Attention-Based Models for Speech Recognition
Jan ChorowskiDzmitry BahdanauDmitriy SerdyukKyunghyun ChoYoshua Bengio
Introduces location-aware attention mechanisms for end-to-end speech recognition, resolving alignment failures across long acoustic inputs and achieving competitive phoneme error rates on TIMIT.
The article addresses the challenge of building fully end-to-end neural systems for speech recognition, where traditional hybrid approaches rely on hand-crafted pronunciation dictionaries, triphone HMMs, and separate language models. Attention-based recurrent sequence generators had succeeded on shorter-sequence tasks such as machine translation, yet they struggled with the much longer and noisier inputs typical of speech, raising questions about whether they could scale reliably.
The article set out to evaluate whether attention mechanisms could be adapted for phoneme recognition on the TIMIT corpus and to develop extensions that would allow the models to handle utterances many times longer than those seen during training.
The authors trained and compared three attention-based recurrent sequence generators on the standard TIMIT train-dev-test split, using 40 mel-filterbank features plus energy and deltas. They started with a content-only attention baseline previously used for translation, then introduced a hybrid mechanism that augments attention scores with convolutional features extracted from the previous alignment, and finally tested a smoothing modification that replaces softmax normalization with a sigmoid to encourage the model to aggregate information from multiple frames. Performance was measured both on the original short test utterances and on artificially concatenated sequences up to ten times longer.
The baseline content-based model reached a competitive 18.7 percent phoneme error rate on the standard test set but rapidly degraded on longer inputs, often aligning only the first 40–50 phones before jumping to the end and cycling. The hybrid model with convolutional location features improved to 18.0 percent error on the original test set and maintained error rates below 20 percent even on utterances formed by repeating or mixing up to eleven original recordings. Adding the smoothing modification further lowered the error to 17.6 percent while preserving robustness to length.
These results demonstrate that a purely neural attention architecture can match or approach the accuracy of conventional pipelines on phoneme recognition without requiring separate alignment or lexicon stages, and that explicit location awareness makes the approach practical for longer audio. The improvement matters because it removes a key obstacle to scaling end-to-end speech systems and suggests the same attention extensions could benefit other sequence tasks that involve long or repetitive inputs.
The article recommends incorporating a language model directly into the generator, extending the approach to direct character or word transcription, and applying the convolutional location features to neural Turing machines or image captioning. Further validation on larger, more varied corpora and real-world long-form audio is needed before deployment decisions can be made; the current experiments rely on a relatively small phoneme dataset and artificially constructed long utterances.
- Paper: Neural Machine Translation by Jointly Learning to Align and Translate, Dzmitry Bahdanau et al. (2015). Introduces the foundational content-based soft attention mechanism in sequence-to-sequence neural networks that the source paper evaluates and extends for acoustic modeling.
- Paper: Sequence to Sequence Learning with Neural Networks, Ilya Sutskever et al. (2014). Establishes the core encoder-decoder recurrent framework for mapping variable-length input sequences to variable-length target sequences.
- Paper: Towards End-To-End Speech Recognition with Recurrent Neural Networks, Alex Graves et al. (2014). Demonstrates early end-to-end recurrent architectures for speech recognition, providing the motivation to eliminate traditional multi-stage phonetic pipelines.
- Paper: Speech Recognition with Deep Recurrent Neural Networks, Alex Graves et al. (2013). Pioneers deep bidirectional LSTM networks and alignment-free objectives on the TIMIT phoneme recognition benchmark used as the baseline in the source.
- Paper: Learning Phrase Representations using RNN Encoder–Decoder for Statistical Machine Translation, Kyunghyun Cho et al. (2014). Presents the foundational RNN Encoder-Decoder model and gated recurrent units for sequence generation that underpin attentional decoders.
- Paper: On the Properties of Neural Machine Translation: Encoder–Decoder Approaches, Kyunghyun Cho et al. (2014). Examines the severe performance degradation of standard encoder-decoder models on long sequence inputs, highlighting the exact length-generalization problem solved in the source.
- Paper: Generating Sequences With Recurrent Neural Networks, Alex Graves (2013). Introduces recurrent sequence generation with location-sensitive Gaussian mixture attention mechanisms that inspired location-based alignments in speech.
- Paper: Long Short-Term Memory, Sepp Hochreiter et al. (1997). Develops the gated Long Short-Term Memory cell architecture necessary for training deep recurrent sequence models over long temporal spans.
- Paper: Listen, Attend and Spell, William Chan et al. (2015). Extends attention-based end-to-end speech recognition to large-scale, full character transcription using a pyramidal encoder and integrated sequence decoding.
- Paper: Deep Speech 2: End-to-End Speech Recognition in English and Mandarin, Dario Amodei et al. (2016). Scales end-to-end neural speech recognition to multi-language industrial settings using deep convolutional recurrent networks.
- Paper: Tacotron: Towards End-to-End Speech Synthesis, Yuxuan Wang et al. (2017). Applies location-aware attention and sequence-to-sequence recurrent modeling to text-to-speech synthesis to generate spectrogram frames from character inputs.
- Paper: Natural TTS Synthesis by Conditioning Wavenet on MEL Spectrogram Predictions, Jonathan Shen et al. (2017). Further refines attention-based acoustic sequence generation for speech synthesis by coupling recurrent mel-spectrogram predictors with neural vocoding.
- Paper: Conformer: Convolution-augmented Transformer for Speech Recognition, Anmol Gulati et al. (2020). Combines convolutional modules with self-attention architectures to modernize end-to-end speech recognition models.
- Paper: Robust Speech Recognition via Large-Scale Weak Supervision, Alec Radford et al. (2023). Scales attention-based sequence-to-sequence speech recognition across massive multilingual and weakly supervised web audio datasets.
- Paper: wav2vec 2.0: A Framework for Self-Supervised Learning of Speech Representations, Alexei Baevski et al. (2020). Develops self-supervised representations for raw audio sequences that improve end-to-end speech recognition efficiency in low-resource conditions.
