REVIEW 3 major objections 4 minor 43 references
Benchmarking Neural Speech Codec Intelligibility with SITool
T0 review · 3 major / 4 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read STOI and ESTOI track listener-rated codec intelligibility; WER does not.
desk verdict SITool is a genuinely useful open-source DRT/MRT toolkit with a careful subjective benchmark, but the WER conclusion rests on an ASR mismatch and the correlation claims need statistical backing. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing instrument is SITool, a Flask-based web application that administers closed-set rhyme tests: listeners choose the word they heard from two options (DRT) or six (MRT). DRT pairs are organized by six distinctive acoustic features (voicing, nasality, sustension, sibilation, graveness, compactness), and scores are chance-corrected with P(c) = (R - W)/(R + W) * 100. Analysis rests on a Linear Mixed-Effects Model over codec condition, talker gender, and wordlist, plus Pearson correlations between mean subjective scores and the objective metrics STOI, ESTOI, and WER.
What would settle it
Run a DRT study with several male and female talkers per wordlist and recompute per-condition correlations; if WER correlates strongly with subjective scores (for example, r > 0.5), or if STOI and ESTOI correlations fall well below 0.9 after gender and wordlist averaging, the paper's central claim is contradicted.
Extended reading notes
Core claim
On the paper's own terms, the discovery is a measurement result: across 13 codecs evaluated with DRT, subjective intelligibility correlates strongly with STOI (Pearson r = 0.958) and ESTOI (r = 0.890) once results are averaged over talker gender and wordlist, while WER is uncorrelated (r = -0.15 averaged). Because the correlation is much weaker before averaging (STOI r = 0.595, ESTOI r = 0.499), the objective metrics fail to capture the gender and wordlist variation that listeners show. The paper further finds that the female reference signal was less intelligible than the male one (85 vs 91.87), and that this gender gap propagates through codec comparisons, so future tests should control talker-specific effects.
Load-bearing premise
The crowdsourced reference recordings are assumed to be equally intelligible across talker genders, yet the female reference scored notably lower (85 vs 91.87), so gender comparisons and some codec rankings could reflect the recordings rather than the codecs.
Editorial extensions
If this is right
- STOI and ESTOI can serve as screening metrics for codec intelligibility when talker gender and wordlist are averaged, but they should not be trusted for per-condition comparisons.
- WER computed with an ASR system is not a valid proxy for DRT intelligibility, at least for single-word closed-set tests.
- Below roughly 1.1 kbps, neural codec intelligibility drops relative to the uncoded signal, so ultra-low-bitrate codecs still have room for improvement in intelligibility.
- Future SITool deployments should include reference signals and pre-test their intelligibility, since talker-specific and wordlist-specific effects can masquerade as codec performance.
- DAC-S, despite a lower bitrate than EVS, showed higher subjective intelligibility for both genders, indicating neural codecs can beat traditional ones on intelligibility.
Reading between the lines
- Because DRT is closed-set and shows ceiling effects, an open-set SUS test might reveal intelligibility differences that STOI and ESTOI averaging hides; the paper itself points toward SUS as future work.
- The gender gap in the reference signal suggests codec rankings could shift if re-tested with multiple talkers per gender, so a natural extension is to build a SITool benchmark with balanced multi-talker stimuli.
- The weak pre-averaging correlation implies that a per-condition intelligibility heatmap from STOI or ESTOI would miss exactly the phoneme-level failures (such as /f/ and /θ/ in graveness items) that subjective DRT surfaces; combining objective metrics with phoneme-level error analysis may be more informative than any single score.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript presents SITool, an open Flask-based web application for conducting DRT and MRT intelligibility tests, and uses it to benchmark 14 codec conditions (13 codecs plus the original reference signal, at various bitrates) with crowdsourced and in-house listeners. It reports phoneme-level feature patterns, compares subjective DRT scores with STOI, ESTOI, and Whisper-based WER, and concludes that STOI/ESTOI correlate with subjective scores only after aggregating over gender and wordlist, while WER does not correlate. The paper also documents a lower female reference intelligibility and wordlist-specific phoneme effects, and discusses the mismatch between objective metrics and subjective gender/wordlist variation.
