Music structure description, i.e. the task of representing the high-level organization of music pieces in a concise, generic and reproducible way, is currently a scientific challenge both algorithmically and conceptually. In this paper, we focus on semiotic structure, i.e. the description of similarities and internal relationships within a music piece, as a low-rate stream of arbitrary symbols from a limited alphabet and we address methodological ques- tions related to annotation. We formulate the labeling task as a blind demodulation problem, whose goal is to identify a minimal set of semiotic codewords, whose realizations within the music piece are subject to a number of connotative variations viewed as modulations. The determination of labels is achieved by combining morphological, paradigmatic and syntagmatic considerations relying respectively on (i) a morphological model of semiotic blocks in order to define their individual properties, (ii) the support of prototypical structural patterns to guide the comparison between blocks and (iii) a methodology for the determination of distinctive features across semiotic classes. Specific notations are introduced to account for unresolvable semiotic ambiguities, which are occasional but must be considered as inherent to the music matter itself. A set of 500 music pieces labeled in accordance with the proposed concepts and annotation conventions is being released with this article.