digital synthesis

[Don Tillman]

   From: "Bjorn Wesen" <bjorn at sparta.lu.se>
   Date: Wed, 29 Oct 1997 21:06:29 +0100

   >How is your "easy FPGA" going to handle aliasing issues due to the
   >waveform and aliasing issues due to FM modulation?

   If you trivially generate waveforms with discontinuities digitally, like
   sawtooth or triangle waves, you end up with aliasing, that is correct. You
   need to make sure the signal is appropriately band limited. You can
   manufacture alias-free basic "analog" waveforms like triangle, square and
   sawtooth by integration of bandlimited impulse trains (the result remain
   bandlimited).

The manufactured alias-free waveforms, like a sawtooth, would need
several versions, say, one for each octave you wanted to play.

But how about a PWM output (a very common VCO feature)?  There the
high frequency content changes depending on the symmetry *and* the
frequency.  How would you handle something like that?

And audio FM modulation creates wild frequencies for a living, so
that's going to alias fiercely.  Audio PWM modulation?  Yikes!

It's a very difficult problem.  Some things that are trivially easy to
do in the analog domain are (maybe) impossible in the digital domain.

   >   There already exist plenty of digital oscillator chips
   >
   >Which are these?

   Any general DSP for example, and related asic implementations with dsp
   cores.

Oh okay, sure,  I thought you mean special purpose, just-plug-them-in
digital VCO chips.

  -- Don