We describe a sequence-to-sequence neural network which can directly generate speech waveforms from text inputs. The architecture extends the Tacotron model by incorporating a normalizing flow into the autoregressive decoder loop. Output waveforms are modeled as a sequence of non-overlapping fixed-length frames, each one containing hundreds of samples. The interdependencies of waveform samples within each frame are modeled using the normalizing flow, enabling parallel training and synthesis. Longer-term dependencies are handled autoregressively by conditioning each flow on preceding frames. This model can be optimized directly with maximum likelihood, without using intermediate, hand-designed features nor additional loss terms. Contemporary state-of-the-art text-to-speech (TTS) systems use a cascade of separately learned models: one (such as Tacotron) which generates intermediate features (such as spectrograms) from text, followed by a vocoder (such as WaveRNN) which generates waveform samples from the intermediate features. The proposed system, in contrast, does not use a fixed intermediate representation, and learns all parameters end-to-end. Experiments show that the proposed model generates speech with quality approaching a state-of-the-art neural TTS system, with significantly improved generation speed.

本研究描述了一种序列到序列的神经网络，可以直接将文本输入转化为语音波形，具备较快的语音生成速度，而且不需要使用中间特征来实现。该模型使用自回归解码器循环与标准化流相结合，对输出波形进行建模，并能够处理长期依赖关系。实验证明，所提出的模型的语音生成质量接近最新的神经网络TTS系统，并具有明显的加速优势。

Wave-Tacotron: 基于端到端的文本到语音合成的光谱图自由方法