A DAPT - AND -A DJUST : O VERCOMING THE L ONG -T AIL P ROBLEM OF M ULTILINGUAL S PEECH R ECOGNITION
Anonymous authors
Paper under double-blind review
A BSTRACT
One crucial challenge of real-world multilingual speech recognition is the longtailed distribution problem, where some resource-rich languages like English have abundant training data, but a long tail of low-resource languages have varying amounts of limited training data.
To overcome the long-tail problem, in this paper, we propose Adapt-and-Adjust (A2), a transformer-based multitask learning framework for end-to-end multilingual speech recognition.
The A framework overcomes the long-tail problem via three techniques: (1) exploiting a pretrained multilingual language model (mBERT) to improve the performance of low-resource languages; (2) proposing dual adapters consisting of both languagespeciﬁc and language-agnostic adaptation with minimal additional parameters; and (3) overcoming the class imbalance, either by imposing class priors in the loss during training or adjusting the logits of the softmax output during inference.
Extensive experiments on the CommonVoice corpus show that A2 signiﬁcantly outperforms conventional approaches.
I NTRODUCTION
Deploying a single Automatic Speech Recognition (ASR) model to recognize multiple languages is highly desired but very challenging for real-world multilingual ASR scenarios due to the wellknown long-tailed distribution challenge, namely, that some resource-rich languages like English have abundant training data, while the majority low-resource languages have varying amounts of training data.
The recent popular end-to-end (E2E) monolingual ASR architecture (Graves et al., 2013; Chan et al., 2015; Vaswani et al., 2017) is promising to achieve state-of-the-art performance for resource-rich languages but suffers dramatically from the long tail of low-resource languages due to the lack of training data.
This paper aims to investigate an end-to-end multilingual ASR framework where a single model is trained end-to-end from a pooled dataset of all target languages to improve the overall performance of multilingual ASR tasks, especially for low-resource languages.
The long-tailed data distribution problem makes building an end-to-end multilingual ASR notoriously challenging.
This imbalanced data setting poses a multitude of open challenges for multi-task training because the distribution of the training data is very skewed.
These challenges stem from two aspects.
First, very limited audio samples are available for low-resource languages, such as Kyrgyz, Swedish, and Turkish, while simultaneously, vast amounts of data exist from highresource languages, such as English, French, and Spanish.
Second, graphemes or subword labels follow a long-tailed distribution in ASR since some labels appear signiﬁcantly more frequently, even for a monolingual setting.
Furthermore, a multilingual system may include languages with writing scripts other than the Latin alphabet, such as Chinese or Cyrillic, that further worsen the skewness.
To further illustrate the long-tail distribution in our study, Figure 1 shows the frequencies of sentence piece tokens in the curated multilingual dataset from CommonVoice (Ardila et al., 2020).
While a standard end-to-end multilingual multi-task training approach can improve overall performance compared with monolingual end-to-end approaches, it cannot fully overcome the longtail problem.
One of the key reasons is the class imbalance issue, which will bias the multilingual model towards the dominant languages.
To address the class imbalance issue, one straightforward approach is to resample the training data (Kannan et al., 2019; Pratap et al., 2020) during batch assembly.
However, such an ad-hoc approach does not fully resolve the underlying long-tail distribution problem, and only a marginal improvement is obtained in practice.
[Figure]
In this paper, we propose the Adapt-and-Adjust (A2) framework for multilingual speech recognition using a speech transformer to address the long-tail data distribution issue.
To adapt the multilingual network to speciﬁc languages with minimal additional parameters, both language-speciﬁc and language-agnostic adapters are used to augment each encoder and decoder layer.
While the language-speciﬁc adapters focus on adapting the shared network weights to a particular language, a common adapter is proposed to learn some shared and language-agnostic knowledge for better knowledge transfer across languages.
Moreover, to increase the relative margin between logits of rare versus dominant languages, we perform class imbalance adjustments during multilingual model training or inference by revisiting the classic idea of logit adjustment (Zhou & Liu, 2006).
Class imbalance adjustment (Collell et al., 2016; Cui et al., 2019; Menon et al., 2020) is applied by adjusting the logits of the softmax input with the class priors.
Lastly, for better language modeling, a distilled mBERT (Devlin et al., 2019) is converted to an autoregressive transformer decoder to jointly explore the multilingual acoustic and text space to improve the performance of low-resource languages.
