On 4/4/06, Tom Cook <[EMAIL PROTECTED]> wrote: > > What are balanced inputs? > > > The typical balanced system has three conductors. One is an earth and is > not used for carrying signal. The signal is the voltage difference between > the other two wires. Neither of these is held at any particular reference > voltage at either end (ie. neither is connected to ground). By taking the > difference between the two wires, you cancel out any noise induced on both > of the wires. The idea is that any electromagnetic field in the area will > induce (at least approximately) the same noise on both wires, because they > are very close together. Then when you take the difference between the two > wires you cancel out the induced noise. This is not a very good > explanation; see eg > http://www.dself.dsl.pipex.com/ampins/balanced/balanced.htm > for a fuller one.
No, I get the idea. In the digital domain, I am familiar with this as a "differential pair." > Phantom power is achieved by putting 48VDC on BOTH signal conductors, > relative to the third conductor. The thing at the other end can then take > 48 volts between a signal line and earth, and send a signal back in the > difference between the signal lines. This difference induced between the > lines is small enough (typically 1mV) that it doesn't much affect the 48V > supply. Needless to say, there is lots of coupling/decoupling involved. Definitely something only an external box could provide. How will users feel about having the external box? Good? Bad? > > Well, a channel is either going to be input or output. ADC and DAC > > are totally different things. It might be possible to turn off a DAC > > well enough that you could have an ADC and DAC on every connector. > > But that might cause reflections and introduce noise. As for software > > configuration, that's up to the software developer how they want to > > rename channels; that's not a hardware issue. > > > I might be wrong, but I don't think reflections are a big deal on a PCB that > is only handling 100kHz tops. Also the ADC/DAC don't need to be directly > connected, since you are almost certainly going to want a buffer op-amp > going either way. Still need some way to switch between them. I'm more concerned about noise and oscillations. I think you're right that a simple reflection over less than a centimeter won't be a problem for us. If we can tri-state the transmitter, and the receiver has sufficiently high impedence, I don't see why we couldn't connect them in pairs to let software select the direction. Only the transmitter needs to be switched off, while we'll just ignore the receiver when we're transmitting. How do you tri-state an analog signal without distorting it? Just cutting power to the opamp probably isn't enough because of whatever will happen to the analog signal when it goes into the output of an opamp. > > USB wouldn't have enough bandwidth for so many channels at once. > > Sure, there's lots of kinds of processing where the data would never > > leave the card, but in many cases, you're going to be shipping data > > back and forth with the application. You need high bandwidth for > > that, and even PCI might not be quite fast enough if all channels are > > uncompressed. > > > ? > > I make 192kHz x 24bits to be 4608000 bits per second. Sixty channels of > that is 276.48 Mb/s. All of USB2, 1394 and gigabit ethernet are capable of > that. The trick is probably getting the latency down. I see your point. We're going to have some sort of latency anyhow due to the external box, though. > > Now, I can already hear you typing, "But you can do all your processing in > software! Even better, in the FPGA!" And do you know what? You are RIGHT. > But sound engineers don't think like that. Each one has his own effects > box that is THE best effects box IN THE WORLD and the only way to stop him > using it is to prise it from his cold, lifeless grip. We give the customer what the customer wants. :) > > Excellent. Plus, for anything missing, we can work together to add it. > > > > It's also worth noting that there are a number of CODEC chips out there that > do a lot of this for you. The idea is to integrate everything together in one place. That may require that we reimplement things. Plus, for some signal processing, we may want more general DSP functionality, either via a real DSP or something of our own design. _______________________________________________ Open-graphics mailing list [email protected] http://lists.duskglow.com/mailman/listinfo/open-graphics List service provided by Duskglow Consulting, LLC (www.duskglow.com)
