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Convolutional Recurrent Neural Networks forPolyphonic Sound Event Detection
Emre Cakr, Giambattista Parascandolo, Toni Heittola, Heikki Huttunen, and Tuomas Virtanen
AbstractSound events often occur in unstructured environ-ments where they exhibit wide variations in their frequencycontent and temporal structure. Convolutional neural networks(CNN) are able to extract higher level features that are invariantto local spectral and temporal variations. Recurrent neural net-works (RNNs) are powerful in learning the longer term temporalcontext in the audio signals. CNNs and RNNs as classifiershave recently shown improved performances over establishedmethods in various sound recognition tasks. We combine thesetwo approaches in a Convolutional Recurrent Neural Network(CRNN) and apply it on a polyphonic sound event detection task.We compare the performance of the proposed CRNN methodwith CNN, RNN, and other established methods, and observe aconsiderable improvement for four different datasets consistingof everyday sound events.
Index Termssound event detection, deep neural networks,convolutional neural networks, recurrent neural networks
IN our daily lives, we encounter a rich variety of soundevents such as dog bark, footsteps, glass smash and thunder.Sound event detection (SED), or acoustic event detection, dealswith the automatic identification of these sound events. Theaim of SED is to detect the onset and offset times for eachsound event in an audio recording and associate a textualdescriptor, i.e., a label for each of these events. SED hasbeen drawing a surging amount of interest in recent yearswith applications including audio surveillance , healthcaremonitoring , urban sound analysis , multimedia eventdetection  and bird call detection .
In the literature the terminology varies between authors;common terms being sound event detection, recognition, tag-ging and classification. Sound events are defined with pre-determined labels called sound event classes. In our work,sound event classification, sound event recognition, or soundevent tagging, all refer to labeling an audio recording withthe sound event classes present, regardless of the onset/offsettimes. On the other hand, an SED task includes both on-set/offset detection for the classes present in the recording
G. Parascandolo and E. Cakir contributed equally to this work.The authors are with the Department of Signal Processing, Tampere
University of Technology (TUT), Finland e-mail: firstname.lastname@example.org.The research leading to these results has received funding from the
European Research Council under the European Unions H2020 FrameworkProgramme through ERC Grant Agreement 637422 EVERYSOUND.
G. Parascandolo has been funded by Googles Faculty Research award.The authors wish to acknowledge CSC IT Center for Science, Finland, for
computational resources.The paper has a supporting website athttp://www.cs.tut.fi/sgn/arg/taslp2017-crnn-sed/Manuscript received July 12, 2016 (revised January 19, 2016).
and classification within the estimated onset/offset, which istypically the requirement in a real-life scenario.
Sound events often occur in unstructured environments inreal-life. Factors such as environmental noise and overlappingsources are present in the unstructured environments and theymay introduce a high degree of variation among the soundevents from the same sound event class . Moreover, therecan be multiple sound sources that produce sound eventsbelonging to the same class, e.g., a dog bark sound eventcan be produced from several breeds of dogs with differentacoustic characteristics. These factors mainly represent thechallenges over SED in real-life situations.
SED where at most one simultaneous sound event is de-tected at a given time instance is called monophonic SED.Monophonic SED systems can only detect at most one soundevent for any time instance regardless of the number of soundevents present. If the aim of the system is to detect all theevents happening at a time, this is a drawback concerningthe real-life applicability of such systems, because in sucha scenario, multiple sound events are very likely to overlapin time. For instance, an audio recording from a busy streetmay contain footsteps, speech and car horn, all appearing asa mixture of events. An illustration of a similar situation isgiven in Figure 1, where as many as three different soundevents appear at the same time in a mixture. A more suitablemethod for such a real-life scenario is polyphonic SED, wheremultiple overlapping sound events can be detected at any giventime instance.
SED can be approached either as scene-dependent or scene-independent. In the former, the information about the acousticscene is provided to the system both at training and test time,and a different model can therefore be trained for each scene.In the latter, there is no information about the acoustic scenegiven to the system.