We conduct experiments and establish a benchmark from the CommonVoice corpus with a realistic long-tailed distribution of different languages.
The extensive experiments show that A signiﬁcantly outperforms conventional approaches for end-to-end multilingual ASR.
Our key contributions are as follows:
• We propose Adapt-and-Adjust (A2), a novel end-to-end transformer-based framework for real-world multilingual speech recognition to overcome the long-tail challenge; • We demonstrate the effectiveness of utilizing a pretrained multilingual language model as a speech decoder to improve multilingual text representations and language adapters to better share the learned information across all languages.
To the best of our knowledge, this work is the ﬁrst to adapt a pretrained multilingual language model for multilingual ASR.
• We show that incorporating class priors during training or inference is effective and essential to addressing the long-tail issue in multilingual training.
• We establish a reproducible multilingual speech recognition benchmark with long-tailed distributions of 11 languages from different language families for the research community.
We will release the code of our implementations and the datasets as a benchmark to facilitate the research addressing the long-tail problem of multilingual ASR.
A DAPT - AND -A DJUST F RAMEWORK 2. O VERVIEW
Figure 2 gives an overview of the proposed A2 framework for end-to-end multilingual ASR tasks.
Our framework improves a basic transformer-based sequence-to-sequence model by proposing three key novel contributions: (1) an mBERT-based decoder, (2) language adapters, and (3) classimbalance adjustments.
First of all, we extend the basic transformer framework by replacing the vanilla decoder with an mBERT model for better language modeling, particularly for low-resource languages.
Second, the common and language-speciﬁc adapters are added to each encoder and decoder layer to learn both the shared and language-speciﬁc information for better adaptation
[Figure]
capability. perform class imbalance adjustments during the training or inference phase, in which the logits to the softmax layer are adjusted with the class priors estimated from the data.
From a Bayesian perspective, the classiﬁer after the logit adjustment minimizes a balanced error where both the head and tail classes contribute equally to the training.
2. B ASE M ODEL : H YBRID CTC-A TTENTION S PEECH T RANSFORMER
A sequence-to-sequence speech transformer model (Dong et al., 2018;
Kim et al., 2016; Karita et al., 2019b)
based on the hybrid CTC-Attention network is used for acoustic modeling.
It takes in the acoustic features x ∈ R T × F and outputs the sentence piece tokens y , where T and F denote the sequence length and feature dimension.
The encoder consists of several 2D convolution layers followed by self-attention layers.
The convolution layers are used to extract more robust features before they are sent to the transformer.
The decoder layers have two attention mechanisms, one for self-attention and the other for the encoder output.
The network is trained in an autoregressive manner by predicting the next token given the current output.
In addition, the CTC layer (Graves et al., 2006) is added to the encoder output to serve as a regularizer to the attention model.
Training Multi-task loss L MTL (Watanabe et al., 2018; Karita et al., 2019b), combining the CTC loss (Graves et al., 2006) and attention loss L ATTN , is used to train the speech transformer.
The multitask loss is computed as an interpolation of the two losses with a hyper-parameter λ ( 0 ≤ λ ≤ 1 ):
[Equation]
where p y is the label distribution after label smoothing (M¨uller et al., 2019) to prevent the model from making over-conﬁdent predictions.
Kullback-Leibler divergence loss (KL) (Kullback &
Leibler, 1951), is used for the attention loss.
Decoding Beam search is used to predict the sentence pieces without any additional language models.
The decoding score is computed as a weighted sum of both the CTC and attention network probabilities using β as the decoding parameter to balance them (Karita et al., 2019a):
[Equation]
where y (cid:48) is the decoded sequence so far.
2. M ULTILINGUAL BERT AS T RANSFORMER D ECODER
For better language modeling, especially for low-resource languages, mBERT is used as the transformer decoder.
Since mBERT was initially trained on text data, it is essential to augment the decoder output with a cross-attention layer to the encoder output for each mBERT layer and its self-attention layer to “align” the two spaces for the speech recognition task, and jointly explore the acoustic and text space.
This is because the text space may diverge signiﬁcantly from the acoustic space of the encoder output.
Autoregressive mBERT Figure 3 depicts the changes made to mBERT so that it can be used as an autoregressive transformer decoder.
We copy embeddings and self-attention parameters of mBERT into a randomly initialized decoder model.
Let t denote the current decoding step.
The autoregressive decoder takes the current input token y t to predict the next token y t +1 .
The mBERT embedding layer converts the input token to a vector representation.
