Hold on a moment, now. Most the earlier discussion in this Thread has been about stepper motors and it is clearly a stepper motor in the picture in Reply #19.
How have DC motors come into the discussion?
Are you saying that the optical encoder is detecting the rotation of a DC motor and the data is being used to control stepper motors?
It would make it so much easier to offer useful advice if we know what you are really trying to do.
How many encoder steps per second need to be detected?
...R