  General quantum measurements are represented by instruments. In this paper
the mathematical formalization is given of the idea that an instrument is a
channel which accepts a quantum state as input and produces a probability and
an a posteriori state as output. Then, by using mutual entropies on von Neumann
algebras and the identification of instruments and channels, many old and new
informational inequalities are obtained in a unified manner. Such inequalities
involve various quantities which characterize the performances of the
instrument under study; in particular, these inequalities include and
generalize the famous Holevo's bound.
