Summary: The servodesign problem is considered in the framework of $H\sb \infty$ control. It is formulated as a kind of mixed sensitivity problem, aiming at constructing a feedback system which has the low sensitive characteristics and stability robustness with respect to plant variations. It is shown that if we put modes corresponding to the dynamics of the signal generator into the weighting function then the resulting controller must eventually possesses an internal model. Considered are both the output feedback case and the case where some of the states or all the states of the plant are directly measurable without measurement noise. In the latter case, it is revealed that under some mild conditions the order of the controller is reduced by the number of independent observations.