More ramblings about VCDO...

[Tim Ressel]

To all -- 
I have considered taking a Xilinx FPGA and creating a basic NCO
(Numerically-Controlled Oscillator). It would clock at about 2-5 MHz and put out
phase information i.e. a sawtooth wave. Put that phase info into a D/A and you
have a ramp. Add some XOR gates inside the FPGA and it puts out triangle. A PWM
is easily done with a digital magnitude comparator inside the chip. All we need
is a sine wave, which can be accomplished several ways. You would need a small
processor to take up MIDI and analog signals and set the NCO accordingly. 

This approach offers several advantages. Locking oscillators is a snap (all run
from the same clock), phase offsets and phase modulation can be done with
precision. NO drift. Synch is pretty much a thing of the past, or to look at it
another way, synch is handled by the processor which handles the NCO.

The output could be encoded serially to take advantage of the many Delta-Sigma
D/As out there. Two converters would give you all four outputs, or one chip
gives ramp and triangle, with standard analog handling the sine and PWM
conversions. 

I'll be taking a class in Verilog/VHDL in late August. Perhaps this will be my
project for the class.

Tim Ressel -- Hardware DQ
Hewlett-Packard
Verifone Division
916-630-2541
tim_r1 at verifone.com