Fixed-rate encoding of nonstationary information sources

An expression is obtained for the optimum distortion theoretically attainable when an information source with finite alphabet is encoded at a fixed rate with respect to a single-letter fidelity criterion. The expression is demonstrated by means of an appropriate coding theorem and converse; This new result generalizes the coding theorem of Shannon for stationary ergodic sources, the coding theorem of Gray-Davisson for stationary nonergodic sources, that of Gray-Saadat for asymptotically mean stationary sources, and that of Ziv for an individual sequence.