Subsequently, the crossattention layer takes the encoder output h enc as the key and value, and the self-attention output as the query, and computes the attention output.
Vocabulary Mapping
The vocabulary size of the vanilla mBERT is too large (119,547 tokens) for training the end-to-end speech recognition system.
Therefore, vocabulary mapping is performed to reduce the number of targets for the speech transformer.
In this work, sentence pieces (SP) (Kudo, 2018) are used as the target tokens for all the end-to-end ASR systems.
The SP models are trained on the transcriptions with a preset vocabulary size.
In this work, we use a shared set of 5, tokens as the multilingual system’s vocabulary.
The minimum number in the token set for the sentence piece model is 150 for all the monolingual systems, except Chinese with 2,265 tokens.
The generated sentence piece tokens are then matched against the mBERT token set.
During training, the embeddings of all tokens in the mBERT vocabulary are initialized with mBERT embeddings.
2. L ANGUAGE A DAPTERS
Similar to Kannan et al.
(2019), lightweight residual language adapters are used to mitigate the data imbalance issues.
As shown in Figure 4, in addition to the language-speciﬁc adapter for capturing the language-intrinsic knowledge, a common shared adapter is also trained to learn language-agnostic information in the multilingual data; we call these Dual-Adapters .
The language-speciﬁc and common adapters are denoted as A lang and A com , respectively.
Each adapter of layer l consists of a down-projection layer W l d , followed by a ReLU activation function, and an up-projection layer
W lu .
The adapters take h l as the input, where h l is the self attention output of layer l .
We compute the output of Adapter ( h l ) as follows for both the language-speciﬁc and common adapters:
[Equation]
Afterwards, the ﬁnal adapter output o l = o
l lang + o l com .
Then, o l is used as the input to the next encoder or decoder layer.
We create a language mask to specify the language-speciﬁc adapters.
[Figure]
2. S ENTENCE P IECE C LASS I MBALANCE A DJUSTMENTS
In addition to the architecture extension with language adapters, the class imbalance problem is addressed to incorporate the class priors during training or inference via logit adjustments.
Derived from a Bayesian classiﬁer point of view in Menon et al.
(2020) for computer vision tasks, the softmax classiﬁer with adjusted logits as input minimizes the balanced error across all classes.
A natural adjustment is to scale the logits
f y ( x ) by the inverse of their class prior π y .
In log domain, the logit adjustment can be performed as follows:
[Equation]
where τ > 0 is a hyper-parameter.
The adjustment can be viewed as applying a class-dependent offset to re-weight each logit according to its class prior.
Class priors
The class priors are the natural frequencies of the sentence piece tokens estimated from the multilingual training data.
To form a valid prior distribution, smoothing is applied to the raw counts according to Equation 6 for zero occurrence tokens:
[Equation]
where C is the total number of counts for all labels, n 0 is the number of labels with zero occurrences,
N is the number of classes and
c i is the raw count of class i .
Training phase class imbalance adjustments To incorporate the priors during training, the logits f dec y t of the last decoder layer are adjusted before softmax according to the following:
[Equation]
The adjusted softmax output vector p adj y of the sequence is used to compute the KL loss and perform the backward propagation to update the model.
y t − 1 is the previous label available only during training.
To reduce the training and inference discrepancy, scheduled sampling (Bengio et al., 2015) is commonly used for sequential classiﬁcation tasks like speech recognition.
During later training iterations, instead of using the ground truth label y t − 1 for computing the logits, y (cid:48) t − 1 is chosen from the maximum prediction output of the current model to simulate the inference:
[Equation]
If the scheduled sampling is used, the adjusted logits at step t will have inﬂuence over all of the following tokens in the current sequence.
This is a crucial difference from the image classiﬁcation task in Menon et al.
If τ is set to be 1, the training phase logit adjustment becomes similar to the widely used label smoothing technique M¨uller
et al.
However, in conventional label smoothing, the prior π y is usually a uniform distribution that is independent of the data.
The logit adjustment applies a class-speciﬁc “smoothing” based on the class prior, and has been shown to be superior to the baseline with the standard label smoothing.
Inference phase class imbalance adjustments Alternatively, the class priors can be incorporated during inference via logit adjustments.
The decoding score is computed as follows:
[Equation]
During beam search, the attention decoding scores p adj y are computed in the same way as the scheduled sampling from the adjusted logits.
