NeurIPS 2020

Unsupervised Sound Separation Using Mixture Invariant Training

Review 1

Summary and Contributions: This paper introduces a method for training a sound source separation system from mixed signals and without the need for separated source signals at train time, i.e., in an unsupervised way. The idea is simple, generic and can be applied to any kind of neural architecture. Experiments on speech separation, speech enhancement and universal sound separation tasks using different datasets reveal that the method yields adequate results and can even be competitive with state-of-the-art supervised methods in mismatched conditions.

Strengths: To the best of my knowledge, this is the first unsupervised method for single channel source separation that yields competitive results with supervised methods in mismatched condition, which represents an important milestone for the field. The introduction and methodological parts of the paper are sound, clearly written and easy to follow. The simplicity and genericity of the method make it widely applicable, and unlock the possibility of training source separationmethods on massive, in-the-wild datasets.

Weaknesses: The main weakness of the paper is the experiment section 4 which is sometimes handwavy, hard to follow, and misses details, as detailed in the feedback section below.

Correctness: The claims, method and empirical methodology are mostly correct, up to the remarks listed below.

Clarity: The paper is well written and easy to follow except for section 4, as detailed below.

Relation to Prior Work: The relation to prior wor is clearly and correctly discussed in the paper.

Reproducibility: Yes

Additional Feedback: # After reading the other reviews and authors' rebuttal, I stand by my score of 8. All the reviewers were in favor of accepting, I think this is a strong paper whose strengths largely outweight its minor flaws. 1) In Fig. 2, the meanings of "matched" and "mismatched" are not cleary explained. I think mismatched means "trained with Libri2Mix and tested on WSJ0-2mix" and the other way around, but it is never clearly written. 2) Section 4.1.: The experimental design in this section seems inadequate, because the 1+1 source mixture case exactly corresponds to the supervized case. How do the authors distinguish between the "% of unsupervised data" in the training set and "unsupervised 1+1 mixtures" (whatever it means)? 3) L221: "we also consider using supervised data from a mismatched domain" -> which supervised data from which domain? Please detail this. 4) The way the dataset crafted by the authors was designed lacks details. WIll it be released for reproducibility? 5) Section 4.3: how was it ensured that the intersection of FUSS and the dataset was empty? 6) L274: what is the meaning of this phrase: "randomly zeroing out one of the supervised mixtures with probability p0" ? 7) L278: The terms MSi and SS are not properly explained. In particular, what is the meaning of the "single source mixture case" (L301) ? 8) L303: What is the meaning of "an additional 'separation consistency' loss" ? 9) L315: What is the meaning of "cases where sources occur together independently" ? 10) L326: "this remains challening because of the lack of ground truth": this is not a very good argument because there exists plenty of real datasets where individual sound sources are recorded separately from the same microphone array in the same environment, and can then be easiliy mixed together for testing purpose (e.g. the numerous ChiME challenges). Typos: -L76: improving performance universal sound separation -> improving performance of.. -L86: form -> from.

Review 2

Summary and Contributions: Proposes and demonstrates an effective technique for learning to separating multi-speaker audio by predicting mixtures of mixtures.

Strengths: 1. The idea is relatively simple yet insightful and evidently works well. 2. The problem is well motivated and certainly unsupervised source separation is an important problem. 3. Good empirical demonstrations on a variety of audio separation tasks. Overall, this is a nice paper, a simple yet clever approach to unsupervised source separation with well executed experiments and well written.

Weaknesses: It would be useful for the authors to comment on whether or not this algorithm can be used in tasks other than just audio and if so why not experiment on some to show that it generalizes to mixed images as well.

Correctness: Yes.

Clarity: Nicely written.

Relation to Prior Work: Yes

Reproducibility: Yes

Additional Feedback:

Review 3

Summary and Contributions: This paper proposed an unsupervised method, referred to as remixing and permutation invariant training (RemixPIT), for the sound separation task. The traditional supervised approaches use synthetic mixtures to do the training, which suffers from the big gap between the training data and real data. In RemixPIT, models trained on the mixture of mixtures require to separate them into several sources and remix the sources to approximate the original mixtures. Moreover, RemixPIT can be used with the supervised paradigm parallelly. RemixPIT has the advantage of leveraging the vastly available data in the wild.