Significance. At the level of a systems and benchmark paper, this is a useful contribution: SITool fills a real gap as an open, deployable tool for standardized DRT/MRT tests, and the study design is careful, with trap questions, a clean gold-standard reference, a synthetic lower anchor, language screening, and LMM/repeated-measures ANOVA analyses. The DRT feature heatmaps and the gender/wordlist interaction analyses are informative for codec developers. However, the central statistical claim about WER is not yet supported, because the WER probe is not valid for the isolated-word DRT stimuli and because the correlation analysis lacks formal inference. With the WER claim repaired or carefully qualified, the toolkit's value stands.
major comments (3)
- [§4.2, Table 1, §4.3] The conclusion that 'only STOI and ESTOI, not WER, significantly correlate with subjective results' is not supported by the WER measurements as reported. Whisper-large is applied to isolated, single-word DRT utterances, and Table 1 shows a WER of 0.25 (female) and 0.19 (male) on the unprocessed reference signal; the authors themselves note in §4.2 that 'single-word utterances pose a challenge to Whisper.' Under those conditions, the near-zero WER correlation (§4.3: r = -0.11/-0.15) is an artifact of an ASR system failing on the task in general, not evidence that WER is unrelated to intelligibility for these codecs. The authors should re-compute WER with an ASR system validated on isolated-word or closed-set stimuli, or with a human word-recognition baseline on the same stimuli, or explicitly restrict the conclusion to the fact that their chosen ASR configuration did not track the subjective scores. As written, the headline comparative claim needs a load-bearing revision.
- [§4.3] Pearson correlations are reported without significance tests, confidence intervals, or any correction for the non-independence of observations repeated over codec conditions, wordlists, and genders, even though the abstract uses the word 'significantly.' The increases from r=0.595 to r=0.958 for STOI and from r=0.499 to r=0.890 for ESTOI after averaging are described descriptively, but the number of independent points after averaging is only the number of codec conditions (approximately 14), and with no interval estimate the strength of the claim is unclear. Please add bootstrap or nested correlation confidence intervals, test whether the averaged correlations differ from zero and from each other, and report the N underlying each correlation.
- [§4.1] The gender comparison is built on reference materials that already differ in intelligibility (female M=85, SD=9.66 vs male M=91.87, SD=8.23), and the authors acknowledge in §4.1 that 'these gender-specific differences already exist at the reference level.' Since the paper uses gender and wordlist interactions to argue that objective metrics fail to capture subjective variation, the analysis would be more convincing with a per-talker or per-word-pair control for the reference-level gender difference. The limitation is stated, but it should be treated as a constraint on the reported gender-related rankings rather than only as a future-work comment.
minor comments (4)
- [§4.2] The first sentence contains a typo: 'We show ojective results' should read 'We show objective results.'
- [Table 1] The 'kHz' column lists values of 8, 16, and 24 for different codecs; please clarify whether this denotes the internal codec sampling rate before resampling to the 16 kHz test rate, so that the column is not misread as the test condition.
- [Figure 3] The heatmap text for feature accuracies is very small, and the six distinctive features are only implicit from the DRT literature; adding a sentence that defines graveness, compactness, and the other features would help readers interpret the diagnostic analysis.
- [§1 and §3.1] The comparison with existing DRT/MRT tools is brief; a short capabilities table contrasting SITool with the Qualtrics scripts of [15] and the NIST MRT tool would make the contribution claim more concrete.
Circularity Check
No significant circularity: the paper's claims rest on direct subjective measurements and external objective metrics, with no fitted parameters or self-referential derivation chain.
