Simultaneous translation (also known as real-time or streaming translation) is the task of generating translations incrementally given partial input only. Simultaneous translation enables interesting applications such as automatic simultaneous interpretation or international conference translations. Simultaneous systems are typically evaluated with respect to quality and latency. This year, we will have 2 tracks and 3 language pairs:

  • Text-to-Text: translating the output of a streaming ASR system in real-time from English to German, English to Japanese, and English to Mandarin Chinese.
  • Speech-to-Text: translating speech into text in real-time from English to German, English to Japanese, and English to Mandarin Chinese.

We want to highlight the differences with respect to last edition:

  • for the text-to-text track, we will use the output of a streaming ASR system as input instead of the gold transcript. As a result, both text-to-text and speech-to-text systems will be ranked together for a given language pair.
  • we are adding Mandarin Chinese as a target language.
  • we are adding an experimental manual evaluation for English-to-German real-time translation
  • we are adding human interpretation benchmark for English-to-German speech translation
  • in order to reduce the number of conditions, we use only segmented input; the manual evaluation will run on reconstructed full documents

We encourage participants to enter all tracks when possible. We also encourage participants to contrast cascaded and end-to-end solutions for the Speech-to-Text track.

Streaming ASR System

We will use an Emformer model implemented in torchaudio. The model is trained on English-German MuST-C v2 (see training recipe). Example usage is available as an ASR demo script (see also screencast for an example). This means that the input to the text-to-text track will be cased and punctuated.

In order to evaluate the corresponding speech-to-text latency for a text-to-text system, you may use SimulEval with an additional argument pointing to a timestamp file. See more details in the SimulEval documentation.

You can find the dev and tst-COMMON sets decoded with the streaming ASR model here, along with instructions on how to run the model. Please use these files in order to determine the latency regime of your text-to-text systems.


This year, we will use automatic evaluation very similar to the last year and we will trial a manual evaluation for the English-to-German track.

Automatic Evalution

We will use a very similar system as last year for evaluation. The system’s performance will be evaluated in two ways:

  • Translation quality: we will use BLEU.
  • Translation latency: we will use standard metrics for simultaneous machine translation including average proportion (AP), average lagging (AL) and differentiable average lagging (DAL).

Like last year, the evaluation implementation will use the SimulEval toolkit. For latency measurement, we will contrast computation aware and non computation aware latency metrics. See the SimulEval description for how those metrics are defined. Note that the definition of average lagging has been modified from the original definition (see section 3.2 in the SimulEval description). The latency is calculated on word level for En-De systems and character level for En-Ja systems and En-Zh systems.

The participants will submit a Docker image (see below for an example) and the organizers will run the image in a controlled environment, specifically an p3.2xlarge AWS instance (see details in

Ranking for Automatic Evaluation

We will evaluate translation quality with detokenized BLEU and latency with AP, AL and DAL. The systems will be ranked by the translation quality with different latency regimes. Three regimes, low, medium and high, will be evaluated. Each regime is determined by a maximum latency threshold. The thresholds are determined by non computation-aware AL (NCA-AL), which represents the delay to the perfect real time system, but all three latency metrics, AL, DAL and AP will be reported. Based on analysis on the quality-latency tradeoffs for the baseline systems, the thresholds are set as follows:

English-German (both speech-to-text and text-to-text):

  • Low Latency: AL < = 1000
  • Medium Latency: AL < = 2000
  • High Latency: AL < = 4000


  • Low Latency: AL < = 2500
  • Medium Latency: AL < = 4000
  • High Latency: AL < = 5000


  • Low Latency: AL < = 2000
  • Medium Latency: AL < = 3000
  • High Latency: AL < = 4000

The submitted systems will be categorized into different regimes based on the NCA-AL calculated on the MuST-C English-German, English-Mandarin and English-Japanese test sets (tst-COMMON), while the translation quality will be calculated on the blind test set. We require participants to submit at least one system for each latency regime. Participants are encouraged to submit multiple systems for each regime in order to provide more data points for latency-quality tradeoff analyses. If multiple systems are submitted, we will keep the one with the best translation quality for ranking. In addition, within each latency regime, we will also measure computation aware AL and rank systems accordingly. Finally, we will report latency-quality trade-off curves for non computation aware AL and for computation aware AL in the findings paper.

