This paper describes a voice conversion system designed with the aim of improving the intelligibility and pleasantness of oesophageal voices. Two different systems have been built, one to transform the spectral magnitude and another one for the fundamental frequency, both based on DNNs. Ahocoder has been used to extract the spectral information (mel cepstral coefficients) and a specific pitch extractor has been developed to calculate the fundamental frequency of the oesophageal voices. The cepstral coefficients are converted by means of an LSTM network. The conversion of the intonation curve is implemented through two different LSTM networks, one dedicated to the voiced unvoiced detection and another one for the prediction of F0 from the converted cepstral coefficients. The experiments described here involve conversion from one oesophageal speaker to a specific healthy voice. The intelligibility of the signals has been measured with a Kaldi based ASR system. A preference test has been implemented to evaluate the subjective preference of the obtained converted voices comparing them with the original oesophageal voice. The results show that spectral conversion improves ASR while restoring the intonation is preferred by human listeners.

State of the art systems for voice conversion have been shown to generate highly natural sounding converted speech. Voice conversion techniques have also been applied to alaryngeal speech, with the aim of improving its quality or its intelligibility. In this paper, we present an attempt to apply a voice conversion strategy based on phonetic posteriorgrams (PPGs), which produces very high quality converted speech, to improve the characteristics of esophageal speech. The main advantage of this PPG based architecture lies in the fact that it is able to convert speech from any source, without the need to previously train the system with a parallel corpus. However, our results show that the PPG approach degrades the intelligibility of the converted speech considerably, especially when the input speech is already poorly intelligible. In this paper two systems are compared, an LSTM based one-to-one conversion system, which is referred to as the baseline, and the new system using phonetic posteriorgrams. Both spectral parameters and f0 are converted using DNN (Deep Neural Network) based architectures. Results from both objective and subjective evaluations are presented, showing that although ASR (Automated Speech Recognition) errors are reduced, original esophageal speech is still preferred by subjects.

Oesophageal speakers face a multitude of challenges, such as difficulty in basic everyday communication and inability to interact with digital voice assistants. We aim to quantify the difficulty involved in understanding oesophageal speech (in humanhuman and human-machine interactions) by measuring intelligibility and listening effort. We conducted a web-based listening test to collect these metrics. Participants were asked to transcribe and then rate the sentences for listening effort on a 5-point Likert scale. Intelligibility, calculated as Word Error Rate (WER), showed significant correlation with user rated effort. Speaker type (healthy or oesophageal) had a major effect on intelligibility and effort. Listeners familiar with oesophageal speech did not have any advantage over non familiar listeners in correctly understanding oesophageal speech. However, they reported lesser effort in listening to oesophageal speech compared to non familiar listeners. Additionally, we calculated speakerwise mean WERs and they were significantly lower when compared to an automatic speech recognition system.

Communication is a huge challenge for oesophageal speakers, be it for interactions with fellow humans or with digital voice assistants. We aim to quantify these communication challenges (both human-human and human-machine interactions) by measuring intelligibility and Listening Effort (LE) of Oesophageal Speech (OS) in comparison to Healthy Laryngeal Speech (HS). We conducted two listening tests (one web-based, the other in laboratory settings) to collect these measurements. Participants performed a sentence recognition and LE rating task in each test. Intelligibility, calculated as Word Error Rate, showed significant correlation with self-reported LE ratings. Speaker type (healthy or oesophageal) had a major effect on intelligibility and effort. More LE was reported for OS compared to HS even when OS intelligibility was close to HS. Listeners familiar with OS reported less effort when listening to OS compared to nonfamiliar listeners. However, such advantage of familiarity was not observed for intelligibility. Automatic speech recognition scores were higher for OS compared to HS.

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