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STT En Conformer-CTC Medium

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Conformer-CTC-Medium model for English Automatic Speech Recognition, Trained on NeMo ASRSET
Latest Version
April 4, 2023
108.16 MB

Model Overview

This collection contains medium size versions of Conformer-CTC (around 30M parameters) trained on NeMo ASRSet with around 16000 hours of english speech. The model transcribes speech in lower case english alphabet along with spaces and apostrophes.

Model Architecture

Conformer-CTC model is a non-autoregressive variant of Conformer model [1] for Automatic Speech Recognition which uses CTC loss/decoding instead of Transducer. You may find more info on the detail of this model here: Conformer-CTC Model.


The NeMo toolkit [3] was used for training the models for over several hundred epochs. These model are trained with this example script and this base config.

The tokenizers for these models were built using the text transcripts of the train set with this script.

The checkpoint of the language model used as the neural rescorer can be found here. You may find more info on how to train and use language models for ASR models here: ASR Language Modeling


All the models in this collection are trained on a composite dataset (NeMo ASRSET) comprising of several thousand hours of English speech:

  • Librispeech 960 hours of English speech
  • Fisher Corpus
  • Switchboard-1 Dataset
  • WSJ-0 and WSJ-1
  • National Speech Corpus (Part 1, Part 6)
  • VCTK
  • VoxPopuli (EN)
  • Europarl-ASR (EN)
  • Multilingual Librispeech (MLS EN) - 2,000 hours subset
  • Mozilla Common Voice (v7.0)

Note: older versions of the model may have trained on smaller set of datasets.


The list of the available models in this collection is shown in the following table. Performances of the ASR models are reported in terms of Word Error Rate (WER%) with greedy decoding.

Version Tokenizer Vocabulary Size LS test-other LS test-clean WSJ Eval92 WSJ Dev93 NSC Part 1 MLS Test MLS Dev MCV Test 6.1 Train Dataset
1.6.0 SentencePiece Unigram 1024 5.9 2.6 2.3 3.7 6.3 8.7 7.9 11.8 NeMo ASRSET
1.0.0 SentencePiece Unigram 128 7.0 2.9 2.5 3.9 7.0 - - - NeMo ASRSET
rc1.0.0 WordPiece 128 7.4 3.0 - - - - - - LibriSpeech

You may use language models to improve the accuracy of the models. The WER(%) of the latest model with different language modeling techniques are reported in the follwoing table.

Language Modeling Training Dataset LS test-other LS test-clean Comment
N-gram LM LS Train + LS LM Corpus 4.6 2.0 N=10, beam_width=128, n_gram_alpha=1.0, n_gram_beta=1.0
Neural Rescorer(Transformer) LS Train + LS LM Corpus 4.4 1.9 N=10, beam_width=128
N-gram + Neural Rescorer(Transformer) LS Train + LS LM Corpus 4.1 1.8 N=10, beam_width=128, n_gram_alpha=1.5, n_gram_beta=1.5

How to Use this Model

The model is available for use in the NeMo toolkit [3], and can be used as a pre-trained checkpoint for inference or for fine-tuning on another dataset.

Automatically load the model from NGC

import nemo.collections.asr as nemo_asr
asr_model = nemo_asr.models.EncDecCTCModelBPE.from_pretrained(model_name="stt_en_conformer_ctc_medium")

Transcribing text with this model

python [NEMO_GIT_FOLDER]/examples/asr/ \
 pretrained_name="stt_en_conformer_ctc_medium" \


This model accepts 16000 KHz Mono-channel Audio (wav files) as input.


This model provides transcribed speech as a string for a given audio sample.


Since this model was trained on publically available speech datasets, the performance of this model might degrade for speech which includes technical terms, or vernacular that the model has not been trained on. The model might also perform worse for accented speech.


[1] Conformer: Convolution-augmented Transformer for Speech Recognition

[2] Google Sentencepiece Tokenizer

[3] NVIDIA NeMo Toolkit


License to use this model is covered by the NGC TERMS OF USE unless another License/Terms Of Use/EULA is clearly specified. By downloading the public and release version of the model, you accept the terms and conditions of the NGC TERMS OF USE.