Support for Class Based n-gram Language Models in Riva (WFST Decoder)#

Language models are used in speech recognition tasks to help disambiguate which word is more likely to occur given it is preceded by a sequence of words. Conventional n-gram models, model the probability of a word occurring given it is preceded by a sequence of (n-1) words.

However, in many domains, there is a likelihood of having a sequence of words that may be similar in nature.

Consider for example the phrases “I would like to fly from New York to Seattle” and “I would like to fly from London to Paris”, the probabilities of these two sentences are generally only dependent on how often people fly between these
places. Otherwise, the probabilities of occurrence of these two sentences in a domain involving booking of flights is equally probable.

This requires support for building n-gram models that consider word classes. Word classes can also be considered as entities or entity types (as in NER), but are more generic in nature.

WFST Decoding#

Support for word classes with n-grams is not trivial. One alternative could be to generate using BNF grammars. However, this could easily increase the size of the LM and could still cause issues with multi-word word classes.

An alternative approach is to use Weighted-Finite State Transducers (WFSTs). WFSTs as used in speech recognition consists of three main components:

  • Grammar WFST (G.fst) - A grammar WFST encodes word sequences in a language/domain. In general, this is basically a language model represented as a weighted finite state acceptor.

  • Lexicon WFST (L.fst) - A lexicon WFST encodes the mapping between a sequence of tokens (phonemes/BPE/WPE or other CTC units) to the corresponding word.

  • Token WFST (T.fst) - The token WFST maps a sequence of frame-level CTC labels to a single lexicon unit (phonemes/BPE/WPE or other CTC units).

These three WFSTs are composed together with other operations to form TLG.fst. The sequence of operations are T min ( det ( L G ) ) where:

  • refers to the composition operation

  • min refers to minimization

  • det refers to determinization

For more information, refer to “Speech Recognition with Weighted Finite-State Transducers” by Mohri, Pereira and Riley (in Springer Handbook on SpeechProcessing and Speech Communication, 2008).

Supporting Class Language Models in the WFST Framework#

Support for class language models in the WFST framework can be summarized as modifying G.fst to support paths in the acceptor that refers to word classes. This requires that:

  • The lexicon has all entries from both the language model and the entries in each of the word classes. This is important as otherwise the words in the word classes will not be recognized.

  • The n-gram model should have the relevant phrases built from word class grammars. For example, in the above example the word class grammar would be “I would like to fly from #entity:city to #entity:city. With #entity:city being the label for the word class corresponding to the cities with airports. This can be achieved in the following two ways:

    • Build the language model from text containing standard text as well as word class labels

    • n-gram LM interpolation/mixing of two LMs, one from standard text and the other from domain specific grammars.

The latter option provides for better weighting between in-domain and out of domain LMs. Refer to the tutorial “How To Train, Evaluate, and Fine-Tune an n-gram Language Model” for LM model interpolation using KenLM. Alternatively, SRILM (license required) can also be used here.

The process for supporting word classes in the WFST framework basically involves replacement of the arcs representing the class labels, with FSTs created from the entities present in the word classes. Given a G.fst containing word class labels (#entity:<class>), and the files (filenames <class>.txt) containing the list of entities for the class. The list of entities is not restricted to words and could be phrases.

For example, in the above example, cities.txt would contain at the very least the following:

$> cat city.txt
new york

Process to Create TLG.fst Supporting Word Classes#

Checkout the class_lm branch from The following scripts are available in the src/riva/decoder/scripts/prepare_TLG_fst directory path.

  • Generate the lexicon, the list of tokens/units, and the list of entities from the arpa file. This is accomplished using the script. The syntax for the command is as follows: --asr_model <path_to_nemo_model> --lm_path <path_to_arpa_file> --grammars <path_to_folder_containing_grammars> <units_txt_path:output> <lexicon_path:output> <classlabels_path:output>

This generates the list of units(tokens), the lexicon, and the list of word classes (to be used as disambiguation symbols).

  • Generate the FSTs, T.fst, L.fst and G.fst, extend and compose to form TLG.fst. --units <units_txt_path> --extra_word_disamb <classlabels_path> <lexicon_path>  <path_to_arpa_file> <lang_folder_path:output>

When run successfully, the above command will generate a graph folder in <lang_folder_path> with the name graph_CTC_<lm_name>, which contains all the files relevant for decoding with WFST.


The script can help evaluate the WFST decoder before being used in the Riva ServiceMaker build. This would be useful to debug any issues with TLG.fst.

The script can be run as follows: ./ <model_path> <graph_path> <path_containing_audio_files> <results_txt_file:output>

Deploying in Riva#

The generated TLG.fst can be used with the Kaldi decoder in Riva. To build the Riva ASR service using the generated TLG.fst, run:

# Syntax: riva-build <task-name> output-dir-for-rmir/model.rmir:key dir-for-riva/model.riva:key
! docker run --rm --gpus 0 -v $MODEL_LOC:/data $RIVA_SM_CONTAINER -- \
    riva-build speech_recognition \
        /data/rmir/asr_offline_conformer_ctc.rmir:$KEY \
        /data/$MODEL_NAME:$KEY \
        --offline \
        --name=asr_offline_conformer_wfst_pipeline \
        --ms_per_timestep=40 \
        --chunk_size=4.8 \
        --left_padding_size=1.6 \
        --right_padding_size=1.6 \
        --nn.fp16_needs_obey_precision_pass \
        --featurizer.use_utterance_norm_params=False \
        --featurizer.precalc_norm_time_steps=0 \
        --featurizer.precalc_norm_params=False \
        --featurizer.max_batch_size=512 \
        --featurizer.max_execution_batch_size=512 \
        --decoder_type=kaldi \
        --decoding_language_model_fst=/data/graph/TLG.fst \
        --decoding_language_model_words=/data/graph/words.txt \
        --kaldi_decoder.asr_model_delay=5 \
        --kaldi_decoder.default_beam=17.0 \
        --kaldi_decoder.max_active=7000 \
        --kaldi_decoder.determinize_lattice=true \
        --kaldi_decoder.max_batch_size=1  \
        --language_code=en-US \
        --wfst_tokenizer_model=<far_tokenizer_file> \
        --wfst_verbalizer_model=<far_verbalizer_file> \