Johan Pauwels

A novel chroma representation of polyphonic music based on multiple pitch tracking techniques

It is common practice to map the frequency content of music onto a chroma representation, but there exist many different schemes for constructing such a representation. In this paper, a new scheme is proposed. It comprises a detection of salient frequencies, a conversion of salient frequencies to notes, a psychophysically motivated weighting of harmonics in support of a note, a restriction of harmonic relations between different notes and a restriction of the deviations from a predefined pitch scale (e.g. the equally tempered western scale). A large-scale experimental evaluation has confirmed that the novel chroma representation more closely matches manual chord labels than the representations generated by six other tested schemes. Therefore, the new chroma representation is expected to improve applications such as song similarity matching and chord detection and labeling.

Evaluating automatically estimated chord sequences

In this paper, we perform an in-depth evaluation of a large number of algorithms for chord estimation that have been submitted to the MIREX competitions in 2010, 2011 and 2012. Therefore we first present a rigorous scheme to describe evaluation methods in a sound, unambiguous way that extends previous work specifically to take into account the large variance in chord estimation vocabularies and to perform evaluations on select sets of chords. Then we take a look at the evaluation metrics used so far and propose some alternative ones. Finally, we use these different methods to get a deeper insight into the strengths of each of the competing algorithms and show that the choice of evaluation measure greatly influences the ranking.

Combining Musicological Knowledge About Chords and Keys in a Simultaneous Chord and Local Key Estimation System

In this paper, we present a probabilistic framework for the simultaneous estimation of chords and keys from audio. The framework is formulated in terms of acoustic models for both keys and chords, and a prior model that contains musicological knowledge about chords and keys. The latter consists of a compound of four components: a duration and a change model for both keys and chords. This division allows us to modify each of the components separately and to choose the most appropriate sources of knowledge for each of them. Furthermore, this makes it easier to interpret their role and their relevance in the estimation procedure. We compared multiple configurations of our system, increasing in complexity. This has permitted us to explore the relation between keys and chords, and the importance of integrating prior musicological knowledge into an automatic estimation system. It was found that chord estimation scores mostly depend on the integration of durational knowledge, while key estimation also requires prior information about the broader context.

Improving the key extraction performance of a simultaneous local key and chord estimation system

Sinificant improvements of a previously developed key and chord extraction system are proposed. The major improvement is the introduction of a separate acoustic model, designed to verify local key hypotheses. The conducted experimental evaluation shows that the presented system improves the state of the art in local key estimation. Our experimental study further demonstrates that the chord estimation performance is already quite robust, whereas the key estimation performance still happens to be sensitive to a number of factors. In particular, we present figures that illustrate the significant impact of the embedded musicological model and the duration of the processed excerpt on the key estimation accuracy.

Segmenting music through the joint estimation of keys, chords and structural boundaries

In this paper, we introduce a new approach to music structure segmentation that is based on the joint estimation of structural segments, keys and chords in one probabilistic framework. More precisely, the boundaries of a structure segment are determined by detecting key changes and by utilizing the difference in prior probability of chord transitions according to their position in a structural segment. In contrast to many of the recent approaches to structural segmentation, this system does not work with self-similarity matrices, although it has been designed to integrate this kind of approach into the framework at a later stage. However, just the current version of the system, using only the estimated harmony, is already producing encouraging results, especially with respect to the precise localization of the boundaries.

The Influence of Chord Duration Modeling on Chord and Local Key Extraction

In this paper, we investigate the effect of different types of chord duration modeling on the performance of a simultaneous chord and local key extraction system. Two hypotheses are examined, (1) whether the introduction of multiple states per key-chord combination makes any difference as it changes the prior duration from a geometric distribution to a negative binomial distribution, and (2) whether making the prior mean duration of a key-chord combination a function of the chord has a positive effect. We found that the introduction of multiple states per key-chord has no influence on neither key nor chord extraction performance, but making the mean duration a function of the chord interpreted in its key leads to an increase in the key estimation capabilities.

Modeling musicological information as trigrams in a system for simultaneous chord and local key extraction

In this paper, we discuss the introduction of a trigram musicological model in a simultaneous chord and local key extraction system. By enlarging the context of the musicological model, we hoped to achieve a higher accuracy that could justify the associated higher complexity and computational load of the search for the optimal solution. Experiments on multiple data sets have demonstrated that the trigram model has indeed a larger predictive power (a lower perplexity). This raised predictive power resulted in an improvement in the key extraction capabilities, but no improvement in chord extraction when compared to a system with a bigram musicological model.

Improved template based chord recognition using the CRP feature

The task of chord recognition in music signals is often based upon pattern matching in chromagrams. Many variants of chroma exist and quality of chord recognition is related to the feature employed. Chroma Reduced Pitch (CRP) features are interesting in this context as they were designed to improve timbre invariance for the purpose of query retrieval. Their reapplication to chord recognition, however, has not been successful in previous studies. We consider that the default parametrisation of CRP attenuates some tonal information, as well as timbral, and consider alternatives to this default. We also provide a variant of a recently proposed compositional chroma feature, adapted for music pieces, rather than one instrument. Experiments described show improved results compared to existing features.