Section: New Results
Source Separation and Localization
Source separation, sparse representations, probabilistic model, source localization
Source separation is the task of retrieving the source signals underlying a multichannel mixture signal.
About a decade ago, state-of-the-art approaches consisted of representing the signals in the time-frequency domain and estimating the source coefficients by sparse decomposition in that basis. These approaches rely only on spatial cues, which are often not sufficient to discriminate the sources unambiguously. Over the last years, we proposed a general probabilistic framework for the joint exploitation of spatial and spectral cues , which generalizes a number of existing techniques including our former study on spectral GMMs . We showed how it could be used to quickly design new models adapted to the data at hand and estimate its parameters via the EM algorithm, and it became the basis of a large number of works in the field, including our own. In the last years, improvements were obtained through the use of prior knowledge about the source spatial covariance matrices , , , knowledge on the source positions and room characteristics , or a better initialization of parameters thanks to specific source localization techniques .
This accumulated progress lead, in 2015, to two main achievements: a new version of the Flexible Audio Source Separation Toolbox, fully reimplemented, was released  and we published an overview paper on recent and going research along the path of guided separation in a special issue of IEEE Signal Processing Magazine devoted to source separation and its applications . This two achievements formed the basis of our work in 2016, exploring intensively the concrete use of these tools and principles in real-world scenarios, in particular within the voiceHome project (see Section 6.13).
Towards Real-world Separation and Remixing Applications
Participants : Nancy Bertin, Frédéric Bimbot, Ewen Camberlein, Romain Lebarbenchon.
In 2015, we began a new industrial collaboration, in the context of the VoiceHome project, aiming at another challenging real-world application: natural language dialog in home applications, such as control of domotic and multimedia devices. As a very noisy and reverberant environment, home is a particularly challenging target for source separation, used here as a pre-processing for speech recognition (and possibly with stronger interactions with voice activity detection or speaker identification tasks as well). In 2016, we publicly released a realistic corpus of room impulse responses and utterances recorded in real homes, and presented it during the Interspeech conference . We also continued benchmarking and adapting existing localization and separation tools to the particular context of this application, worked on a better interface between source localization and source separations steps, and investigated new means to reduce the latency and computational burden of the currently available tools (low-resolution source separation preserving speech recognition improvement, automatic selection of the best microphones, joint localization and multichannel speech / non speech classification prior to any separation).
In november 2016, we started investigating a new application of source separation to sound respatialization from Higher Order Ambisonics (HOA) signals, in the context of free navigation in 3D audiovisual contents. This work is conducted in a collaboration with the IRT b<>Com, through the Ph.D. of Mohammed Hafsati (co-supervised by Nancy Bertin, RÃ©mi Gribonval).
Implicit Localization through Audio-based Control for Robotics
Participant : Nancy Bertin.
Main collaborations (audio-based control for robotics): Aly Magassouba and François Chaumette (Inria, EPI LAGADIC, France)
Acoustic source localization is, in general, the problem of determining the spatial coordinates of one or several sound sources based on microphone recordings. This problem arises in many different fields (speech and sound enhancement, speech recognition, acoustic tomography, robotics, aeroacoustics...) and its resolution, beyond an interest in itself, can also be the key preamble to efficient source separation. Common techniques, including beamforming, only provides the direction of arrival of the sound, estimated from the Time Difference of Arrival (TDOA) . This year, we have particularly investigated alternative approaches, either where the explicit localization is not needed (audio-based control of a robot) or, on the contrary, where the exact location of the source is needed and/or TDOA is irrelevant (cosparse modeling of the acoustic field, see Section 7.1.2).
In robotics, the use of aural perception has received recently a growing interest but still remains marginal in comparison to vision. Yet audio sensing is a valid alternative or complement to vision in robotics, for instance in homing tasks. Most existing works are based on the relative localization of a defined system with respect to a sound source, and the control scheme is generally designed separately from the localization system.
In contrast, the approach that we investigate over the last three years focuses on a sensor-based control approach. We proposed a new line of work, by considering the hearing sense as a direct and real-time input of a closed loop control scheme for a robotic task. Thus, and unlike most previous works, this approach does not necessitate any explicit source localization: instead of solving the localization problem, we focus on developing an innovative modeling based on sound features. To address this objective, we placed ourselves in the sensor-based control framework, especially visual servoing (VS) that has been widely studied in the past .
Last year, we established an analytical model linking the Interaural Time Difference (ITD) sound features and control input of the robot, defined and analyzed robotic homing tasks involving multiple sound sources, and validated the proposed approach by simulations and experiments with an actual robot . This year, we consolidated these results and extended the range of applicative tasks  and obtained similar results (including theoretical and experimental) for the Interaural Level Difference (ILD), in combination with the absolute energy level . Another set of experiments, presented during the IROS workshop  was successfully carried with a humanoid robot, notably without any measurement nor modeling of the robot's Head Relative Transfer Functions (HRTF). This work was mainly lead by Aly Magassouba, who defended his Ph.D. (co-supervised by Nancy Bertin and François Chaumette) in December 2016.
Emerging activities on Virtually-Supervised Sound Localization
Participants : Antoine Deleforge, Clément Gaultier, Saurabh Kataria.
Audio source localization consists in estimating the position of one or several sound sources given the signals received by a microphone array. It can be decomposed into two sub-tasks : (i) computing spatial auditory features from raw audio input and (ii) mapping these features to the desired spatial information.
Extracting spatial features from raw audio input: The most commonly used features in binaural (two microphones) sound source localization are frequency-dependent phase and level differences between the two microphones. To handle the presence of noise, several sources, or reverberation, most existing methods rely on some kind of aggregation of these features in the time-frequency plane, often in a heuristic way. In , we introduced the rectified binaural ratio as a new spatial feature. We showed that for Gaussian point-source signals corrupted by stationary Gaussian noise, this ratio follows a complex -distribution with explicit parameters. This new formulation provides a principled, statistically sound and efficient method to aggregate these features in the presence of noise. Experiments notably showed the higher robustness of these features compared to traditional ones, in the task of localizing heavily corrupted speech signals.
Mapping features to spatial information: Existing methods to map auditory features to spatial properties divide into two categories. Physics-driven methods attempt to estimate an explicit mapping based on an approximate physical model of sound propagation in the considered system. Data-driven methods bypass the use of a physical model by learning the mapping from a training set, obtained by manually annotating features extracted from real data. We proposed a new paradigm that aims at making the best of physics-driven and data-driven approaches, referred to as virtually-supervised acoustic space mapping , . The idea is to use a physics-based room-acoustic simulator to generate arbitrary large datasets of room-impulse responses corresponding to various acoustic environments, adapted to the physical audio system at hand. We demonstrated that mappings learned from these data could potentially be used to not only estimate the 3D position of a source but also some acoustical properties of the room . We also showed that a virtually-learned mapping could robustly localize sound sources from real-world binaural input, which is the first result of this kind in audio source localization .