Hi Maciel,
This version (with step-dir pulses as output) is potentially absolute winner
Now I can use my driver modules (
http://www.pololu.com/catalog/product/1182) with your system, and the interface circuit might be much simpler than before.
Looking forward to test..
EDIT:
I have suggestion: Could you try and use mic input of those USB audio adapters for encoder quadrature signals (DC coupled. of course..)?