Türkçe Otomatik Konuşma Tanıma Sistemi için Dil Modeli Optimizasyon Yöntemi

Türkçe gibi sınırlı kaynaklara sahip dillerle karşı karşıya kaldığında mevcut Otomatik Konuşma Tanıma (ASR: Automatic Speech Recognition) modelleme stratejisi hala büyük bir performans düşüşü yaşıyor. Özellikle Dil modeli, akustik modeli yeterince desteklemediğinde Kelime Hata Oranı (WER: Word Error Rate) yükselmektedir. Bu yüzden, sağlam bir Dil modeli (LM: Language Model) mevcut corpus'dan kelime bağıntıları oluşturarak ASR performansını iyileştirmeye güçlü bir katkı sağlar. Ancak Türkçenin sondan eklemeli yapısı nedeniyle sağlam bir dil modeli geliştirmek zorlu bir görevdir. Bu çalışmada, sınırlı kaynaklara sahip Türkçe ASR'nin WER performansını iyileştirmek için cümle düzeyinde bir LM optimizasyon yöntemi önerilmiştir. Önerilen yöntemde Markov varsayımlarından elde edilen sabit bir kelime dizisi yerine, kelime dizisinin cümle oluşturma olasılığı hesaplanmıştır. Kelime dizisi olasılığını elde etmek için n-gram ve atlama gramı özelliklerine sahip bir yöntem sunulmuştur. Önerilen yöntem hem istatistiksel hem de Yapay Sinir Ağı (ANN: Artificial Neural Network) tabanlı LM'ler üzerinde test edilmiştir. Sadece kelimeler değil, aynı zamanda alt kelime seviyesi kullanılarak yapılan deneylerde, Dilsel Veri Konsorsiyumu (LDC: Linguistic Data Consortium) aracılığıyla paylaşılan iki Türkçe korpus (ODTÜ ve Boğaziçi) ve HS olarak adlandırdığımız özel olarak oluşturduğumuz ayrı bir korpus kullanılmıştır. İstatistik tabanlı LM'den elde edilen deneysel sonuçlara göre, ODTÜkcorpusda %0,5 WER artışı, Boğaziçi korpusda %1.6 WER azalması ve HS kopusta %2,5 WER azalması gözlemlenmiştir. İleri Beslemeli Sinir Ağları tabanlı LM'de ODTÜ corpusda %0,2, Boğaziçi korpusda %0,8 ve HS korpusda %1.6 WER düşüşleri gözlendi. Ayrıca Tekrarlayan Sinir Ağı - Uzun Kısa Süreli Bellek tabanlı LM'de ODTÜ korpusda %0,6, Boğaziçi korpusda %1.1 ve HS korpusda %1.5 WER düşüşleri gözlendi. Sonuç olarak önerilen yöntem Turkçe ASR’de kullanılan LM'lere uygulandığında WER azalmış ve ASR'nin toplam performansı artmıştır.

Anahtar Kelimeler:

Türkçe Otomatik konuşma tanıma, Türkçe korpus, Türkçe dil modelleme, Türkçe dil modeli optimizasyonu

A Language Model Optimization Method for Turkish Automatic Speech Recognition System

The current Automatic Speech Recognition (ASR) modeling strategy still suffers from huge performance degradation when faced with languages with limited resources such as Turkish. Especially when the Language Model (LM) does not support the Acoustic Model (AM) sufficiently, the Word Error Rate (WER) increases. Therefore, a robust LM makes a strong contribution to improving ASR performance by generating word relations from the existing corpus. However, developing a robust language model is a challenging task due to the agglutinative nature of Turkish. Therefore, within the scope of the study, a sentence-level LM optimization method is proposed to improve the WER performance of Turkish ASR. In the proposed method, instead of a fixed word sequence obtained from the Markov assumptions, the probability of the word sequence forming a sentence was calculated. A method with n-gram and skip-gram properties is presented to obtain the word sequence probability. The proposed method has been tested on both statistical and Artificial Neural Network (ANN) based LMs. In the experiments carried out using, not only words but also sub-word level, two Turkish corpora (METU and Bogazici) shared via Linguistic Data Consortium (LDC) and a separate corpus, which we separate corpus that we specially created as HS was used. According to the experimental results obtained from statistical-based LM, 0.5% WER increases for the METU corpus, 1.6% WER decreases for the Bogazici corpus, and a 2.5% WER decrease for the HS corpus were observed. In the Feedforward Neural Networks (FNN) based LM, WER decreases were observed 0.2% for the METU corpus, 0.8% for the Bogazici corpus, and 1.6% for the HS corpus. Also, in the Recurrent Neural Network (RNN)-Long Short Term Memory (LSTM) based LM, WER decreases were observed 0.6% for METU corpus, 1.1% for the Bogazici corpus and 1.5% for the HS corpus. As a result, when the proposed method was applied to the LMs required for ASR, WER decreased, and the total performance of ASR increased.

Keywords:

Turkish Automatic speech recognition, Turkish language model, Turkish language model score optimization, Turkish corpus,

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