full rationale
The paper reports an empirical benchmark of 13 speech codecs using DRT and MRT subjective tests, then compares those directly measured intelligibility scores with objective metrics (STOI, ESTOI, WER). There is no derivation chain in which a quantity is defined in terms of a target result, no parameter fitted to a subset and then renamed as a prediction, and no invocation of a uniqueness theorem or ansatz smuggled in via self-citation. The subjective DRT scores are obtained from human listeners via the newly introduced SITool; the objective metrics are computed by pre-existing external algorithms (STOI, ESTOI, Whisper ASR). The correlation analysis simply compares two independently obtained sets of measurements. Self-citations appear only for implementation details, e.g., the ForwardTacotron-based synthesis system 'with extensions similar to [35]' and StyleMelGAN as vocoder [36], and for an ESTOI reference [5, 6, 7]; none of these support the central correlation claims or define the outcome variables. The concern that Whisper's high WER on clean references (0.25 female, 0.19 male) may weaken the validity of the WER-based conclusion is a measurement-appropriateness issue, not circularity, because the WER values are not constructed from, or fitted to, the subjective DRT scores. The paper is self-contained against external benchmarks, and the central claim (STOI and ESTOI correlate with subjective DRT scores while WER does not) is an empirical finding from independent data rather than a reduction to its own inputs.
Assumptions & free parameters
assumptions (5)
- domain assumption Standard DRT/MRT word lists and scoring (ITU-T P.807) measure speech intelligibility for the tested codec conditions.
- domain assumption Crowdsourced listeners who pass B1/B2 English screening and use headphones give reliable intelligibility judgments.
- domain assumption STOI and ESTOI are valid objective predictors of intelligibility for neural and traditional codec outputs, including across resampling.
- domain assumption Whisper large word error rate on isolated rhyming words is a meaningful objective intelligibility measure.
- domain assumption The audio stimuli from Lechler and Wojcicki [15] are suitable and free of talker or recording confounds.
Cite this review
Pith. "Pith review of Benchmarking Neural Speech Codec Intelligibility with SITool." pith.science (2026). https://pith.science/paper/R6OAVFHV
@misc{pith2026250601731,
author = {Pith},
title = {Pith review of: Benchmarking Neural Speech Codec Intelligibility with SITool},
year = {2026},
howpublished = {\url{https://pith.science/paper/R6OAVFHV}},
note = {Machine review of arXiv:2506.01731}
}
read the original abstract
Speech intelligibility assessment is essential for evaluating neural speech codecs, yet most evaluation efforts focus on overall quality rather than intelligibility. Only a few publicly available tools exist for conducting standardized intelligibility tests, like the Diagnostic Rhyme Test (DRT) and Modified Rhyme Test (MRT). We introduce the Speech Intelligibility Toolkit for Subjective Evaluation (SITool), a Flask-based web application for conducting DRT and MRT in laboratory and crowdsourcing settings. We use SITool to benchmark 13 neural and traditional speech codecs, analyzing phoneme-level degradations and comparing subjective DRT results with objective intelligibility metrics. Our findings show that, while neural speech codecs can outperform traditional ones in subjective intelligibility, only STOI and ESTOI - not WER - significantly correlate with subjective results, although they struggle to capture gender and wordlist-specific variations observed in subjective evaluations.
Figures
Reference graph
Works this paper leans on
-
[1]
Introduction Neural speech codecs have gained increasing interest for their ability to outperform traditional coding schemes, especially at very low bitrates [1]. Unlike traditional approaches, they use data-driven methods that avoid costly tuning via listening tests [1] but may hallucinate content due to their generative na- ture. Thus, performance evalu...
-
[2]
Benchmarked Speech Codecs We evaluate the intelligibility of various publicly available state-of-the-art neural codecs and, for a full performance evalu- ation, compare them with prominent traditional speech codecs. In this work, we consider LPCNet3 [20] at 1.6 kbps, an early solution that combines classical signal processing and deep neural networks to d...
work page Pith review arXiv 2025
-
[3]
SITool - Speech Intelligibility Toolkit for Subjective Evaluation We developed a standalone Flask-based web application that supports both DRT and MRT testing and can be deployed in laboratory settings or on crowdsourcing platforms like Ama- zon Mechanical Turk (AMT) 5. Our tool allows the integration of consent forms, language proficiency assessments, pr...
-
[4]
Results 4.1. Subjective Speech Intelligibility Evaluation Subjective intelligibility results for male and female talkers are shown in Figure 2. The results show that the female reference signal (M=85, SD=9.66) has a noticeably lower intelligibility score than the male reference signal (M=91.87, SD=8.23). This pattern can be observed across most codecs, ex...
