[sdiy] Xilinx 3E board comments

[Eric Brombaugh]

Paul Maddox wrote:
> The *BIG* advantage with FPGAs is that you can do several things at 
> the same time.
> You could easily run 32 phase accumulator oscillators simultaneously, 
> givine each oscillator a 50Mhz update rate (what aliasing?)

Good point. That gives you ~0.01Hz resolution. It also puts Nyquist at 
25 MHz, so the aliased harmonics will be down ~67dB worst-case by the 
time they fold back into the audio range. Not quite CD-quality, but 
better than the 45dB I was seeing.

Then you either have to build a (fairly simple) decimating FIR to get 
the sample rate down to the audio range to output through a good-quality 
DAC, or use a Delta-Sigma converter running in logic at full speed like 
the one Jim Patchell has on the fpga.synth.net site. (has anyone tried 
that yet?)

Eric