隱藏式馬可夫模型與類神經網路模型在語音辨識的研究領域上，各佔有一席之地，本篇論文提出一種混合兩種模型的新方法，以結合兩者的優點，提高辨識率及應用的範圍。實驗方面，以國語數字“0”到“9”當做訓練及測試的語音資料，並分成語者相關系統及語者無關系統兩部分進行。所提出的新方法以HMM-NN-Net、HMM-HMM-Net與NN-NN-Net三種狀態模型系統予以實現，並將實驗結果與隱藏式馬可夫模型相比較。 在語者相關系統方面，三種系統的辨識率皆在九成以上，其中HMM-NN-Net、NN-NN-Net狀態模型更可達百分之百，並且在經過對收斂條件適當的調整後，HMM-NN-Net狀態模型的辨識率以微幅的差距超越隱藏式馬可夫模型。 在語者無關系統方面，HMM-NN-Net狀態模型以94.25﹪的辨識率領先其他模型，進一步證明了新方法的可行性。同時，利用HMM-NN-Net與NN-NN-Net兩種狀態模型的比較，對類神經網路收斂問題，做完整的分析。 Hidden Markov model (HMM) was widely used for speech recognition and has been proved useful in dealing with the statistical and sequential aspects of the speech signal. However, their discriminative properties are weak if they are trained with the maximum likelihood. On the other hand, neural networks (NN) have powerful classification capability but are not well-suited for dealing with time-varying input patterns. In this study, a hybrid HMM-NN speech recognition system that combines the advantages of both models is presented. Three neural net state models, HMM-NN-Net, HMM-HMM-Net and NN-NN-Net, are developed for the proposed hybrid HMM-NN system. All the experimental results are compared with the one obtained from HMM. In the speaker-dependent experiment, the recognition rates of all the three models are above the level of 90 percent. Furthermore, in spite of the results of HMM-HMM-Net models, all error rates approach to zero after adjusting the criterion. In the speaker-independent case, HMM-NN-Net model achieves a recognition rate of 94.25 percent and has the best performance compared with other models. Besides, NN-NN-Net model requires less training time than HMM-NN-Net model although its recognition capability cannot compete with HMM-NN-Net model. The experimental results indicate that the hybrid HMM-NN recognition system based on HMM-NN-Net model improves the performance of traditional HMM system. It is also found that the criterion of neural net state models was related to the recognition capability.