-
[5]
Conclusions We benchmarked 13 speech codecs, from traditional to state-of- the-art GAN-based codecs, on intelligibility using the newly de- veloped SITool. Unlike speech quality evaluation, DRT offered a more detailed analysis of phoneme-specific degradations and distinctive acoustic features. Our results show that below 1100 bps, the intelligibility of n...
-
[6]
M. Kim and J. Skoglund, “Neural speech and audio coding: Modern AI technology meets traditional codecs [special issue on model-based and data-driven audio signal processing],”IEEE Sig- nal Processing Magazine, vol. 41, no. 6, pp. 85–93, 2025
work page 2025
-
[7]
Methods for subjective determination of transmission quality,
ITU-T Rec. P.800, “Methods for subjective determination of transmission quality,” 1996
work page 1996
-
[8]
ITU-R Recommenda- tion BS.1534-3: Method for the subjective assessment of interme- diate quality level of audio systems,
International Telecommunication Union, “ITU-R Recommenda- tion BS.1534-3: Method for the subjective assessment of interme- diate quality level of audio systems,” October 2015
2015
Show all 43 references
-
[9]
ViSQOL v3: An open source production ready objec- tive speech and audio metric,
M. Chinen, F. S. Lim, J. Skoglund, N. Gureev, F. O’Gorman, and A. Hines, “ViSQOL v3: An open source production ready objec- tive speech and audio metric,” in 2020 twelfth international con- ference on quality of multimedia experience (QoMEX) . IEEE, 2020, pp. 1–6
2020
-
[10]
An algorithm for predicting the in- telligibility of speech masked by modulated noise maskers,
J. Jensen and C. H. Taal, “An algorithm for predicting the in- telligibility of speech masked by modulated noise maskers,” IEEE/ACM Transactions on Audio, Speech, and Language Pro- cessing, vol. 24, no. 11, pp. 2009–2022, 2016
2009
-
[11]
Neural speech coding for real-time communications using constant bitrate scalar quantization,
A. Brendel, N. Pia, K. Gupta, L. Behringer, G. Fuchs, and M. Mul- trus, “Neural speech coding for real-time communications using constant bitrate scalar quantization,” IEEE Journal of Selected Topics in Signal Processing, 2024
2024
-
[12]
High-fidelity audio compression with improved RVQGAN,
R. Kumar, P. Seetharaman, A. Luebs, I. Kumar, and K. Kumar, “High-fidelity audio compression with improved RVQGAN,” in Advances in Neural Information Processing Systems , A. Oh, T. Naumann, A. Globerson, K. Saenko, M. Hardt, and S. Levine, Eds., vol. 36. Curran Associates, Inc...
2023
-
[13]
Speech quality evaluation of neural audio codecs,
T. Muller, S. Ragot, L. Gros, P. Philippe, and P. Scalart, “Speech quality evaluation of neural audio codecs,” in Proc. Interspeech, 2024
2024
-
[14]
Quantifying the relation between speech quality and speech intelligibility,
J. E. Preminger and D. J. V . Tasell, “Quantifying the relation between speech quality and speech intelligibility,” Journal of Speech, Language, and Hearing Research, vol. 38, no. 3, pp. 714– 725, 1995
1995
-
[15]
The SUS test: A method for the assessment of text-to-speech synthesis intelligibility us- ing semantically unpredictable sentences,
C. Beno ˆıt, M. Grice, and V . Hazan, “The SUS test: A method for the assessment of text-to-speech synthesis intelligibility us- ing semantically unpredictable sentences,” Speech communica- tion, vol. 18, no. 4, pp. 381–392, 1996
1996
-
[16]
ANSI/ASA S3.2-2020 method for measuring the intelligibility of speech over communication systems,
Acoustical Society of America, “ANSI/ASA S3.2-2020 method for measuring the intelligibility of speech over communication systems,” 2020
