u-HuBERT: A Unified HuBERT

u-HuBERT stands for “Unified Hidden Unit BERT” which is a unified self-supervised pre-training framework that can leverage unlabeled speech data of many different modalities for pre-training, including both uni-modal and multi-modal speech. u-HuBERT was proposed by Meta AI in 2022 and published in this paper: “A Single Self-Supervised Model for Many Speech Modalities Enables Zero-Shot Modality Transfer” by the same author who introduced HuBERT and AV-HuBERT.


Architecture-wise, u-HuBERT has the same architecture as AV-HuBERT which consists of a set of modality-specific feature extractors. The video encoder is a modified ResNet-18 model that takes image frames of $88 \times 88$ pixels sampled at $25$ frame-per-second and produce a sequence of $512$ dimensional embeddings at the same frame rate. The audio encoder is filter bank features sampled at $100Hz$, stacked every $4$ frames to match the video frame rate ($25\ fps$). Both audio and video representations are concatenated together frame-by-frame and linearly projects them to the embedding dimension of the shared Transformer. The shared Transformer has $24$ layers, with $16$ heads, embedding dimension $1024$, and FFN dimension $4096$, using the pre-norm residual connection setup:

Unlike AV-HuBERT which was pre-trained using multi-modal (i.e audio-visual) data, u-HuBERT is pre-trained using both audio-only and multi-modal speech data forming modality-agnostic features. To encourage the modality-agnosticism even more, the modality dropout (which was used in the audio-visual fusion block in AV-HuBERT) is extended and applied to data by randomly dropping a subset of the modalities, effectively creating multiple copies of the data with the same target but different input modalities.


In this paper, u-HuBERT pre-training was performed on three different datasets; the first two datasets are audio-visual and were used with AV-HuBERT, while the last one is an audio-only dataset. These three datasets are:

  • LRS3: Lib-Reading Sentences 3 with $433$ hours of English audio-visual speech.

  • VC2-En: VoxCeleb2-En with $1,326$ hours of English YouTube audio-visual speech filtered from VoxCeleb2.

  • TED-LIUM 3: with $452$ hours of English audio collected from TED talks.

AV-HuBERT was pre-trained for five iterations; the first iteration was done using audio-only features, and the following four iterations were done using audio-visual features extracted from the pre-trained model in the previous iteration. To pre-train u-HuBERT, the pre-trained AV-HuBERT at iteration 4 was used as a feature extractor on the audio-only data. Then, u-HuBERT were pre-trained on the combined data (audio-visual data + audio-only data) for $1M$ updates using the Adam optimizer and a learning rate of $0.002$. Gradient norm is clipped at $1.0$ to stabilize training. Modality dropout with a probability of $0.25$ was applied. To improve noise robustness, they applied online noise augmentation where each utterance is corrupted with additive noise at $0dB$ sampled from MUSAN dataset with a probability of $0.25$.

After pre-training is done, the model should return modal-agnostic representations. To make sure that’s the case, they extracted audio, visual, and audio-visual features from LRS3 validation utterances and run t-SNE algorithm and the results are shown below:

It is visually evident that the audio, visual, audio-visual features have very similar distributions when using modality dropout (top row), contrary to those from the model without modality dropout (bottom row).


Once the model is pre-trained, they removed the cluster prediction head (the linear layer on top of transformer) and added a downstream task-specific prediction head. The pre-trained model can be fine-tuned on labeled multi-modal speech, uni-modal speech, or speech with mixed modalities. More importantly, the modalities included in the fine-tuning data do not necessarily cover the modalities seen during pre-training, yet the fine-tuned model can still handle the downstream task on all pre-trained modalities, which they referred to as zero-shot modality generalization.

Speech Recognition

LRS3 “trainval” and “pretrain” were used for fine-tuning. Noise augmentation was done similar to the pre-training. A $9$-layer randomly initialized Transformer decoder with $8$ attention heads and $1024$/$409\ $embedding/FFN dimensions is appended to the pre-trained u-HuBERT, and the entire model is fine-tuned with a cross-entropy loss predicting the next text token given the input and the previous text tokens. Text is represented as unigram-based subword units with a vocabulary size of $1,000$. The whole list of hyper-parameters as shown in the following table:

The following table compares fine-tuning from scratch, as well as from pre-trained models with and without modality dropout, using labeled audio, visual, or audio-visual speech, and highlights the results of zero-shot modality transfer in purple shades. Also, it shows the speech recognition WER results on LRS3 test, where:

  • PT: pre-trained model (on LRS3 and VC-2).

  • PT mod-drop-p: modality dropout is applied in pre-training

  • Ft mod-drop-p: modality dropout is applied in fine-tuning.

  • FT mod: denotes the type of labeled data used for fine-tuning, AV stands for “Audio-Visual”, A for “Audio”, and V for “video”.

From the past table, we can see the following:

  • Models without pre-training are significantly worse.

  • Modality dropout is important for pre-training and fine-tuning, especially when evaluating on uni-modal data (i.e audio-only or video-only).

  • Applying modality dropout during fine-tuning leads to consistent improvement on noisy AV test set. This is likely because occasionally dropping the audio stream encourages the model to rely more on the visual input which improves noise robustness.

  • Applying modality dropout during pre-training leads to consistent improvement on zero-shot cases, i.e models pre-trained on audio-visual data and fine-tuned on either audio-only or video-only data.