E XPERIMENTS 3. E XPERIMENTAL S ETUP
Dataset The CommonVoice dataset (Ardila et al., 2020) is a multilingual corpus collected by
Mozilla.
Similar to Conneau et al. (2020), we use 11 languages: English (en), Spanish (es), French (fr), Italian (it), Kyrgyz (ky), Dutch (nl), Russian (ru), Swedish (sv), Turkish (tr), Tatar (tt), and
Chinese (zh) .
The dataset is split into training, dev, and eval sets according to the ESPnet recipe.
The transcriptions are tokenized using the SentencePiece model with the unigram algorithm.
We train our SentencePiece model using our transcription data, and the size of the vocabulary is shown in Table 10.
We then add special tokens, such as < unk > , < sos > , < eos > , and a blank token for the CTC objective.
The data split is shown in Table 6.
Network conﬁgurations We use six transformer encoder layers with a hidden size of 2048 units and eight attention heads, each with an attention dimemsion of 256.
For the decoder, distilmBERT (Sanh et al., 2019) is used.
The mBERT decoder consists of six transformer decoder layers with a hidden size of 3072 and an attention dimension of 756, and four attention heads.
We train our model with a batch size of 32 and accumulate the gradient in two steps to have a larger batch size using a single GPU NVIDIA V100 16GB.
The models are trained with the Adam optimizer with a warm-up step of 25000.
In particular, for balanced sampling, we take six samples for each language and construct a balanced batch by accumulating the gradient 11 times.
Training and Evaluation We evaluate our model using beam-search with a beamwidth of 10 and λ = 0 .
3 and β = 0 .
The hyper-parameter τ is set to 0.3 for both the training and inference phase class imbalance adjustments.
The multilingual models are trained with 150K iterations.
We compute the average over the last ten checkpoints as the decoding model.
For the monolingual setting, we stop after 100 epochs of training.
Models are evaluated using the character error rate (CER) to simplify the evaluation and to have a universal metric for all languages.
Baselines As baseline approaches, we consider the following:
Monolingual : we train monolingual models; SMT (Standard Multilingual Training), we randomly sample the batch from the data distribution; BS (Balanced Sampling), we sample the same number of utterances for each language in a batch so that they have roughly equal contributions to the training; LAN-Speciﬁc Adapters : language-speciﬁc adapters proposed by Kannan et al.
(2019); and LID : (language ID) conditioning with one-hot language vectors proposed by Li et al.
3. R ESULTS
In Table 1, we present the test results on the CommonVoice dataset.
Compared to the monolingual models, even the SMT models improve the performance of the low-resource languages signiﬁcantly.
In other words, SMT is a decent multilingual baseline to be compared with.
We conjecture that this may be because the multilingual models can capture common sub-phonetic articulatory features (Kirchhoff et al., 2002; Metze, 2005; Livescu et al., 2007; Wang & Sim, 2014) that are shared by different languages and are beneﬁcial for low-resource languages recognition.
Balanced Sampling We observe the same trend as in Kannan et al.
(2019): compared to the SMT, the tail language performance is signiﬁcantly boosted.
However, the performance of the head languages suffers due to fewer occurrences during training. The model is clearly over-ﬁtted to the tail languages due to up-sampling, for example, the CER performance on the training data of the “ky” and “sv” languages is signiﬁcantly lower than the evaluation data (3.4% and 4.2% training CERs vs 13.4% and 22.8% evaluation CERs).
Consequently, the overall performance is the same as SMT.
In fact, even after balanced sampling, the sentence piece tokens still have a long-tailed distribution, as shown in Appendix C.
[Table]
Language Adapters We next compare the language adaptation techniques, the LAN-Speciﬁc
Adapters (Kannan et al., 2019), the one-hot language vector (Li et al., 2018) [Table] and the Dual-Adapters.
Note that all adapters are based on BS + mBERT, which has better performance than the BS-only model.
Adding the language-speciﬁc adapters without common adapters signiﬁcantly outperforms the BS baseline, with a 0.9% absolute performance gain.
Another way of injecting language information is to augment a one-hot language vector.
Interestingly, applying sentence piece class imbalance adjustment (LID + Adjust-Train) to the language vector signiﬁcantly improves the CER.
Sentence Piece Class Imbalance Adjustment
The long-tail distribution issue is addressed with class imbalance adjustments.
Both the training and inference phase adjustments provide a signiﬁcant performance gain over the LAN-Speciﬁc Adapters, with 1% absolute CER reduction.
