PocketSphinx on Android

Caution! This tutorial has not been tested for quite some time. It is possible that it no longer works with the latest versions of Android and the developer tools.

This tutorial describes a demo app that is available on GitHub.

PocketSphinx Android demo

In order to run the demo app we recommend to use Android Studio. You can download the Android Studio IDE and sdk from the official download page.

Building and running from Android Studio

In order to obtain the demo in the IDE, please select to checkout a project from VCS, select GitHub and enter the project URL: https://github.com/cmusphinx/pocketsphinx-android-demo.

Once the project is set up, your IDE will update and download all dependencies automatically. You should now be able to run the project.

After starting the app the recognizer will take some time to initialize. After initialization it will wait for the keyword “oh mighty computer”. Once this keyword is detected, it will ask you to select the demo – “digits”, “weather” or “phones”. The “digits” demo recognizes digits from 0 to 9, the “weather” demo recognizes weather forecasts and the “phones” demo demonstrates phonetic recognition.

To try a certain demo, import this project into the IDE and run it as usual. In case of errors please check the logcat for further details.

Building and running from Eclipse

We do not support the Eclipse project anymore, please consider an SDK upgrade.

Building and running from the command line

You can also build the project with the gradle build system.

  1. Clone the repo from Github:
    git clone https://github.com/cmusphinx/pocketsphinx-android-demo.git.
  2. Attach your physical device or setup a virtual device.
  3. Create a file local.properties to point to sdk folder:
    sdk.dir = /home/user/android/sdk.
  4. Run gradle installDebug. It will build and install the application on the device.
  5. Manually run the application from the device application menu.

Using pocketsphinx-android

Referencing the library in an Android project

The library is distributed as an Adroid Archive (AAR) which includes both the binary so files for different architectures and independent Java code.

In Android Studio you need to include the AAR into your project. Just go to File > New > New module and choose Import .JAR/.AAR Package.

You can also add the AAR to your project using the command line as described in this stackoverflow post.

Once the AAR is imported as module into the project, make sure it is listed as a dependency of a main module in app/build.gradle:

dependencies {
    compile project(':aars')

Setting permissions

In order to store asset files your application must have WRITE_EXTERNAL_STORAGE permission. To record audio you need RECORD_AUDIO permission. Please note that since Android 6.0, RECORD_AUDIO is not automatically enabled, but must be confirmed in the application settings manually.

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.RECORD_AUDIO" />

Including resource files

The standard way to ship resource files with your application in Android is to put them in the assets/ directory of your project. In order to make them available for pocketsphinx, these files should have physical path. However, as long as they are in .apk they don’t have such a physical path.

The Assets class from pocketsphinx-android provides a method to automatically copy asset files to the external storage of the target device. edu.cmu.pocketsphinx.Assets#syncAssets synchronizes resources by reading items from the assets.lst file which is located on the top assets/. Before copying it matches the MD5 checksums of an asset and a file on external storage with the same name in case it exists.

It only copies the files if there is incomplete information (no file on the external storage, .md5 files are not available) or if there is a hash mismatch. The PocketSphinxAndroidDemo contains an ant script that generates assets.lst as well as the .md5 files, look for assets.xml.

Please note that the assets might be out of sync if the ant build script didn’t run properly in your build process. So, you should make sure that the script runs or to create the md5 files and assets.lst yourself.

To integrate the assets sync into your application do the following:

  1. Copy the models/assets.xml build file from the demo application into your application into the same folder app.
  2. Edit the app/build.gradle build file to run assets.xml, just as in the Android demo:
ant.importBuild 'assets.xml'
preBuild.dependsOn(list, checksum)

That should do the trick. You can now verify that the assets.lst file was created and that the md5 files are updated.

Sample application

The classes and methods of pocketsphinx-android were designed to resemble the same workflow used in pocketsphinx, except that basic data structures are turned into classes and functions that work with these structures are turned into methods of the corresponding classes. So, if you are familiar with pocketsphinx you should feel comfortable with pocketsphinx-android, too.

The SpeechRecognizer is the main class to access decoder functionality. It is created with the help of a SpeechRecognizerSetup builder. A SpeechRecognizerBuilder allows to configure the main properties as well as other parameters of the decoder. The parameters’s keys and values are the same as those that are passed to the pocketsphinx binaries on the command-line. Read more about tweaking the performance of pocketsphinx here.

recognizer = defaultSetup()
	    .setAcousticModel(new File(assetsDir, "en-us-ptm"))
	    .setDictionary(new File(assetsDir, "cmudict-en-us.dict"))

The decoder configuration is a lengthy process that contains IO operations, so it’s recommended to run it inside an async task.

A decoder supports multiple named searches which you can switch in runtime:

// Create keyword-activation search
recognizer.addKeyphraseSearch(KWS_SEARCH, KEYPHRASE);

// Create grammar-based searches
File menuGrammar = new File(assetsDir, "menu.gram");
recognizer.addGrammarSearch(MENU_SEARCH, menuGrammar);

// Next search for digits
File digitsGrammar = new File(assetsDir, "digits.gram");
recognizer.addGrammarSearch(DIGITS_SEARCH, digitsGrammar);

// Create language model search
File languageModel = new File(assetsDir, "weather.dmp");
recognizer.addNgramSearch(FORECAST_SEARCH, languageModel);

Once you set up the decoder and added all the searches you can start recognition with:


You will get notified on the speech end event in the onEndOfSpeech callback of the recognizer listener. Then you could call recognizer.stop() or recognizer.cancel(). Latter will cancel the recognition, former will cause the final result to be passed in the onResult callback.

During the recognition you will receive partial results in the onPartialResult callback.

You can also access other Pocketsphinx methods that are wrapped in Java classes in swig. For details check for the Decoder, Hypothesis, Segment and NBest classes.

Building pocketsphinx-android

Pocketsphinx is provided with prebuilt binaries and it’s challenging to compile it on various platforms. Unless you fully understand what you are doing, you should rather not build it yourself. We recommend to use prebuilt binaries instead.

Build dependencies

Building steps

You need to checkout sphinxbase, pocketsphinx and pocketsphinx-android and put them in the same directory.

├─ pocketsphinx
├─ sphinxbase
└─ pocketsphinx-android

Older versions might be incompatible with the latest pocketsphinx-android, so you need to make sure you are using the latest versions. You can use the following commands to checkout the repositories:

git clone https://github.com/cmusphinx/sphinxbase
git clone https://github.com/cmusphinx/pocketsphinx
git clone https://github.com/cmusphinx/pocketsphinx-android

After arranging the directories you need to update the file local.properties in the project root and define the following properties:

  • sdk.dir – the path to the Android SDK
  • ndk.dir – the path to the Android NDK

For example:


After everything is set, run gradle build. This will create pocketsphinx-android-5prealpha-debug.aar and pocketsphinx-android-5prealpha-release.aar in build/outputs/aar.

Building an application with pocketsphinx Building a dictionary