In another experiment, they fine-tuned the model on LibriSpeech dataset, which is an audio-only dataset from reading audio books that is out-of-domain relative to the pre-training data (which were mainly TV talks). The following table shows the performance degradation compared to fine-tuning on in-domain LRS3 data, the model still performs decently on zero-shot transfer scenarios.

In comparison to other state-of-the-art models LRS3 test set, the following table shows that u-HuBERT performance is on par or better than the best other models. Furthermore, compared to AV-HuBERT, u-HuBERT can be pre-trained on additional unimodal data to yield even better performance.

In the past table, u-HuBERT is the only model that can be fine-tuned on any modality (audio or video). All other models are modal-specific.

Spanish Speech Translation

To simulate speech translation setting, they translated the English transcriptions of LRS3 “trainval” set into Spanish using a trained machine translation model creating paired (English audio, English video, Spanish text) data. They used the same decoder for speech translation as for speech recognition, where a $9$-layer randomly initialized Transformer is appended to the pre-trained u-HuBERT encoder to decode English audio-visual data into Spanish text.

Unigram-based sentencepiece units with a vocabulary of 1,000 were sued as decoding output units. The overall fine-tuning and decoding paradigms remain same as speech recognition. A detailed list of the hyper-parameters used can be found in the following table:

The performance of u-HuBERT on speech translation can be seen in the following table; where you can see that u-HuBERT pre-training not only greatly improves translation quality, but also shows strong performance in zero-shot modality transfer. Within u-HuBERT, modality dropout is an essential component for acquiring such capacity.


In the paper, authors decided to ablate some of the hyper-parameter choices. For faster experimentation, they used the base model and pre-trained it on the LRS3 dataset and fine-tuned the model on the “trainval” set. The following is a summarization of the different experimentation.

Modality Dropout Ratio

To better understand the impact of pre-training modality dropout configurations on zero-shot modality transfer, they pre-trained a BASE model for $200K$ updates and then fine-tuned on “trainval” set. Four modality dropout configurations were considered, represented by the probability of using both streams ($p_{av}$), audio stream ($p_{a}$) and video stream ($p_{v}$). Results are summarized in the following table:

From the past table, we can deduce that when fine-tuning on audio-visual or audio-only data, setting the $\mathbf{p}_{\mathbf{v}}$ higher leads to slightly worse performance on audio-visual and audio-only test sets. In contrast, it leads to slightly better performance when fine-tuning on visual-only data.

Pre-training with Additional Audio-only Data

They pre-trained the BASE model on audio-visual + audio-only data for different numbers of updates, namely $(100,\ 200,\ 400,\ 600)K$ updates. The baseline (PT on AV) was pre-trained on audio-visual LRS3 only, while the proposed model (PT on AV+A) was pre-trained on the same audio-visual data as the baseline plus the VC2-En audio data. Results are shown below:

The previous figure shows that adding audio-only data consistently improves the ASR performance at all number of updates. In contrast, the performance of “PT on AV+A” lags behind “PT on AV” on audio-visual and visual speech recognition when a model is trained for fewer than $400K$ updates.

Nevertheless, when a model is trained for longer ($\mathbf{600K}$ updates), mixed-modal pre-training outperforms the baseline on all three tasks. The observation suggests that u-HuBERT can benefit from training on more data and continue improving with more updates, while the performance of the baseline saturates earlier due to the lack of data.

AV Dropout Vs A-only Dropout

The following table shows the impact of fine-tuning modality dropout vs the audio-only dropout on the model’s performance on the speech recognition task. From the table, we can see that increasing the dropout doesn't change the performance by much.

Number of Frozen Layers

Next, they studied the impact of freezing the first few layers of u-HuBERT $L_{frz}$ when fine-tuning. Setting this effectively treats the first $L_{frz} = (0,\ 6,\ 12,\ 18,\ 24)$ layers as fixed feature extractor, while the following layers are considered pre-trained and jointly optimized with the added prediction head during fine-tuning.

$L_{frz} = 24$ means the whole encoder is frozen and the 9-layer decoder is the only part that is being fine-tuned.

Number of Frozen Steps

Next, they studied the impact of freezing u-HuBERT for the the first few updates $N_{frz}$ when fine-tuning. These tried $N_{frz} = (0,\ 15,\ 30,\ 45,\ 60)K$ updates and the results are reported in the following table:

We can observe similar trends as those when varying the number of layers to freeze: not freezing at all leads worse performance on all input modalities. Freezing for $\mathbf{30K}$ updates out of $\mathbf{60K}$ total updates leads to good balance for all modalities.

Learning Rates

The following table shows the impact of different learning rates on u-HuBERT performance:

Layer’s Ability for Feature Extraction

Finally, To better understand how u-HuBERT learns modality-agnostic features, they experimented the clustering quality of different layers per modality of a pre-trained u-HuBERT model. They reported the Phone Normalized Mutual Information (PNMI) reported in HuBERT of layer-wise clusters per modality quantized by audio-only ($C^{a}$ ), video-only ($C^{v}$), audiovisual ($C^{av}$) and all-combined ($\cup_{m}C^{m}$) codebook, respectively:

To the model pre-trained with modality-dropout (left-side), the features of its intermediate layers become more agnostic to modality as the layer goes deeper, shown from the diminishing gap between its cross-modality and modality-specific performance in later layers. Furthermore, the final layer is best suited to achieve a unified model that generalizes across modalities.