Note that we will use tst-COMMON from the v2.0 release of MuST-C in order to determine the latency regime.

Manual Evaluation

The English-to-German track will include manual evaluation of simultaneous speech translation for at least one variant of submitted system for each participating team (based on the selection by the team).

The evaluation will consist in playing the source sound/video with live text captions to speakers fluent in the source English and native in the target German, and collecting “continuous ranking”. This method is described in Section 3.1.1 (page 22) in the master thesis by Dávid Javorský.

As a benchmark, human interpretations presented in the exact same form of live text captions, will be scored in the same setting.

Training and Development Data

You may use the same training and development data available for the Offline Speech Translation task. Specifically, please refer to the Allowed Training Data and the Past Editions Development Data sections.

Baseline Implementation and Example

English-to-German Speech-to-Text Translation

You can find a baseline and instructions on how to reproduce it here. Our final evaluation will be run inside Docker. To run an evaluation with Docker, first build a Docker image from the Dockerfile. Here is an example Dockerfile for the baseline:

FROM ubuntu:20.04


RUN apt-get update && apt-get install -y build-essential git python3 python3-pip libsndfile1
RUN git clone /fairseq
RUN pip3 install torch torchaudio vizseq soundfile sentencepiece sacrebleu=="1.5.1"
WORKDIR /fairseq
RUN pip3 install -e .
RUN git clone /SimulEval
WORKDIR /SimulEval
RUN pip3 install -e .
RUN ln -s /usr/bin/python3 /usr/bin/python

ENTRYPOINT simuleval \
    --agent $AGENT \
    --source $SRC_FILE \
    --target $TGT_FILE \
    --output $OUTPUT \
    --scores $EXTRA_AGENT_ARGS

Assuming your current directory contains the Dockerfile above, you can run the following commands to run the baseline evaluation inside Docker:

WORKDIR=<WORKING DIRECTORY> # `input` contains source/target files and wav files, `models` contains databin and the checkpoint
EXTRA_AGENT_ARGS="--data-bin $DATA --config $CONFIG_YAML --model-path $MODEL"

docker build -t iwslt2021_simulst_baseline:latest .
docker run -e AGENT="$AGENT" -e SRC_FILE="$SRC_FILE" -e TGT_FILE="$TGT_FILE" -e EXTRA_AGENT_ARGS="$EXTRA_AGENT_ARGS" -e OUTPUT="$OUTPUT" -v $FAIRSEQ:/SimulEval/fairseq -v $WORKDIR/input:/SimulEval/input -v $WORKDIR/models:/SimulEval/models -it iwslt2021_simulst_baseline

If you encounter a bus error similar to this issue, you can try adding --shm-size 8G to the docker run command.

When submitting your system, please make sure it works for the MuST-C dev and test sets. During the official evaluation, we will run the submitted system with the blind set.

English-to-Japanese Speech-to-Text Translation

You can find a baseline and instructions on how to reproduce it here.

English-to-Japanese Text-to-Text Translation

You can find a baseline and instructions on how to reproduce it here.

System Submission

Note: the deadline has been extended to March 29 2022 EOD anywhere on earth.

Participants are required to run the evaluation on the dev and tst-COMMON MuST-C sets and report the results as part of the submission. This is to make sure that the submitted systems work so that organizers can run them as well. The submission files should be packed into a zip or tar.gz file and uploaded to Dropbox prior to the deadline. The submission files should include instructions on how to run the system in a README or file as well as all the necessary files (Docker image, checkpoints, vocabulary, etc.) for the organizers to be able to run the system.


Results are reported in the findings paper. We’ve also made available the system logs.




  • Katsuhito Sudoh (NAIST)
  • Satoshi Nakamura (NAIST)
  • Ondřej Bojar, Věra Kloudová, Dávid Javorský (Charles University)
  • Barry Haddow (University of Edinburgh)
  • Jiatong Shi (CMU)
  • Shinji Watanabe (CMU)
  • Xutai Ma (Johns Hopkins University, Meta)
  • Maha Elbayad (Meta)
  • Changhan Wang (Meta)
  • Hongyu Gong (Meta)
  • Juan Pino (Meta)