On 9/8/05, Jack Carroll <[EMAIL PROTECTED]> wrote: > The devil is on the analog side. 16-bit DACs ain't simple, and > integrating them into board layouts and building good anti-aliasing filters > ain't simple. If you're talking about high sample rate, presumably you're > pushing for performance, so you'd want at least a 4th order Bessel filter. > That means careful selection of fast op amps and close-tolerance capacitors > with low dielectric absorption. If we're willing to go that far, we might > want to consider upgrading to 18-bit DACs; the audio kind aren't a whole lot > more expensive.
As I understand it, you don't really use DACs. You use a high-speed pulse-width modulated digital signal that you run through an analog filter. For instance, if you had an alternating digital signal at, say, 200 Khz, but you ran it through a low-pass filter that didn't let anything above 50KHz get through, you'd get a slightly (perhaps) ripply signal somewhere in between 0 and 1. We could drive a digital signal at 100MHz, varying the pulse width and duty cycle and get very clean audio out of the filter. And we are pretty sure that that's how a lot of audio is done. We just don't know what the usual solutions are exactly. > Also, the spec/cost tradeoffs will surely take some time on the > discussion list. I'd say this is best split off as a sub-project, with a > target schedule aimed at absorption into the ASIC board layout. The > development board should continue as planned, without further feature > changes. An analog group might be able to read the app notes from Analog > Devices and Crystal Semiconductor and work something up, while the Verilog > code is getting debugged. I'm no digital audio specialist, but with good > app notes from the parts suppliers I might be able to contribute some > circuit design work here. I can cookbook an active filter. Or perhaps we should table it for now and when OGC's are in production, start up a whole, separate OAP to design a high-end card. > Another thing: With space allocated for two DVI connectors on the > back panel, will there be enough real estate left for 6 channels worth of > audio connectors? I don't think we want a multi-pin connector and a > breakout cable. No. You'd put it on a separate face-plate. > But let me ask a more basic question. What's the benefit of > integrating sound into a graphics board, instead of just recommending a > sound board that's known to play well under Linux or using what's on the > motherboard? I'm not being sarcastic, I really don't know the answer to > that. That, as a matter of fact, is exactly our question, and it seems to be answered: No point in audio on OGP. _______________________________________________ Open-graphics mailing list [email protected] http://lists.duskglow.com/mailman/listinfo/open-graphics List service provided by Duskglow Consulting, LLC (www.duskglow.com)
