 3/15/06, James Richard Tyrer <[EMAIL PROTECTED]> wrote: > I am looking at the analog parts of the schematic and haven't found > anything yet. > > I had some questions about the design. I have answered some by > downloading some data sheets. But still have some remaining questions > about the video DACs. > > It seems that we have a very expensive solution for this so I was > wondering about the required specs. > > 1. What is our max clock rate for the DACs?
330MHz for the one being populated. > > 2. What is the bandwidth required for the video out buffers. Can you explain what you mean by "video output buffer"? The total memory bandwidth is 1.6*10**9 pixels per second, while the max video for one head is only 330*10**6. > Note that in theory if we use shunt peaked video out buffers that #2 > would be twice #1 for analog video but it would be normal to have an > additional frequency margin. We're going to have no trouble at all with getting pixels out of memory onto the screen. With both heads at 330 megapixel/sec digital (which requires a bit more bandwidth than analog due to reduced blanking), that's less than 660 million pixels per second. That's 40% of our bandwidth, but you won't even notice that until you start using the more advanced 3D features that suck bandwidth. > > There is digital so you would probably want the video out signal to have > the 3rd harmonic for good rise time. If so, you need 6x +/- and TV > video buffers wouldn't do the job. It now occurs to me that when you say "bandwidth", you're talking about an analog thing, in terms of signal line transmissions and all that. That's Howard's thing, I'm afraid, so I really can't talk much about it. I'm not even sure what you mean when you say "3rd harmonic". :) > I couldn't find a video SRAM in the design? I would want this as a > feature in the final board and would prefer 12 bit per color. The video framebuffer is stored in graphics memory (Those DDR ram chips on there) and read out as we're scanning the display. And you're going to get 8 bits per color. Remember that OGA is somewhat of a minimalist design, targeted at performing well for desktops. Neato stuff like programmable shaders and higher RGB color depth is reserved for later generations, when we can AFFORD it. I could possibly provide a drawing mode where green is 12 bits, reducing red and blue to 6 bits, but you won't get this for video output. A design decision was made VERY early on where pixels are fixed at 32-bit ARGB. Even 8-bit pseudocolor mode is just a trick of the host interface. _______________________________________________ Open-graphics mailing list [email protected] http://lists.duskglow.com/mailman/listinfo/open-graphics List service provided by Duskglow Consulting, LLC (www.duskglow.com)
