# Enrichment of Oesophageal Speech: Voice Conversion with Duration-Matched Synthetic Speech as Target
## 6 Discussion  
In this study, we have employed and evaluated a novel DNN-based voice conversion system aimed at enriching OS. The evaluations involved subjective (preference test) as well as objective (STOI, ASR) aspects. The evaluations were performed on unprocessed-enriched pairs of samples from four OS speakers. Additionally, objective results of the proposed system were compared with those of our previous experiments. In the ASR evaluations, the results of WER improvement for the proposed system was not unanimous. However, for all the 3 ASR systems and all 4 speakers, our proposed system had better PWC scores compared to unprocessed OS. This means that our enrichment resulted in the ASR systems recognising more number of words in comparison to unprocessed OS. Correlation analysis suggests that more errors were found in speakers who underwent the laryngectomy more recently. This is expected, as these speakers have had less time to train and practice the techniques of OS production. This explains the higher WER and lower PWC for speakers 04M3 and 16M3 compared to the other two more proficient speakers. STOI as an objective intelligibility measure is usually applied in cases where already available clean signals are degraded with noise. In the case of OS, the original signal itself is degraded. Although the duration-matched SS is clean and aligns with the original OS and the enriched outputs, it cannot be considered as a clean reference in the true sense. Moreover, the reference SS signal was also the target of the VC process. Therefore, STOI scores in this case may be interpreted as a measure of similarity with the target SS (which is the goal of a VC task) rather than objective intelligibility. Nonetheless, for all the four speakers, a higher STOI value for the proposed system (over 62% to 73%) compared to unprocessed OS (45% to 55%) indicates that there was an improvement in intelligibility. Comparisons with previous methods also revealed improved intelligibility with the proposed method. The preference test revealed a preference for the proposed method for three of the four OS speakers. For the remaining fourth and the least proficient speaker, there was no preference for the proposed system, but no preference for unprocessed OS either. The listeners were mostly undecided. This is possibly because the speaker's intelligibility is poor, and although the conversion system helped in improving some spectral characteristics, some other characteristics of the speaker such as the long duration of the stimuli, phones and silences, and the resulting slow and unnatural rhythm, were present in the converted version too. In a previous experiment of enriching OS using a DNN-based VC system [31], there was an improvement in ASR scores. However, the listeners preferred a system that performed a simple fundamental frequency transformation, which did not improve ASR or intelligibility scores. Similarly, in another experiment [32], we did not achieve intelligibility improvement or a preference for the proposed method. Both the above experiments involved only one OS speaker (02M3), who also was one of the most intelligible speakers of the dataset. In the current study, with a novel strategy and more speakers, we have shown that our OS enrichment method improves intelligibility in addition to being preferred by listeners. The source-target alignment process is particularly problematic for OS and that was a limitation in the aforementioned previous systems. Using a duration-matched target and skipping the error-prone alignment process helped in overcoming that limitation and improving results. The proposed system can possibly be improved by using newer DNN VC technologies and newer speech synthesis systems. The speech synthesis method used in this study is relatively old and was specifically chosen for its ability to generate speech with forced durations. Although newer DNN based speech synthesis systems are of better quality, they do not have this ability.