The gains are mostly due to the improved performance of the head languages, although tail languages also beneﬁt from the logit adjustments.
More importantly, the gap between the monolingual and multilingual performance for the head languages is greatly reduced, leading to a better “balanced error” performance.
This strongly justiﬁes the importance of class imbalance adjustments.
Compared with BS, A2 also avoids over-ﬁtting to the tail languages, CERs on “ky” and “sv” are 8.2% and 23.6%, which are closer to evaluation CERs.
Compared to SMT with random sampling, A2 has a signiﬁcant better averaged CER performance with a modest cost for the two resource rich languages “fr” and “en”.
Some example transcriptions and detailed analysis using different A2 modules are given in Appendix E.
3. A BLATION S TUDIES 3.3. M ULTILINGUAL BERT
The effectiveness of mBERT is presented in Table 2.
The performance of mBERT depends heavily on the quality of the acoustic models.
Without adapters or logit adjustments, the improvement over
BS is marginal, and mBERT performance is even worse if standard multilingual training is used.
This may indicate that, with better acoustic models like A2, the text space of the vanilla mBERT is better aligned with the acoustic space, which leads to improved recognition performance across all languages, especially for low-resource languages.
It is also interesting to note that, even without adapters or balanced sampling, the “SMT + mBERT + Adjust-Train” system yields the same overall performance as the best adapter system (BS + mBERT + Dual-Adapters). [Table] [Table]
L ANGUAGE A DAPTERS
In this section [Table] we investigate the impacts of different language adapter conﬁgurations.
The results and parameter sizes are given in Table 3.
Generally speaking, adding adapters into the decoder layers is not as effective as in the encoder layers, indicating adaptation of the acoustic space is much more effective than of the text space.
Therefore, considering the extra computation and parameters, it is advisable to apply only the encoder adapters with just a 2M increase in parameters
[Table]
[Table]
We investigate the effectiveness of the common language adapters [Table] as shown in Table 4.
The DualAdapters outperform the language-speciﬁc adapters signiﬁcantly, by a 0.5% absolute CER reduction, indicating knowledge transfer with the common adapter is effective. According to their written
[Table]
scripts, we also divide the 11 languages into language groups, e.g., Latin, Chinese characters and
Cyrillic scripts.
This is based on the fact that some languages do not share any sentence pieces with each other.
For example, the Chinese sentence pieces are only used by the “zh” language.
They can also be groups into language families, e.g., Romance, Chinese, Turkic, Germanic.
This group focuses more on the similarities in low-level language lexica, grammars, and pronunciations, which are usually subsumed under the end-to-end multilingual architectures.
According to one group, languages that belong to the same cluster do not necessarily belong to the same cluster in the other group.
For example, Tartar and Turkish are both Turkic languages. However, Tartar uses the Cyrillic script, and Turkish uses the Latin alphabet. All languages in the same group share the same
Dual-adapters, and the adapters are trained with all language members.
In general, grouping by language families is better than grouping by written scripts because grouping by language families is more consistent with the encoder adapters for adapting the acoustic space, and the importance of encoder adapters is shown in Table 3.
Compared with the individual language adapters, sharing language adapters by language families helps the low-resource languages performance, e.g., “sv” of the Germanic group, “ky” and “tr” of the Turkic group because more data are used to train language the group adapters.
However, this also comes with a cost to the resource-rich languages.
3.3. S ENTENCE P IECE C LASS -I MBALANCE A DJUSTMENTS : T RAINING VS . I NFERENCE
The two logit adjustments are compared in Table 5.
For the SMT systems, training phase adjustment shows a clear advantage over inference phase adjustment.
Under the convex assumption, the solution of the two adjustment approaches is the same.
However, deep neural network optimization is a nonconvex problem, so they may converge to different local minima.
Under SMT, the model is heavily biased towards the head classes due to random sampling.
Training phase class imbalance adjustment can help the training to place more focus on the tail classes, leading to much better balanced and lower error.
With better acoustic models, e.g., language adapters, the inference phase adjustment can better calibrate the raw classiﬁcation scores and yield similar performance to the training phase adjustment. Lastly, we show the effects of τ on inference phase logit adjustment in Appendix D
[Table]
[Table]
R ELATED W ORK
Long-Tail Conventional approaches to addressing the long-tail problem are focused on data sampling methods (Kubat & Matwin, 1997; Chawla et al., 2002; Wallace et al., 2011).