2020
-
[17]
Intelligibility and acceptability testing for speech technology,
A. Schmidt-Nielsen, “Intelligibility and acceptability testing for speech technology,” Applied speech technology , pp. 194–231, 1995
1995
-
[18]
Subjective test methodology for assessing speech intelligibility,
ITU-T Rec. P.807, “Subjective test methodology for assessing speech intelligibility,” 2016
2016
-
[19]
Subjective evaluation of speech quality with a crowdsourcing approach,
ITU-T Rec. P.808, “Subjective evaluation of speech quality with a crowdsourcing approach,” 2021
2021
-
[20]
Crowdsourced multilingual speech intelligibility testing,
L. Lechler and K. Wojcicki, “Crowdsourced multilingual speech intelligibility testing,” in ICASSP, 2024, pp. 1441–1445
2024
-
[21]
Towards the next generation of web-based experiments: A case study as- sessing basic audio quality following the ITU-R recommendation BS.1534 (MUSHRA),
M. Schoeffler, F.-R. St ¨oter, B. Edler, and J. Herre, “Towards the next generation of web-based experiments: A case study as- sessing basic audio quality following the ITU-R recommendation BS.1534 (MUSHRA),” in Proc. Web Audio Conference , Paris, France, January 2015
2015
-
[22]
Drt/***: Programs to administer and score the diagnostic rhyme test,
R. J. Calistri and H. J. Kallman, “Drt/***: Programs to administer and score the diagnostic rhyme test,”Behavior Research Methods, Instruments, & Computers, vol. 18, no. 1, pp. 57–58, 1986
1986
-
[23]
A short- time objective intelligibility measure for time-frequency weighted noisy speech,
C. H. Taal, R. C. Hendriks, R. Heusdens, and J. Jensen, “A short- time objective intelligibility measure for time-frequency weighted noisy speech,” in ICASSP, 2010, pp. 4214–4217
2010
-
[24]
Asr-based speech intelligibility pre- diction: A review,
M. Karbasi and D. Kolossa, “Asr-based speech intelligibility pre- diction: A review,”Hearing Research, vol. 426, p. 108606, 2022
2022
-
[25]
LPCNet: Improving neural speech synthesis through linear prediction,
J.-M. Valin and J. Skoglund, “LPCNet: Improving neural speech synthesis through linear prediction,” inICASSP. IEEE, 2019, pp. 5891–5895
2019
-
[26]
Soundstream: An end-to-end neural audio codec,
N. Zeghidour, A. Luebs, A. Omran, J. Skoglund, and M. Tagliasacchi, “Soundstream: An end-to-end neural audio codec,” IEEE/ACM Transactions on Audio, Speech, and Lan- guage Processing, vol. 30, pp. 495–507, 2022
2022
-
[27]
Neural dis- crete representation learning,
A. van den Oord, O. Vinyals, and k. kavukcuoglu, “Neural dis- crete representation learning,” in Advances in Neural Information Processing Systems, I. Guyon, U. V . Luxburg, S. Bengio, H. Wal- lach, R. Fergus, S. Vishwanathan, and R. Garnett, Eds., vol. 30. Curran Associates, ...
2017
-
[28]
Generative speech coding with predictive variance regularization,
W. B. Kleijn, A. Storus, M. Chinen, T. Denton, F. S. C. Lim, A. Luebs, J. Skoglund, and H. Yeh, “Generative speech coding with predictive variance regularization,” in ICASSP, 2021, pp. 6478–6482
2021
-
[29]
High fidelity neural audio compression,
A. D ´efossez, J. Copet, G. Synnaeve, and Y . Adi, “High fidelity neural audio compression,” Transactions on Machine Learning Research, 2023
2023
-
[30]
Low bitrate high-quality rvqgan- based discrete speech tokenizer,
S. Shechtman and A. Dekel, “Low bitrate high-quality rvqgan- based discrete speech tokenizer,” in Interspeech 2024, 2024, pp. 4174–4178
2024
-
[31]
Generative de-quantization for neural speech codec via latent diffusion,
