Improving End-To-End Speech Synthesis With Local Recurrent Neural Network Enhanced Transformer

Although Transformer based neural end-to-end TTS model has demonstrated extreme effectiveness in capturing long-term dependencies and achieved state-of-the-art performance, it still suffers from two problems. 1) limited ability to model sequential and local structures in sequences; 2) heavily rely on position embeddings that have limited effect but require an amount of design efforts. In this paper, we introduce local recurrent neural network (Local-RNN) into Transformer to make full use of the advantages of both RNN and Transformer while mitigating their drawbacks. The sequential and local structures could be effectively modeled by Local-RNN, while the long-term dependencies could be captured by Transformer without any use of position embeddings. Subjective evaluation results show our proposed model outperforms baseline (Transformer) with a gap of 0.12 in MOS and achieves close to human quality (4.34 vs. 4.45 in MOS) on general test. Case level intelligibility test also show an absolute improvement of 6.5% in case level intelligibility rate over the baseline on a challenging test.