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Kaldi model converter to ONNX

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Kaldi-ONNX Converter

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Kaldi-ONNX is a tool for porting Kaldi Speech Recognition Toolkit neural network models to ONNX models for inference. With the converted ONNX model, you can use MACE to speedup the inference on Android, iOS, Linux or Windows devices with highly optimized NEON kernels (more heterogeneous devices will be supported in the future).

This tool supports converting both Nnet2 and Nnet3 models. Almost all components in Nnet2 and Nnet3 models are supported, and the available components are listed in converter/

Besides DNN, many speech recognition models are using RNN or TDNN networks. To make them more portable and efficient, this tool will convert these RNN and TDNN networks to DAG-like networks.


1. Dependencies

pip install -r requirements.txt

2. Prepare models

This tool only supports Kaldi's text model as an input.

If you have a binary model, Kaldi's

tool can help you get a text one:

Nnet2 Model

path/to/kaldi/src/nnet2bin/nnet-am-copy --binary=false          \
                                        final.raw text.mdl

Nnet3 Model

path/to/kaldi/src/nnet3bin/nnet3-copy --binary=false          \
                                      --raw=true              \
                                      --prepare-for-test=true \
                                      final.raw text.mdl

Don't forget to use the

option to optimize the model.

Before converting, you need to use

to get leftcontext\rightcontext\modulus, which is required for converting.

More details about kaldi's tools are in Kaldi's Documentation.

3. Convert

One command to convert:

python converter/ --input=models/kaldi/model_name.mdl  \
                            --output=models/onnx/model_name.onnx \
                            --transition-model=path/to/save/transition_model.trans \
                            --conf=path/to/save/configuration.conf \
                            --chunk-size=20 \
                            --left-context=left_context \
                            --right-context=right_context \
                            --modulus=modulus \
                            --subsample-factor=subsample_factor \

4. Graph review

After converting, there is a graphic tool for you to review the onnx model: ONNX Model Viewer.

5. Deployment

To run the ONNX models, an inference framework with Kaldi specific operators support is needed. Currently, MACE has Kaldi-ONNX support which is primarily optimized for mobile phones and IoT devices.

For more details about deployment, please refer to MACE documents.

6. Validation

Since MACE already supports most frequently used components in Kaldi. We can use it to validate converted Kaldi models. The validation process is giving the same inputs to Kaldi and MACE's computation and check if they will output the same results.

Generate random input

We provide a tool to generate the same random input data for Kaldi and MACE's computation.

python tools/ --input-dim=40 \
                                --chunk-size=20 \
                                --left-context=left_context \
                                --right-context=right_contex \
                                --kaldi-input_file=path/to/kaldi/input/test_input.ark \

'inputdim' and 'chunksize' are model's input dim and chunk, can be specified by case. 'kaldiinputfile' and 'maceinputfile' are paths for saving generated data files.

Compute in Kaldi

Kaldi has command line tools for computing the model's propogation.


path/to/kaldi/src/nnet2bin/nnet-am-compute  --chunk-size=chunk_size \
                                            path/to/kaldi/model/file/final.mdl \
                                            ark,t:/path/to/kaldi/input/file/test_input.ark \


path/to/kaldi/src/nnet3-compute  --frames-per-chunk=chunk_size \
                                 path/to/kaldi/model/file/final.mdl \
                                 ark,t:/path/to/kaldi/input/file/test_input.ark \

Convert output data file

After running

, we'll get Kaldi's output data in text format . Because MACE supports numpy data file as input or output, we need to convert Kaldi's output data file to numpy format.

The script tools/ will help you doing this convertion.

python tools/  --input=path/to/kaldi/data/file.ark \

Prepare config file for MACE

Here is an example yaml config file for fisher english model.

# fisher_english.yml

library_name: fisher_english_8 target_abis: [armeabi-v7a, arm64-v8a] model_graph_format: file model_data_format: file models: fisher_english_8: platform: onnx model_file_path: model_sha256_checksum: e27d8147995b0a68e1367d060dc4f41c0f434043992a52548ff961e4e1e87e6c subgraphs: - input_tensors: - 0 input_shapes: - 1,20,140 output_tensors: - 17 output_shapes: - 1,20,7880 backend: kaldi input_data_formats: NONE output_data_formats: NONE validation_inputs_data: - validation_outputs_data: - backend: kaldi runtime: cpu limit_opencl_kernel_time: 0 nnlib_graph_mode: 0 obfuscate: 0


Convert and validate the model in MACE:

cd path/to/mace
python tools/ convert --config=path/to/fisher_english.yml
python tools/ run --config=path/to/fisher_english.yml --validate

The command will give you the similarity between MACE's and Kaldi's output results.

More details about how to use MACE, please step to MACE documents.

5. Examples

We have converted numbers of Kaldi's Nnet2 and Nnet3 models, and put them in MACE Model Zoo.


  • GitHub issues: bug reports, usage issues, feature requests


Apache License 2.0.


Python code should conform to PEP8 Style Guide for Python Code.

You can use pycodestyle to check the style.

pycodestyle $(find . -name "*.py")

Any kind of contribution is welcome. For bug reports, feature requests, please just open an issue without any hesitation. For code contributions, it's strongly suggested to open an issue for discussion first.


kaldi-onnx learned a lot from the following projects during the development: * Kaldi Speech Recognition Toolkit, * ONNX.

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