Previous work on sound events has been mostly focusedon sound event classification, where audio clips consist-ing of sound events are classified. Apart from establishedclassifierssuch as support vector machines , deeplearning methods such as deep belief networks , convo-lutional neural networks (CNN) , ,  and recurrentneural networks (RNN) ,  have been recently proposed.Initially, the interest on SED was more focused on monophonicSED. Gaussian mixture model (GMM) - Hidden Markovmodel (HMM) based modelingan established method thathas been widely used in automatic speech recognitionhasbeen proposed to model individual sound events with Gaussianmixtures and detect each event through HMM states usingViterbi algorithm , . With the emergence of more
Fig. 1: Sound events in a polyphonic recording synthesizedwith isolated sound event samples. Upper panel: audio wave-form, lower panel: sound event class activity annotations.
advanced deep learning techniques and publicly available real-life databases that are suitable for the task, polyphonic SEDhas attracted more interest in recent years. Non-negative matrixfactorization (NMF) based source separation  and deeplearning based methods (such as feedforward neural networks(FNN) , CNN  and RNN ) have been shown toperform significantly better compared to established methodssuch as GMM-HMM for polyphonic SED.
Deep neural networks  have recently achieved remark-able success in several domains such as image recogni-tion , , speech recognition , , machine trans-lation , even integrating multiple data modalities such asimage and text in image captioning . In most of thesedomains, deep learning represents the state-of-the-art.
Feedforward neural networks have been used in mono-phonic  and polyphonic SED in real-life environments by processing concatenated input frames from a small timewindow of the spectrogram. This simple architecturewhilevastly improving over established approaches such as GMM-HMMs  and NMF source separation based SED ,presents two major shortcomings: (1) it lacks bothtime and frequency invariancedue to the fixed connectionsbetween the input and the hidden unitswhich would allowto model small variations in the events; (2) temporal context isrestricted to short time windows, preventing effective modelingof typically longer events (e.g., rain) and events correlations.
CNNs  can address the former limitation by learningfilters that are shifted in both time and frequency , lackinghowever longer temporal context information. Recurrent neu-ral networks (RNNs), which have been successfully applied toautomatic speech recognition (ASR)  and polyphonic SED, solve the latter shortcoming by integrating informationfrom the earlier time windows, presenting a theoreticallyunlimited context information. However, RNNs do not easilycapture the invariance in the frequency domain, rendering ahigh-level modeling of the data more difficult. In order tobenefit from both approaches, the two architectures can becombined into a single network with convolutional layersfollowed by recurrent layers, often referred to as convolutionalrecurrent neural network (CRNN). Similar approaches com-bining CNNs and RNNs have been presented recently in ASR, ,  and music classification .
In this paper we propose the use of multi-label convolutional
recurrent neural network for polyphonic, scene-independentsound event detection in real-life recordings. This approachintegrates the strengths of both CNNs and RNNs, which haveshown excellent performance in acoustic pattern recognitionapplications , , , , while overcoming their indi-vidual weaknesses. We evaluate the proposed method on threedatasets of real-life recordings and compare its performanceto FNN, CNN, RNN and GMM baselines. The proposedmethod is shown to outperform previous sound event detectionapproaches.
The rest of the paper is organized as follows. In Section IIthe problem of polyphonic SED in real-life environments isdescribed formally and the CRNN architecture proposed forthe task is presented. In Section III we present the evaluationframework used to measure the performance of the differentneural networks architectures. In Section IV experimentalresults, discussions over the results and comparisons withbaseline methods are reported. In Section V we summarizeour conclusions from this work.
A. Problem formulation
The aim of polyphonic SED is to temporally locate and labelthe sound event classes present in a polyphonic audio signal.Polyphonic SED can be formulated in two stages: soundrepresentation and classification. In sound representation stag