On Thu, Sep 08, 2005 at 12:22:26PM -0400, Timothy Miller wrote:
> So, Howard and I were talking and he suggested that it would be a
> shame to have TV-out without audio, so we decided it would be good to
> bring this up on the list.
>
> There's no way we could sell a low-end audio card by itself. To make
> a sellable product in that arena where people would actually BUY it,
> we'd have to go to mega-high-end. And we don't have the resources for
> that right now. BUT, adding simple digital audio to the graphics chip
> MIGHT not be such a big deal.
>
> So, what we'd like to ask people is what we would need to do to
> achieve that. We're thinking 6-channel 16-bit, high sample-rate. We
> THINK that all we'd have to do for an audio channel is some sort of
> PCM on a digital output connected to some sort of simple RC filter,
> but we don't know what the existing solutions are.
>
> We also need to assess the impact of this on our time and silicon resources.
>
> If it's a simple matter of having the graphics chips DMA controller
> pull blocks of data from main memory and push them through some sort
> of mixer and something that converts from analog samples to a digital
> wave form that's fed into an RC filter, we can probably do that
> without too much fuss. (Typically, the RC filters would go on a
> separate board with a ribbon cable connected to a header, for anyone
> who wanted to do the add-on.)
>
> Comments?
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.
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.
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.
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.
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