Paper:
Deep Voice 2: Multi-Speaker Neural Text-to-Speech
We introduce a technique for augmenting neural text-to-speech (TTS) with lowdimensional trainable speaker embeddings to generate different voices from a single model. As a starting point, we show improvements over the two state-ofthe-art approaches for single-speaker neural TTS: Deep Voice 1 and Tacotron. We introduce Deep Voice 2, which is based on a similar pipeline with Deep Voice 1, but constructed with higher performance building blocks and demonstrates a significant audio quality improvement over Deep Voice 1. We improve Tacotron by introducing a post-processing neural vocoder, and demonstrate a significant audio quality improvement. We then demonstrate our technique for multi-speaker speech synthesis for both Deep Voice 2 and Tacotron on two multi-speaker TTS datasets. We show that a single neural TTS system can learn hundreds of unique voices from less than half an hour of data per speaker, while achieving high audio quality synthesis and preserving the speaker identities almost perfectly.
3. Using these two single-speaker models as a baseline, we demonstrate multi-speaker neural speech synthesis by introducing trainable speaker embeddings into Deep Voice 2 and Tacotron.
We organize the rest of this paper as follows. Section 2 discusses related work and what makes the contributions of this paper distinct from prior work. Section 3 presents Deep Voice 2 and highlights the differences from Deep Voice 1. Section 4 explains our speaker embedding technique for neural TTS models and shows multi-speaker variants of the Deep Voice 2 and Tacotron architectures. Section 5.1 quantifies the improvement for single speaker TTS through a mean opinion score (MOS) evaluation and Section 5.2 presents the synthesized audio quality of multi-speaker Deep Voice 2 and Tacotron via both MOS evaluation and a multi-speaker discriminator accuracy metric. Section 6 concludes with a discussion of the results and potential future work.

Review:
This paper presents a solid piece of work on the speaker-dependent neural TTS system, building on previous works of Deep Voice and Tacotron architecture. The key idea is to learn a speaker-dependent  embedding vector jointly with the neural TTS model. The paper is clearly written, and the experiments are presented well. 
My comments are as follows.
-- the speaker embedding vector approach is very similar to the speaker code approach for speaker adaptation studied in ASR, e.g.,
Ossama Abdel-Hamid, Hui Jiang, "Fast speaker adaptation of hybrid NN/HMM model for speech recognition based on discriminative learning of speaker code", ICASSP 2013
This should be mentioned in the paper. ASR researchers later find that using fixed speaker embeddings such i-vectors can work equally well (or even better). It would be interesting if the authors could present some experimental results on that. The benefit is that you may be able to do speaker adaptation fairly easily, without learning the speaker embedding as the presented approach. 
--in the second of paragraph of Sec. 3, you mentioned that the phoneme duration and frequency models are separated in this work. Can you explain your motivation?
--the CTC paper from A. Graves should be cited in section 3.1. Also, CTC is not a good alignment model for ASR, as it is trained by marginalizing over all the possible alignments. For ASR, the alignments from CTC may be very biased from the ground truth. I am curious how much segmentation error do you have, and have you tried some other segmentation models, e.g., segmental RNN?
--in Table 1, can you present comparisons between Deep Voice and Tocotron with the same sample frequency?
--is Deep Voice 2 trained end-to-end, or the modules are trained separately? It would be helpful to readers to clarify that.
--the speaker embedding are used as input to several different modules, some ablation experiments in the appendix would be helpful to see how much difference it would make for each module.