Hi Arhangel. Thanks for the reply. Maybe you might be right. the encoders that I have put out very good defined square wave pulses at 55 pules an inch. I expect my maximum travel speed to be 2 inches per second. As I accelerate the PWM output should increase smoothly and it isn't. I am not sure how using comparators would help me? Other people I've talked to have suggested using linear encoders i.e 0 to 5 volts out with increase in speed and feeding the output into the A to D convertors. Just anothe Idea for me to try. Thanks