Recently, the long-tail distribution issue has regained interest for neural network models (Menon et al., 2020), and several approaches have been proposed, such as weight normalization (Kang et al., 2019), adaptive margin (Cao et al., 2019), and equalized loss (Tan et al., 2020).
Adapters Adapters were ﬁrst proposed to learn domain-speciﬁc representations in computer vision in a parameter-efﬁcient way (Rebufﬁ et al., 2017).
They were subsequently adopted for NLP tasks to avoid ﬁne-tuning a new model for each task by training an adapter module for each task while sharing the pre-trained language model parameters (Houlsby et al., 2019; Lin et al., 2020).
Invertible adapters were proposed in Pfeiffer et al.
(2020a;b) to effectively adapt an existing pretrained multilingual model to unseen languages for multi-task cross-lingual transfer.
Multilingual ASR E2E architectures like LAS (Toshniwal et al., 2018) and the Recurrent Neural
Transducer (Kannan et al., 2019) have been used for building a multilingual ASR system for a group of Indian languages.
In Kannan et al.
(2019), language adapters are used to tackle the data imbalance problem, although the improvement from using language adapters is marginal compared to the language vector augmentation.
Acoustic vector quantization is also used in the recent work by Conneau et al.
(2020) on multilingual ASR.
A massive multilingual ASR study with more than 50 languages and more than 16,000 hours of speech is presented in Pratap et al.
The two main techniques are data resampling and language clusters, which bear some similarities with our balanced sampling and language adapters.
Unfortunately, their datasets are not publicly available.
C ONCLUSION
In this paper, we introduce Adapt-and-Adjust (A2), an end-to-end transformer-based framework to address the crucial challenge of long-tail data distribution issues in multilingual speech recognition.
A2 consists of three major components, namely, language adapters, class imbalance adjustments, and a pretrained multilingual language model.
Extensive experiments on the CommonVoice corpus show that A2 signiﬁcantly outperforms conventional approaches for end-to-end multilingual speech recognition due to 1) the better acoustic modeling with adapters and class imbalance adjustments; and 2) the better language modeling with pretrained multilingual BERT.
A A2 A LGORITHM
The full algorithm of the A2 framework with training phase sentence piece class imbalance adjustments is outlined in Algorithm 1.
Algorithm 1 Adapt-and
Require: D : long-tailed multilingual data
Require: θ : an encoder-decoder model
Require: α , λ : step size hyperparameters 1: randomly initialize θ 2: copy mBERT pretrained language model to decoder parameters in θ 3: compute class priors π from D 4: while not done do 5:
Sample batch of multilingual utterances x ∼ D 6:
Generate language adapter mask m using the language tag in x 7:
Compute encoder hidden states h enc using x and m by encoder forwarding 8:
Compute logits f dec y t using h enc and m by decoder forwarding 9:
Adjust logit according to Equation10:
Compute CTC posteriors p ctc ( y | h enc ) 11:
Compute attention loss L ATTN in Equation 12:
Compute multi-task loss L MTL using p ctc ( y | h enc ) , L ATTN , and λ in Equation13:
Update model θ ← θ − α ∇ θ L MTL 14: end while
B ASR M ODEL S TRUCTURE
The encoders and decoders of the transformer-based speech recognition model used in this study is presented here. [Figure] enc dec dec
Encoder Layer Figure 5 shows the structure of the encoder layer.
An adapter layer is added after the layer norm and self-attention module.
We also apply two residual connections after both the self-attention layer and the adapter layer:
[Equation]
[Equation]
(13) where h l − 1 enc is the encoder hidden states of the previous layer l −
Decoder Layer Figure 6 shows the structure of decoder layer with the adapter.
We place the adapter layer after the cross-attention model.
[Equation]
where h l − 1 dec is the decoder hidden states of the previous layer
[Table]
and h l dec is the output the current layer
[Table]
C D ATASET D ISTRIBUTION
Table 6 shows the data split used for training, validation [Table] and testing.
Table 7 shows the frequencies of the labels in the original data and our estimates on the balance sampling.
There are only a few data with an occurrence of more than 50k (around 1%).
The class distribution on the balance sampling is shifted to the 10k-50k range; thus, there are more labels in the middle of the distribution.
[Table]
[Table]
[Table]
Figure 7 and Figure 8 visualizes the long-tail distribution in more detailed.
