  The performance of automatic speech recognition (ASR) systems has advanced
substantially in recent years, particularly for languages for which a large
amount of transcribed speech is available. Unfortunately, for low-resource
languages, such as minority languages, regional languages or dialects, ASR
performance generally remains much lower. In this study, we investigate whether
data augmentation techniques could help improve low-resource ASR performance,
focusing on four typologically diverse minority languages or language variants
(West Germanic: Gronings, West-Frisian; Malayo-Polynesian: Besemah, Nasal). For
all four languages, we examine the use of self-training, where an ASR system
trained with the available human-transcribed data is used to generate
transcriptions, which are then combined with the original data to train a new
ASR system. For Gronings, for which there was a pre-existing text-to-speech
(TTS) system available, we also examined the use of TTS to generate ASR
training data from text-only sources. We find that using a self-training
approach consistently yields improved performance (a relative WER reduction up
to 20.5% compared to using an ASR system trained on 24 minutes of manually
transcribed speech). The performance gain from TTS augmentation for Gronings
was even stronger (up to 25.5% relative reduction in WER compared to a system
based on 24 minutes of manually transcribed speech). In sum, our results show
the benefit of using self-training or (if possible) TTS-generated data as an
efficient solution to overcome the limitations of data availability for
resource-scarce languages in order to improve ASR performance.
