OpenEars 2.5 is out

See the details here. OpenEars is now able to support speech recognition with English, Spanish, Mandarin Chinese, French, German, and Dutch.

Pocketsphinx wrappers with SWIG for Ruby and Javascript

There is a big demand in support of speech recognition API in various programming environments. Python is getting a lot of traction, Ruby is still widely used among ROR developers, then Javascript is getting more importance, even among non-web programmers. Surprisingly, Node is quite popular in robotics and embedded development these days. Then there is Java on servers and Java on Android, C# on Windows mobile and desktop, C# in mono and Unity3d, a wide-spread game platform. Then there is GStreamer multimedia framework, also quite popular due to support for various complex media processing pipelines. Last, Microsoft somehow decided to forget C# and recently invented Managed C++ for Windows Phone. Hopefully, they will forget about that beast soon.

It might be reasonable to create a specific wrapper for every case, and there are many ways to write wrappers. For example in Python you can use FFI, cython and few other technologies. But you need to take into account that wrapper is not simply a piece code to invoke methods. Language support goes with documentation on how to use the decoder in particular language, tests, performance evaluations and so on and so forth. It is very complex to maintain even simple bindings for the language, imagine if you need to support 10 of them. There are example of custom wrappers created for CMUSphinx, Pocketsphinx-Ruby based on FFI or Node wrapper for Pocketsphinx. Looking on those efforts you can see that while extremely useful, such frameworks are significantly inconsistent, making it harder to reuse C documentation and C examples, thus increasing maintenance effort to support them all.

There are very good solutions for this problem, one of them is SWIG, a wrapper generator with special API description language. With SWIG one can easily create interfaces in dozen languages and access all pocketsphinx features in a consistent way.

Recently we have implemented a support for SWIG wrappers for several languages like Ruby and Node Javascript. We also supported Python and Java bindings in SWIG for quite some time. As a result, you can get consistent native language interface in many languages:

For example, here is Ruby code:

require 'pocketsphinx'
config = Pocketsphinx::Decoder.default_config()
config.set_string('-hmm', '../../model/en-us/en-us')
config.set_string('-dict', '../../model/en-us/cmudict-en-us.dict')
config.set_string('-lm', '../../model/en-us/en-us.lm.bin')
decoder =
open("../../test/data/goforward.raw") {|f|
    while record =
         decoder.process_raw(record, false, false)
puts decoder.hyp().hypstr()
decoder.seg().each { |seg|
      puts "#{seg.word} #{seg.start_frame} #{seg.end_frame}"

And here is Javascript code

var fs = require('fs');
var ps = require('pocketsphinx').ps;
modeldir = "../../model/en-us/"
var config = new ps.Decoder.defaultConfig();
config.setString("-hmm", modeldir + "en-us");
config.setString("-dict", modeldir + "cmudict-en-us.dict");
config.setString("-lm", modeldir + "en-us.lm.bin");
var decoder = new ps.Decoder(config);
fs.readFile("../../test/data/goforward.raw", function(err, data) {
     if (err) throw err;
     decoder.processRaw(data, false, false);
     it = decoder.seg().iter()
     while ((seg = != null) {
           console.log(seg.word, seg.startFrame, seg.endFrame);

Here is Python code:

from pocketsphinx.pocketsphinx import *
from sphinxbase.sphinxbase import *
modeldir = "../../../model"
config = Decoder.default_config()
config.set_string('-hmm', path.join(modeldir, 'en-us/en-us'))
config.set_string('-lm', path.join(modeldir, 'en-us/en-us.lm.bin'))
config.set_string('-dict', path.join(modeldir, 'en-us/cmudict-en-us.dict'))
decoder = Decoder(config)
stream = open(path.join("../../../test/data", 'goforward.raw'), 'rb')
while True:
  buf =
  if buf:
    decoder.process_raw(buf, False, False)
hypothesis = decoder.hyp()
print ('Best hypothesis: ', hypothesis.hypstr, " model score: ", hypothesis.best_score, " confidence: ", hypothesis.prob)
print ('Best hypothesis segments: ', [seg.word for seg in decoder.seg()])

There are two issues in using this technology. First, SWIG is not always easy for newcomers, the syntax is complex and not always easy to comprehend, however, the whole framework is extremely flexible and enables to implement very complex features like iterators. So additional effort to learn SWIG is definitely justified and we have quite some experience now, so we can help anyone interested to add support for a new language.