To better visualize them, we ignore the classes fewer than 20k
[Figure] [Figure]
D E FFECTS OF τ ON I NFERENCE P HASE L OGIT A DJUSTMENT
Figure 9 depicts the CERs of the systems with different τ for inference phase logit adjustment with the best A2 conﬁguration: balanced sampling with mBERT and Dual-Adapters.
We choose one language from each group, and the other languages demonstrate the same trend.
In Figure 9, the advantage of the logit adjustment is clearly shown compared to the baseline without logit adjustment ( τ = 0 ).
All of the languages achieve their best CER with a τ value of τ = [0 . 3 , 0 . 4] .
Performance saturates with heavier smoothing ( τ > = 0 . 5 ).
[Figure]
E T RANSCRIPTION E XAMPLES
We provide samples of two languages generated from different models, a resource-limited language zh (traditional Chinese scripts and Mandarin spoken in Taiwan) and a resource-rich language en (English).
[Table]
The reference in Table 8 roughly translates to English as “Obstacles and challenges are the sources of power that motivate us.”
The monolingual output is rather poor. Not only did it miss an important character 們 for distinguishing “my” and “our”, the sounding out of the sentence is also quite different from the reference (Fu4 Neng2 Yi
Piao4 Zhan4 Shi4 Wo3 De0 Yuan2 Dong4 Li4).
The standard multilingual training (SMT) improves the monolingual training by predicting the character 與 (Yu3) in place of 一 (Yi2).
In addition, the output of 調 站 (Tiao2 Zhan4) sounds almost the same as the reference 挑 戰 (Tiao3 Zhan4).
(The monolingual system output 票 佔 (Piao4 Zhan4) not only does not make any sense in terms of word meaning, the pronunciation is also quite different from the reference.)
This shows that the multilingual training helps improve the acoustic modelling compared to the monolingual training if the monolingual training data are limited.
The mBERT model further helps the language modeling by correcting 可 能 與 調 站 (literally translates to “possibility and changing stations“) to 困 難 與 挑 戰 (“obstacles and challenges”).
However, mBERT also replaces 機 發 (Ji1 Fa1) to 資 料 (Zi1 Liao4) since 資 料 (“documents/materials”) is a more common word than 機 發 , which is a wrong word with correct pronunciations relative to the reference.
Lastly, the adapters successfully convert 資 料 (Zi1 Liao4) to 機 發 (Ji1Fa1) so that it sounds the same as the reference with one wrong character.
This demonstrates the adapter’s capability in better acoustic modelling.
Finally the errors including the wrong characters in 機 發 (Ji1Fa1) and 員 動 力 (Yuan Dong4 Li4) can be easily corrected by a decent external language model during decoding, e [Table] g., an
RNN-LM trained on the training transcriptions.
[Table]
Table 9 shows the transcriptions of different models applied to an English testing utterance.
Different from Table 8, where only limited training data are available, mBERT does not provide any improvement over SMT, except that it makes the sentence more grammatical by adding a subject, “I”.
The Dual-Adapters improve the performance compared to mBERT by successfully decoding the sentence piece “ENT” in “SPENT”, which is closer to “TWENTY” in terms of characters.
Lastly, the class imbalance adjustment system yields the best CER [Table]
F V OCABULARY C OVERAGE
[Table]
G L OSS D YNAMICS
We plot the validation losses from A2 and the other baselines in Figures 10 and
[Figure]
[Figure]
H F EW -S HOT S ETTING
We also investigate the effectiveness of the A2 framework in the few-shot setting.
We take ﬁvehour speech data of each language as the training data.
The test results are shown in Table [Figure]
Using mBERT as the speech decoder consistently improves the performance in the low-resource monolingual scenario.
Interestingly, for Chinese language “zh”, without using mBERT as the decoder, the model seems to be unable to learn useful information and the mBERT decoder yields an absolute CER reduction of 17.2%.
For multilingual training, marginal performance gains are observed with mBERT decoder, indicating the multilingual language model is complementary to the shared acoustic knowledge learned from the multilingual speech data.
The language adapters lead to consistent and signiﬁcant performance gain across all languages.
Note the sentence piece class imbalance adjustments do not yield signiﬁcant performance boost as in the long-tail multilingual setting in the main text. We believe this is largely due to less severe sentence piece class imbalance when the same amount of training data for each language is used
[Table]
[Table]
I H YPER - PARAMETER S ETTING
The hyper-parameters used in the training are shown in Table
[Table] [Table]