Outline of the MX Standard

MX is a new XML-based format to describe comprehensively heterogeneous music contents. In a single MX file, music symbols, printed scores, audio tracks, computer-driven performances, catalogue metadata, text and graphic contents related to a single music piece are linked and mutually synchronised within the same framework. Heterogeneous contents are organised in a multilayered structure that supports different encoding formats and a number of digital objects for each layer. 1 A New Standard for Music MX, an acronym which stands for Musical application using XML, is the code-name for a new file format whose international standardisation is in progress. Its development follows the guidelines of IEEE P1599, Recommended Practice Dealing with Applications and Representations of Symbolic Music Information Using the XML Language. MX is the result of research efforts at the Laboratorio di Informatica Musicale, or LIM, of the Università degli Studi di Milano. P1599, the IEEE Standard, is sponsored by the Computer Society Standards Activity Board and was launched by the Technical Committee on Computer Generated Music (IEEE CS TC on CGM) [1]. In 2002, a prototypal version of the format was released, originally known as Musical Application using XML, or MAX [2]. This format was discussed at IEEE MAX 2002 international conference. The IEEE final evaluation process, known as balloting, is currently being performed, with the aim of making MX/P1599 an international standard. The most recent version is MX Release Candidate 1, whose DTD and documentation can be downloaded from http://www.mx.dico.unimi.it. Tools for music visualisation [3], content-based retrieval [4], and automatic segmentation [5] are currently available. 2 Key Features of MX MX is an XML-based format. There are many advantages in choosing XML to describe information in general, and music information in particular. For instance, an Outline of the MX Standard 197 XML-based language allows inherent readability, extensibility and durability. It is open, free, easy to read by humans and computers, and can be edited by common software applications. Moreover, it is strongly structured, it can be extended to support new notations and new music symbols, and it can thus become a means of interchange for music with software applications and over the Net. Most of these topics have been treated in [6] and [7]. A comprehensive description of music must support heterogeneous materials. Thanks to the intrinsic capability of XML to provide structures for information, such representations can be organised in an effective and efficient way. MX employs six different layers to represent information, as explained in [8] and shown in Figure 1: • General – music-related metadata, i.e. catalogue information about the piece; • Logic – the logical description of score symbols; • Structural – identification of music objects and their mutual relationships; • Notational – graphical representations of the score; • Performance – computer-based descriptions and executions of music according to performance languages; • Audio – digital or digitised recordings of the piece. Not all layers must, or can, be present for a given music piece. Of course, the higher their number, the richer the musical description. Richness has been mentioned in regard to the number of heterogeneous types of media description, namely symbolic, logic, audio, graphic, etc. But the philosophy of MX allows one extra step, namely that each layer can contain many digital instances. For example, the Audio layer could link to several audio tracks, and the Structural layer could provide many different analyses for the same piece. The concept of multilayered description – as many different types of descriptions as possible, all correlated and synchronised – together with the concept of multi-instance support – as many different media objects as possible for each layer – provide rich and flexible means for encoding music in all its aspects. It is possible to adopt some ad hoc encoding in addition to already existing formats to represent information. In fact, while a comprehensive format to represent music is not available, popular existing standards must be taken into account. This is a not a contradiction because of the two-sided approach of MX to music representation, which is: keep intrinsic music descriptions inside of the MX file – in XML format – and media objects outside of the MX file – in their original format. The symbols that belong to the score, such as chords and notes, are described in XML, in the Logic layer. On the contrary, MP3 files and other audio descriptions are not translated into XML format, rather they are linked and mapped inside the corresponding MX layer, the Audio layer. It should be clear that the description provided by an MX file is flexible and rich, both in regard to the number and to the type of media involved. In fact, thanks to this approach, a single file can contain one or more descriptions of the same music piece in each layer. For example, in the case of an operatic aria, the MX file could house: the catalogue metadata about the piece, its author(s) and genre; the corresponding portion of the libretto; scans of the original manuscript and of a number of printed scores; several audio files containing different performances; related iconographic contents, such as sketches, on-stage photographs, and playbills. Thanks to the