H. Yang, I. Jang, and M. Kim, “Generative de-quantization for neural speech codec via latent diffusion,” in ICASSP, 2024, pp. 1251–1255
2024
-
[32]
Semanticodec: An ultra low bitrate semantic audio codec for general sound,
H. Liu, X. Xu, Y . Yuan, M. Wu, W. Wang, and M. D. Plumb- ley, “Semanticodec: An ultra low bitrate semantic audio codec for general sound,” IEEE Journal of Selected Topics in Signal Pro- cessing, pp. 1–14, 2024
2024
-
[33]
Moshi: A speech-text foundation model for real-time dialogue,
A. D ´efossez, L. Mazar´e, M. Orsini, A. Royer, P. P´erez, H. J´egou, E. Grave, and N. Zeghidour, “Moshi: A speech-text foundation model for real-time dialogue,” 2024
2024
-
[34]
Ultra-low-bitrate speech coding with pretrained transformers,
A. Siahkoohi, M. Chinen, T. Denton, W. B. Kleijn, and J. Skoglund, “Ultra-low-bitrate speech coding with pretrained transformers,” in Interspeech 2022, 2022, pp. 4421–4425
2022
-
[35]
SNAC: Multi- scale neural audio codec,
H. Siuzdak, F. Gr ¨otschla, and L. A. Lanzend¨orfer, “SNAC: Multi- scale neural audio codec,” in Audio Imagination: NeurIPS Work- shop AI-Driven Speech, Music, and Sound Generation, 2024
2024
-
[36]
The adaptive multi-rate speech coder,
E. Ekudden, R. Hagen, I. Johansson, and J. Svedberg, “The adaptive multi-rate speech coder,” in IEEE Workshop on Speech Coding Proceedings. Model, Coders, and Error Criteria (Cat. No.99EX351), 1999, pp. 117–119
1999
-
[37]
The adaptive multi- rate wideband speech codec (AMR-WB),
B. Bessette, R. Salami, R. Lefebvre, M. Jelinek, J. Rotola-Pukkila, J. Vainio, H. Mikkola, and K. Jarvinen, “The adaptive multi- rate wideband speech codec (AMR-WB),” IEEE Transactions on Speech and Audio Processing, vol. 10, no. 8, pp. 620–636, 2002
2002
-
[38]
Standardization of the new 3GPP EVS codec,
S. Bruhn, H. Pobloth, M. Schnell, B. Grill, J. Gibbs, L. Miao, K. J ¨arvinen, L. Laaksonen, N. Harada, N. Naka, S. Ragot, S. Proust, T. Sanda, I. Varga, C. Greer, M. Jel ´ınek, M. Xie, and P. Usai, “Standardization of the new 3GPP EVS codec,” in ICASSP, 2015, pp. 5703–5707
2015
-
[39]
Codec 2 - open source speech coding at 2400 bits/s and below,
D. Rowe, “Codec 2 - open source speech coding at 2400 bits/s and below,”TAPR and ARRL 30th Digital Communications Con- ference, pp. 80–84, 2011
2011
-
[40]
The AudioLabs system for the Blizzard Challenge 2023,
F. Zalkow, P. Sani, M. Fast, J. Bauer, M. Joshaghani, K. Kayyar, E. A. P. Habets, and C. Dittmar, “The AudioLabs system for the Blizzard Challenge 2023,” in Proceedings of the Blizzard Chal- lenge Workshop, Grenoble, France, 2023, pp. 63–68
2023
-
[41]
StyleMelGAN: An efficient high-fidelity adversarial vocoder with temporal adaptive normal- ization,
A. Mustafa, N. Pia, and G. Fuchs, “StyleMelGAN: An efficient high-fidelity adversarial vocoder with temporal adaptive normal- ization,” in ICASSP, Toronto, Canada, 2021, pp. 6034–6038
2021
-
[42]
Dataset of sounds of symptoms associated with respiratory sickness,
S. Patel, A. Rivas, and D. Psaltos, “Dataset of sounds of symptoms associated with respiratory sickness,” Nov 2020. [Online]. Available: osf.io/tmkud
2020
-
[43]
V oice analytics in the wild: Validity and predictive accuracy of common audio- recording devices,
F. Busquet, F. Efthymiou, and C. Hildebrand, “V oice analytics in the wild: Validity and predictive accuracy of common audio- recording devices,” Behavior Research Methods , vol. 56, no. 3, pp. 2114–2134, 2024
2024
Reviewed August 7, 2026 · model on record in the stance chip above.
Discussion (0). Sign in to comment.