Second issue is that it is not always easy to support many language features, async interfaces of Node require a lot of additional work on top of C API. However, such API must be implemented as an extension of SWIG-created interface, making it easy to keep consistent API across languages and frameworks and allowing us to improve C API as well. This is the approach we took with Java on Android, I believe this approach could be successful for other languages.

A separate project node-pocketsphinx which enables simple installation of Node wrapper with npm package manager is a tiny layer above SWIG-created wrapper. You only need to provide package files and you can enjoy full decoder features of NPM Pocketsphinx module. It still misses async API which is important for Node, but we hope to add it soon.

Large varieties of use cases we meet with shared language wrappers are very helpful for C API design as it pointed above. For example, we recently changed word segment iterator C API to provide better consistency across different languages and simplified access. Hopefully, such activity will enable us to create a good stable C API for the upcoming release of CMUSphinx framework version 5.

QtSpeechRecognition API for Qt Using Pocketsphinx

Qt Logo
It is really great to see the wide variety of APIs raising around Pocketsphinx, one recent new one is QtSpeechRecognition API implemented by Code-Q for assistive applications. This undertaking is quite ambitious, the main features include

  • Speech recognition engines are loaded as plug-ins.
  • Engine is controlled asynchronously, causing only minimal load to the
    application thread.
  • Built-in task queue makes plug-in development easier and forces
    unified behavior between engine integrations.
  • Engine integration handles the audio recording, making it easy to use
    from the application.
  • Application can create multiple grammars and switch between them.
  • Setting mute temporarily disables speech recognition, allowing
    co-operation with audio output (speech prompts or audio cues).
  • Includes integration to PocketSphinx engine (latest codebase) as a

You can discuss features and find more details on the following thread in Qt mailing list. You can find the sources in review in qtspeech project, branch wip/speech-recognition.

The implementation already includes pretty interesting features, for example it intelligently saves and restores CMN state for more robust recognition. So let us see how it goes.

New language model binary format

Expectations for the vocabulary size in LVCSR has grown dramatically in recent years. 150 thousand words is a must for modern speech recognizers while 10 years ago most system operated only with 60 thousand words. In several morphologically-rich languages like Russian the vocabulary of such size is critical for good OOV rate, but even in English it is important because of the variety of topics one can expect as an input. With such a large vocabulary ngram language models should store millions of ngrams and their weights, which requires memory efficient data structure that allows fast queries. Ngram language models are also widely used in NLP, machine translation, so this topic got a lot of attention in recent years. Several toolkits for language modeling like SRILM, IRSTLM, MITLM, BerkeleyLM implement special data structures to hold the language model.

CMUSphinx decoders use its own ngram model data structure that support files in ARPA and DMP format. While it has some fancy techniques like trie organization of ngrams, simple weight quantizing and sorted ngram arrays, there is a serious shortcoming. Word ID is limited with uint16 type, so maximum vocabulary is 65k words. Simply replacing the ID type could seriously increase currently used language models sizes. Moreover, current implementation is limited by a maximum ngram order of 3. So it was decided to implement a new state-of-art data structure. KenLM reverse trie data structure was selected as a base for CMUSphinx implementation. “Reverse” means that last word of ngram is looked up first:

In example above trigram “is one of” is queried. Each node contains “next pointer” to the beginning of successors list. Separate array for each ngram order is maintained. Each node contains quantized weights: probability and backoff. Nodes are stored in bit array, i.e. minimum amount of bits required to store next pointers and word IDs are used.

Those ideas where carefully implemented in both sphinxbase and sphinx4 and now the one can use language model of unlimited vocabulary size with improved memory consumption and query speed. It is still possible to use arpa and dmp models, but to enhance loading time, convert your model into trie binary format using sphinx_lm_convert:

sphinx_lm_convert -i your_model.lm.(dmp) -o your_model.lm.bin

Worth to mention that while it is possible to read DMP format, ability to generate DMP files is removed.

New generic 70k language model also landed in trunk. Check out its performance and report how it works on your tasks. It is expected to somewhat boost recognition accuracy decreasing OOV rate. For example in lecture transcription task OOV decreased by factor of 2.

Looking forward for your feedback: opinions, suggestions, bug reports. Say thanks to Vyacheslav Klimkov for the amazing implementation!