AUGMENTING TEXT-BASED MUSIC RETRIEVAL WITH AUDIO SIMILARITY (2009)

We investigate an approach to a music search engine that indexes music pieces based on relatedWeb documents. This allows for searching for relevant music pieces by issuing descriptive textual queries. In this paper, we examine the effects of incorporating audio-based similarity into the text-based ranking process – either by directly modifying the retrieval process or by performing post-hoc audiobased re-ranking of the search results. The aim of this combination is to improve ranking quality by including relevant tracks that are left out by text-based retrieval approaches. Our evaluations show overall improvements but also expose limitations of these unsupervised approaches to combining sources. Evaluations are carried out on two collections, one large real-world collection containing about 35,000 tracks and on the CAL500 set. 1. MOTIVATION AND RELATED WORK In the last years, the development of query-by-description music search engines has drawn increasing attention [1– 5]. Given the size of (commercial) digital music collections nowadays (several millions of tracks), this is not a big surprise. While most “traditional” music retrieval approaches pursue a query-by-example strategy, i.e., given a music piece, find me other pieces that sound alike, queryby-description systems are capable of retrieving relevant pieces by allowing to type in textual queries targeting musical or contextual properties beyond common meta-data descriptors. As this method of issuing queries is the common way to search the Web, it appears desirable to offer this type of functionality also in the music domain. Several approaches to accomplish this goal have been presented – all of themwith a slightly different focus. In [1], Baumann et al. present a system that incorporates metadata, lyrics, and acoustic properties all linked together by a semantic ontology. Queries are analyzed by means of natural language processing and tokens have to be mapped to the corresponding concepts. Celma et al. [2] use a Web crawler focused on audio blogs and exploit the texts from Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. c © 2009 International Society for Music Information Retrieval. the blogs to index the associated music pieces. Based on the text-based retrieval result, also musically similar songs can be discovered. In [3], we propose to combine audio similarity and textual content from Web documents obtained via Google queries to create representations of music pieces in a term vector space. A modification to this approach is presented in [6]. Instead of constructing term vector representations, an index of all downloaded Web documents is created. Relevance wrt. a given query is assessed by querying the Web document index and applying a technique called rank-based relevance scoring that takes into account the associations between music tracks and Web documents (cf. Section 2.1). Evaluations show that this document-centered approach is superior to the vector space approach. However, as this method is solely based on texts from the Web it may neglect important acoustic properties and suffer from effects such as popularity bias. Furthermore, inadequately represented tracks and tracks not present on the Web are penalized by this approach. In this paper, we aim at remedying these shortcomings and improving ranking quality by incorporating audio similarity into the retrieval process. Recently, the method of relevance scoring has also been adapted to serve as a source of information for automatically tagging music pieces with semantic labels. In [5], Barrington et al. successfully combine audio content features (MFCC and Chroma) with social context features (Web documents and last.fm tags) via machine learning methods and therefore improve prediction accuracy. The usefulness of audio similarity for automatic tagging is also shown in [4] where tags from well-tagged tracks are propagated to untagged tracks based on acoustic similarity. The remainder of this paper is organized as follows: In the next section we review methods for Web-based music track indexing and audio-based similarity computation. Section 3 describes two possible modifications of the initial approach that are examined in Section 4. In Section 5, based on these results, we discuss perspectives and limitations of combining Weband audio-based approaches before drawing conclusions in Section 6. 2. INCORPORATED TECHNIQUES In the following, we explain the methods for constructing a Web-based retrieval system and calculating audio similarity, which we combine in Section 3. 2.1 Web-based Indexing and RRS Ranking The idea of Web-based indexing is to collect a high number of texts related to the pieces in the music collection to gather many diverse descriptions (and hence a rich indexing vocabulary) and allow for a large number of possible queries. In our first approach, we aimed at permitting virtually any query by involving Google for query expansion [3]. When introducing rank-based relevance scoring (RRS), we renounced this step in favor of reduced complexity and improved ranking results [6]. From our point it is very reasonable to limit the indexing vocabulary to terms that actually co-occur with the music pieces (which is still very large). Construction of an index with a corresponding retrieval scheme is performed as follows. To obtain a broad basis of track specific texts, for each music piecem in the collectionM , three queries are issued to Google based on the information found in the id3 tags of the music pieces: