Online Speech recognition using RNN-Transducer

Speech to text using RNN Transducer (Graves et al 2013 ) trained on 2000+ hours of audio speech data.

This work is a joint collaboration with @w86763777

Highlights

• First repo demonstrating online decoding capability of RNN Transducer (RNN-T)

• Port RNN-T model to ONNX and OpenVINO

• A large scale training on diverse voice datasets for RNN-T with apex and data parallel

Using this model we can run online speech recognition on Youtube Live video with ( 4 ~ 10 seconds faster than Youtube's caption ) on an 2.3GHz dual-core Intel Core i5 processor.

Visualize alignment of audio and text, similar to paper in Graves et al 2013.

Training Tips

Most of our insights share some similarity with this article: Towards an ImageNet Moment for Speech-to-Text. The difference is between our work and the mentioned article is that we mainly focus in online decoding, hence limit ourselves to RNN Transducer loss with uni-directional recurrent network. Hence, training requires more parameters and resource as we are limited by past audio feature.

Our current best model only achive a WER of 16.3% on Librispeech test-clean, which is still a long way to reach the common baseline of around 5%.

But we still learn some tricks and would like to share with you.

1. Balanced encoder and predictor is important

A good balance between audio encoding and language decoding is important since audio features is much more complicated than text ( in terms of diversity and feature space ). Hence a good rule of thumb is encoder should at least 4 times the capacity of the predictor network.

2. Vocabulary size improve convergence

Contradict to the article mentioned above, we found that larger vocabulary size ( we use BPE as wordpiece vocabulary ) always result in better performance. This is interesting, because CNN based CTC model usually suffers from performance drop when the vocabulary size increase til an extend.

3. Some other tips

Big batch size is better as mentioned in all previous RNN-T papers ( duh )

Train your model as large as possible ( 100M parameters is better )

Time reduction in first and middle layers help to reduce training memory usage but suffers certain performance hit that we haven't had the resource and time to investigate. However, think this can be make up by designing much more efficient model architecture (Maybe GRU for encoder model instead of LSTM ).

Layer norm helps model to handle sudden increase of voice pitch during online decoding, this allows us to skip CMVN preprocessing commonly found in online decoding. But this slows down the convergence speed.

Training RNN-T is slow, and any brave warrior who wish to challenge should be patience and expect to own a good compute resource ( ie workstation many GPUs, 1TB of SSD storage ).

We use Adam optimizer for fast convergence in order to meet the deadline for our final project. We experiement with SGD w momentum but find it extremely slow to converge.

Other results

1. Performance comparsion between Pytorch, ONNX, OpenVINO in inference stage

Evaluation environment

OS: macOS Catalina 10.15.4
CPU: i5-7360U (2C4T, 2.3G →2.6G)
RAM: 8G 2133 MHz LPDDR3
PyTorch: 1.5.0
OpenVINO: 2020.3.194
Data: Random sample 50 audio from LibriSpeech test-clean

We found inference under OpenVINO is two times slower than Pytorch and ONNX runtime. The cause of degrade performance from OpenVINO is unknown, we didn't find any explaination other than lack of optimization for LSTM in OpenVINO.

FrameWork	WER	Avg Encoding Time	Avg Decoding Time	Avg Joint Time	Avg Throughput Per Second
Pytorch	11.08 %	12.289 ms	0.490 ms	0.482 ms	5.797 sec/sec
ONNX	11.08 %	11.850 ms	0.462 ms	0.496 ms	5.989 sec/sec
OpenVINO	11.08%	20.296 ms	0.897 ms	0.594 ms	3.543 sec/sec

2. ImageNet for speech recognition is still far away?

If you want to do online decoding. However, training a offline decoding CNN based CTC model is fast and low memory usage due to the use of CNN module. We were able to fit a 211 M CNN based model in one RTX 2080 with batch size of 8, but struggle to train a 50M RNN-T model on RTX 2080 with the same batch size.

Model	# Param	GPU	Time	Vocab size	Batch size	WER
8 layer Bi-Encoder 1280 hidden size [1]	> 180M	32 x Google TPU 8G ?	7 Days	16k	512	3.6%
CNN Based [2]	211M	8x NVIDIA Tesla V100 32 GB	4.16 Days	27 Character	512 (fp16)	3.7%
6 layer Encoder 1024 hidden size (ours)	50M	4 x NVIDIA RTX 2080-Ti 12G	3 Days	2k	128 (32 * 4) (fp16)	16.3%

Issues:

• Currently dataparallel in pytorch 1.4.0 is broken, so you must use pytorch 1.5.0 and apex in parallel training make sure you have supported cudnn and cuda version

  • a distributed parallel version lightning.py is supported for Pytorch 1.4.0

Install:

• Install torch and torchaudio with compatible version

  • make sure your torch and torchaudio is matched ie : torch==1.4.0 must pair with torchaudio==0.4.0

• Install apex https://nvidia.github.io/apex/amp.html

  git clone https://github.com/NVIDIA/apex
  cd apex
  pip install -v --no-cache-dir --global-option="--cpp_ext" --global-option="--cuda_ext" ./
  

• Install warprnnt-pytorch https://github.com/HawkAaron/warp-transducer

  git clone https://github.com/HawkAaron/warp-transducer
  cd warp-transducer
  mkdir build
  cd build
  cmake -DCUDA_TOOLKIT_ROOT_DIR=$CUDA_HOME ..
  make
  cd ../pytorch_binding
  export CUDA_HOME="/usr/local/cuda"
  python setup.py install
  

• Install other dependencies

  pip install -r requirements.txt
  

Training:

Checkout configuration examples under flagfiles and rnnt/args.py for more details about parameters.

For dataparallel training or single GPU training:

python train.py --config flagfiles/E4D1.txt

For distributed training:

python lightning.py --config flagfiles/E4D1.txt

If the learning rate and batch size is right, you should have a convergence curve as below after 24 hours of training.

Datasets:

Common Voice : 178.621 hrs

mkdir common_voice
cd common_voice
wget https://voice-prod-bundler-ee1969a6ce8178826482b88e843c335139bd3fb4.s3.amazonaws.com/cv-corpus-4-2019-12-10/en.tar.gz
tar -zxvf en.tar.gz

• Download english dataset from https://voice.mozilla.org/en/datasets

• execute preprocess_common_voice.py to convert audio to 16k, PCM 16bits wav files ( this takes around 20 hours )

Youtube Caption : 118 hrs

Librispeech release 1 : 1000 hrs

Download all the tar.gz files from here and unzip files under a directory LibriSpeech, and point your flagfiles to each directory files

check rnnt/args.py for the argument names

TEDLIUM: 118.05 hrs

• Either download release 1 or 3 ( version 1 is smaller )

wget http://www.openslr.org/resources/51/TEDLIUM_release-3.tgz
wget http://www.openslr.org/resources/51/TEDLIUM_release-1.tgz
pip install git+https://github.com/mcfletch/sphfile.git

Data path

```
└──RNN-T/                   # this repo
    ├──train.py
    ├──...
    └──datasets
        ├──common_voice/
        │   ├──clips/               # all the audio
        │   └──train.tsv
        ├──youtube-speech-text/
        │   ├──english/             # all the audio
        │   └──english_meta.csv
        ├──TEDLIUM_release1/
        │   ├──train/
        │   │   └──wav              # all the audio
        │   └──test/
        │       └──wav              # all the audio
        └──LibriSpeech/
            ├──train-clean-360/
            ├──train-clean-100/
            ├──train-other-500/
            └──test-clean/
```

OpenVINO cheat sheet

• Export pytorch model to ONNX format

  python export_onnx.py \
      --flagfile ./logs/E6D2-smallbatch/flagfile.txt \
      --step 15000 \
      --step_n_frame 10
  

• Install OpenVINO inference engine Python API

  sudo -E apt update
  sudo -E apt -y install python3-pip python3-venv libgfortran3
  pip install -r /opt/intel/openvino/deployment_tools/model_optimizer/requirements.txt
  

• Model Optimizer

  • Setup envs

    source /opt/intel/openvino/bin/setupvars.sh
    

  • Encoder

    python3 /opt/intel/openvino/deployment_tools/model_optimizer/mo.py \
        --framework onnx \
        --input_model ./logs/E6D2-smallbatch/encoder.onnx \
        --model_name encoder \
        --input "input[1 10 240],input_hidden[6 1 1024],input_cell[6 1 1024]" \
        --output_dir ./logs/E6D2-smallbatch/
    

  • Decoder

    python3 /opt/intel/openvino/deployment_tools/model_optimizer/mo.py \
        --framework onnx \
        --input_model ./logs/E6D2-smallbatch/decoder.onnx \
        --model_name decoder \
        --input "input[1 1]{i32},input_hidden[2 1 256],input_cell[2 1 256]" \
        --output_dir ./logs/E6D2-smallbatch/
    

  • Joint

    python3 /opt/intel/openvino/deployment_tools/model_optimizer/mo.py \
        --framework onnx \
        --input_model ./logs/E6D2-smallbatch/joint.onnx \
        --model_name joint \
        --input "input_h_enc[1 640],input_h_dec[1 256]" \
        --output_dir ./logs/E6D2-smallbatch/
    

TODO

• Parallelize model training

• Use BPE instead of character based tokenizer, should reduce more memory

• Write checkpointing and tensorboardX logger

• Modify wraprnnt-pytorch to compatible with apex mixed precision

Reference

• Sequence Transduction with Recurrent Neural Networks

• SPEECH RECOGNITION WITH DEEP RECURRENT NEURAL NETWORKS

• Awni implementation of transducer