Abstract: Previous work has shown that for low-resource source languages, automatic speech-to-text translation (AST) can be improved by pretraining an end-to-end model on automatic speech recognition (ASR) data from a high-resource language. However, it is not clear what factors --e.g., language relatedness or size of the pretraining data-- yield the biggest improvements, or whether pretraining can be effectively combined with other methods such as data augmentation. Here, we experiment with pretraining on datasets of varying sizes, including languages related and unrelated to the AST source language. We find that the best predictor of final AST performance is the word error rate of the pretrained ASR model, and that differences in ASR/AST performance correlate with how phonetic information is encoded in the later RNN layers of our model. We also show that pretraining and data augmentation yield complementary benefits for AST.

Introduction: Low-resource automatic speech-to-text translation (AST) has recently gained traction as a way to bring NLP tools to under-represented languages. An end-to-end approach BIBREF0, BIBREF1, BIBREF2, BIBREF3, BIBREF4, BIBREF5, BIBREF6 is particularly appealing for source languages with no written form, or for endangered languages where translations into a high-resource language may be easier to collect than transcriptions BIBREF7. However, building high-quality end-to-end AST with little parallel data is challenging, and has led researchers to explore how other sources of data could be used to help. A number of methods have been investigated. Several of these use transcribed source language audio and/or translated source language text in a multitask learning scenario BIBREF8, BIBREF3, BIBREF5 or to pre-train parts of the model before fine-tuning on the end-to-end AST task BIBREF3. Others assume, as we do here, that no additional source language resources are available, in which case transfer learning using data from language(s) other than the source language is a good option. In particular, several researchers have shown that low-resource AST can be improved by pretraining on an ASR task in some other language, then transferring the encoder parameters to initialize the AST model. For example, Bansal et al. BIBREF4 showed that pre-training on either English or French ASR improved their Spanish-English AST system (trained on 20 hours of parallel data) and Tian BIBREF9 got improvements on an 8-hour Swahili-English AST dataset using English ASR pretraining. Overall these results show that pretraining helps, but leave open the question of what factors affect the degree of improvement. For example, does language relatedness play a role, or simply the amount of pretraining data? Bansal et al. showed bigger AST gains as the amount of English pretraining data increased from 20 to 300 hours, and also found a slightly larger improvement when pretraining on 20 hours of English versus 20 hours of French, but they pointed out that the Spanish data contains many English code-switched words, which could explain the latter result. In related work on multilingual pretraining for low-resource ASR, Adams et al. BIBREF10 showed that pre-training on more languages helps, but it is not clear whether the improvement is due to including more languages, or just more data. To begin to tease apart these issues, we focus here on monolingual pretraining for low-resource AST, and investigate two questions. First, can we predict what sort of pretraining data is best for a particular AST task? Does it matter if the pretraining language is related to the AST source language (defined here as part of the same language family, since phonetic similarity is difficult to measure), or is the amount of pretraining data (or some other factor) more important? Second, can pretraining be effectively combined with other methods, such as data augmentation, in order to further improve AST results? To answer these questions, we use the same AST architecture and Spanish-English parallel data as Bansal et al. BIBREF4, but pretrain the encoder using a number of different ASR datasets: the 150-hour AISHELL corpus of Chinese as well as seven GlobalPhone languages, each with about 20 hours of data. We find that pretraining on a larger amount of data from an unrelated language is much better than pretraining on a smaller amount of data from a related language. Moreover, even when controlling for the amount of data, the WER of the ASR model from pretraining seems to be a better predictor of final AST performance than does language relatedness. Indeed, we show that there is a very strong correlation between the WER of the pretraining model and BLEU score of the final AST model—i.e., the best pretraining strategy may simply be to use datasets and methods that will yield the lowest ASR WER during pretraining. However, we also found that AST results can be improved further by augmenting the AST data using standard speed perturbation techniques BIBREF11. Our best results using non-English pretraining data improve the test set BLEU scores of an AST system trained on 20 hours of parallel data from 10.2 to 14.3, increasing to 15.8 with data augmentation. Finally, we analyze the representations learned by the models and show that better performance seems to correlate with the extent to which phonetic information is encoded in a linearly separable way in the later RNN layers.

Question: How many layers does their model have?
Answer: 10