Strengths: + This paper proposes a new unsupervised approach for sound separation, which can handle the differences in acoustic conditions and distribution of sources between training and inference process. Such a gap is really a big problem in this research field and RemixPIT provides a potential solution to it. + Extensive experiments on WSJ0-2mix and Libri2Mix demonstrates the effectiveness of RemixPIT. Besides, on the universal sound separation tasks, the proposed method also achieves state-of-the-art performance, especially on the reverberant cases. + RemixPIT has the advantage of leveraging the vastly available data in the wild.

Weaknesses: - The authors claim that synthetic mixture data is problematic and the proposed RemixPIT can tackle this problem. Can the authors provide the separation results on the cases of natural sound mixtures? Since the presented experiment results are still based on the synthetic mixtures, as well as the demo examples in the supplementary. - In part 3.2, the authors assume the number of mixtures to be 2 and the optimization of matrix A is based on the brute-force search. If the number of mixtures goes higher, the search space will exponentially rise. Could the authors present a potential solution for it? - For the speech separation experiments in part 4.1, is the rightmost model trained from scratch on the unsupervised training examples, or based on the model parameters trained with part supervised examples? - From Table 1, the improvement over the pure-supervised learning method is minor, especially for FUSS (16hr).

Correctness: The method is correct, but the supporting experiments for the claims can't be found in the paper. Please refer to the weakness part for the details.

Clarity: Yes.

Relation to Prior Work: Yes.

Reproducibility: No

Additional Feedback: ===== Post Rebuttal ====== It's a nice paper. However, as far as I am concerned, this paper just shows the separation results of simulated data. The experiment will be much stronger with recording results. I understand that it is almost impossible to compute the SNRi of simulated data. But other metrics such as WER can help. Moreover, this paper only has a modest improvement over the purely supervised learning method on FUSS. Combining the opinions above, I decided to give 6 for the final rating.

Review 4

Summary and Contributions: The paper proposes an unsupervised method for sound separation. Unlike supervised systems where sound sources are mixed together to form input and reference output pairs, this paper proposes a method which relies only on the mixtures. The proposed method is applied on speech separation, speech enhancement and sound separation.

Strengths: The proposed method is novel and interesting. It is an unsupervised method which opens up the possibility of training sound separation models directly on real world data. The paper also provides some interesting insight and discussions on the proposed method. Demo examples are also provided.

Weaknesses: I think the paper is a bit weak on the experimental side. All experiments are done on synthetically created training data. This pulls down the paper a bit as the main advantage of this unsupervised training is the ability to use real world data. One of the dataset even has not been released yet. Detailed comments are below.

Correctness: Yes

Clarity: yes

Relation to Prior Work: yes

Reproducibility: Yes

Additional Feedback: Overall, I liked this paper. It is a simple yet clever reformulation of PIT to train sound separation models in an unsupervised manner. The unsupervised training method opens up the possibility of learning from real world data. I also liked the discussion provided in the paper. I think the primary limiting factor of this paper is the experimental evaluation. Given that the focus is on unsupervised learning and developing the ability to use real world data, it would have been more convincing to actually show that empirically. All of the experiments are done on synthetically mixed training data. I think there should have been at least one experiment where RemixPIT training is done using some real world data rather than synthetically mixed ones, and compared with supervised training. How much difference between the two exists ? Matched and mismatched conditions in this case would have been also interesting to see. Some additional comments. Using just upto 2 sources in speech separation seems a bit limiting again. In speech separation experiments (Fig 2), what is a statistically significant change in SN-SNRi ? In Fig 2, in some cases the performance goes up while going from fully supervised on one extreme to fully unsupervised on the other ? While one can argue that this might be expected for mismatched-supervised cases, this also happens for matched-supervised cases, e.g. matched-supervised 2 sources. Can authors comment a bit on it ? For speech enhancement experiments, why draw x1 from speech+noise and x2 from noise only ? WHy not train by just relying on the mixtures, speech+noise set ? It would be better to provide some additional details on the speech enhancement experiments. Is it a matched noise type test or mismatched noise type ? ---------- Overall I think this paper should be accepted. Updating the score to reflect that.