Hybrid Acoustic Models Research Articles

Advanced recurrent network-based hybrid acoustic models for low resource speech recognition

Recurrent neural networks (RNNs) have shown an ability to model temporal dependencies. However, the problem of exploding or vanishing gradients has limited their application. In recent years, long short-term memory RNNs (LSTM RNNs) have been proposed to solve this problem and have achieved excellent results. Bidirectional LSTM (BLSTM), which uses both preceding and following context, has shown particularly good performance. However, the computational requirements of BLSTM approaches are quite heavy, even when implemented efficiently with GPU-based high performance computers. In addition, because the output of LSTM units is bounded, there is often still a vanishing gradient issue over multiple layers. The large size of LSTM networks makes them susceptible to overfitting problems. In this work, we combine local bidirectional architecture, a new recurrent unit, gated recurrent units (GRU), and residual architectures to address the above problems. Experiments are conducted on the benchmark datasets released under the IARPA Babel Program. The proposed models achieve 3 to 10% relative improvements over their corresponding DNN or LSTM baselines across seven language collections. In addition, the new models accelerate learning speed by a factor of more than 1.6 compared to conventional BLSTM models. By using these approaches, we achieve good results in the IARPA Babel Program.

A hybrid acoustic model for room impulse response synthesis

The prediction and synthesis of room impulse responses (RIR) has wide application from computer gaming to architectural acoustics. When a level of physical accuracy is important, a single acoustic modeling technique is usually limited by its computational load. Hybrid acoustic models target different time/frequency regions of the RIR with different modeling techniques. This paper introduces a hybrid acoustic model consisting of a physical FDTD model for low-mid frequencies, beam-tracing, and the acoustic radiance transfer method in the early part and late parts at high frequencies respectively. In this work, attention is given to establishing the equivalence of the boundary characteristics in each modeling domain. Good agreement is demonstrated indicating that mixing the separate model responses leads to an energetically consistent RIR.

Using different acoustic, lexical and language modeling units for ASR of an under-resourced language – Amharic

State-of-the-art large vocabulary continuous speech recognition systems use mostly phone based acoustic models (AMs) and word based lexical and language models. However, phone based AMs are not efficient in modeling long-term temporal dependencies and the use of words in lexical and language models leads to out-of-vocabulary (OOV) problem, which is a serious issue for morphologically rich languages. This paper presents the results of our contributions on the use of different units for acoustic, lexical and language modeling for an under-resourced language (Amharic spoken in Ethiopia). Triphone, Syllable and hybrid (syllable-phone) units have been investigated for acoustic modeling. Word and morphemes have been investigated for lexical and language modeling. We have also investigated the use of longer (syllable) acoustic units and shorter (morpheme) lexical as well as language modeling units in a speech recognition system.Although hybrid AMs did not bring much improvement over context dependent syllable based recognizers in speech recognition performance with word based lexical and language model (i.e. word based speech recognition), we observed a significant word error rate (WER) reduction compared to triphone-based systems in morpheme-based speech recognition. Syllable AMs also led to a WER reduction over the triphone-based systems both in word based and morpheme based speech recognition. It was possible to obtain a 3% absolute WER reduction as a result of using syllable acoustic units in morpheme-based speech recognition. Overall, our result shows that syllable and hybrid AMs are best fitted in morpheme-based speech recognition.

Hybrid NN/HMM acoustic modeling techniques for distributed speech recognition

Distributed speech recognition (DSR) where the recognizer is split up into two parts and connected via a transmission channel offers new perspectives for improving the speech recognition performance in mobile environments. In this work, we present the integration of hybrid acoustic models using tied-posteriors in a distributed environment. A comparison with standard Gaussian models is performed on the AURORA2 task and the WSJ0 task. Word-based HMMs and phoneme-based HMMs are trained for distributed and non-distributed recognition using either MFCC or RASTA-PLP features. The results show that hybrid modeling techniques can outperform standard continuous systems on this task. Especially the tied-posteriors approach is shown to be usable for DSR in a very flexible way since the client can be modified without a change at the server site and vice versa.