Robust speaking rate estimation using broad phonetic class recognition Jiahong Yuan and Mark Liberman University of Pennsylvania Mar. 16, 2010

Yuan and Liberman: ICASSP Introduction Speaking rate has been found to be related to many factors (Yuan et al. 2006, Jacewicz et al. 2009): young people > old people northern speakers > southern speakers (American English) male speakers > female speakers long utterances > short utterances emotion, style, conversation topics, foreign accent, etc. Listeners ‘normalize’ speaking rate in speech perception (Miller and Liberman 1979); and speaking rate affects listeners’ attitudes to the speaker and the message (Megehee et al. 2003). Speaking rate also affects the performance of automatic speech recognition. Fast and slow speech lead to a higher word error rate (Siegler and Stern, 1995, Mirghafori et al, 1996).

Yuan and Liberman: ICASSP Introduction The conventional method for building a robust speaking rate estimator is to do syllable detection based on energy measurements and peak picking algorithms (Mermelstein 1975, Morgan and Fosler-Lussier 1998, Xie amd Niyogi 2006, Wang and Narayanan 2007, Zhang and Glass 2009). The studies have utilized full-band energy, sub-band energy, and sub- band energy correlation in syllable detection. Howitt (2000) demonstrated that energy in a fixed frequency band ( Hz) was as good for finding vowel landmarks as the energy at the first formant. Our study on syllable detection using the convex-hull algorithm (Mermelstein 1975) also shows that this frequency band has the best results.

Yuan and Liberman: ICASSP Introduction

Yuan and Liberman: ICASSP Introduction Using automatic speech recognition for speaking rate estimation would be a natural approach, however: The performance of ASR is much affected by speaking rate; ASR only works well when the training and test data are from the same speech genre, dialect, or language. For speaking rate estimation, what is important is not the recognition word error rate (WER) or phone error rate. A recognizer that can robustly distinguish between vowels and consonants would be sufficient.  broad phonetic class recognition for speaking rate estimation

Yuan and Liberman: ICASSP Introduction The broad phonetic classes possess more distinct spectral characteristics than the phones within the same broad phonetic classes. It has been found that almost 80% of misclassified phonemes were within the same broad phonetic class (Halberstadt and Glass 1997). Broad phonetic classes have been applied for improved phone recognition, and have been shown to be more robust in noise (Scanlon et al. 2007, Sainath and Zue 2008). Broad phonetic classes have also been used in large vocabulary ASR to overcome the issue of data sparsity and robustness, e.g., decision tree-based clustering with broad phonetic classes.

Yuan and Liberman: ICASSP Data and Method A broad phonetic class recognizer was built using 34,656 speaker turns from the SCOTUS corpus (~ 66 hours). The speaker turns were first forced aligned using the Penn Phonetics Lab Forced Aligner, and then, the aligned phones were mapped to broad phonetic classes for training. The acoustic models are mono broad-class three-state HMMs. Each HMM has 64 Gaussian Mixture components on 39 PLP coefficients. The language model is broad-class bigram probabilities. To compare, a general monophone recognizer was also built using the same data. The training was done using the HTK Toolkit, and the HVite tool in HTK was used for testing.

Yuan and Liberman: ICASSP Data and Method ClassPhonetic categorization CMU dictionary phones Number of tokens V1Stressed vowelsVowel classes: 1 and 2 447,665 V0unstressed vowelsVowel class: 0336,278 SStops and affricates B CH D G JH K P T 418,994 FFricatives DH F HH S SH TH V Z ZH 352,968 NNasals M N NG 208,178 GGlides and liquids L R W Y 203,683 PPauses and non- speech --149,268

Yuan and Liberman: ICASSP Evaluation on TIMIT There is no standard scoring toolkit for syllable detection evaluation. We follow the evaluation method in Xie and Niyogi (2006): Find the middle points of the vowel segments from the recognition output. A point is counted as correct if it is located within a syllabic segment, otherwise, it is counted as incorrect. If two or more points are located within a syllabic segment, only one of them is counted as correct and the others as incorrect. The incorrect points are insertion errors, and the syllabic segments that don’t have any correct points are deletion errors. Deletion and insertion error rates are both calculated against the number of syllabic segments in the testing data.

Yuan and Liberman: ICASSP Evaluation on TIMIT There are 1,344 utterances and 17,190 syllabic segments in the testing data, which includes all the utterances in the TIMIT test dataset excluding SA1 and SA2 utterances.

Yuan and Liberman: ICASSP Effect of Language Model Language model has a larger effect on monophone recognition than on broad phonetic class recognition. In the following experiments using broad phonetic class models, the grammar scale factor was set to be 2.5.

Yuan and Liberman: ICASSP Error analysis There were 7,448 outside insertions in total, among which: /r, l, y, w/: 3635 (48.8%) /q/: 1411 (18.9%) - “a glottal stop that “may be an allophone of t, or may mark an initial vowel or a vowel-vowel boundary”. The syllabic nasals and laterals, /el, em, en, eng/, and the schwa vowels, /ax, ax-h, ax-r/, are more likely to be deleted. The diphthongs, /aw, ay, ey, ow, oy/, are more likely to have inside insertions.

Yuan and Liberman: ICASSP Error analysis

Yuan and Liberman: ICASSP Evaluation on Switchboard The ICSI manual transcription portion of the Switchboard telephone conversation speech was used for testing. We ran the broad class recognizer on the entire utterances, and let the recognizer handle pauses and non-speech segments in the utterances. To calculate the detected speaking rate, we simply counted the number of vowels, both V1 and V0, in the recognition of an utterance, and divided the number by the length of the utterance.

Yuan and Liberman: ICASSP Evaluation on Foreign Accented English 200 self-introductions selected from the CSLU foreign accented English corpus were used for testing. correlation: 0.898; mean error: -0.01; stddev error: 0.36.

Yuan and Liberman: ICASSP Evaluation on Mandarin Broadcast News 5,000 utterances randomly selected from the Hub-4 Mandarin Broadcast News corpus were used for testing. No language models were involved. correlation:.755;mean error:.055; stddev error:.730.

Yuan and Liberman: ICASSP Conclusion We built a broad phonetic class recognizer, and applied it to syllable detection and speaking rate estimation. Its performance is comparable to state-of-the-art syllable detection and speaking rate estimation algorithms, and it is robust for different speech genres and different languages without tuning any parameters. Unlike the previous algorithms, the broad class phonetic recognizer can automatically handle pauses and non-speech segments. This presents a great advantage for estimating speaking rate in natural speech. With no language models involved, the broad class recognizer still has good performance on syllable detection and speaking rate estimation, which opens up many opportunities for application.