# SoundTouch in Android

## Compiling SoundTouch for Android

SoundTouch source code package contains "Android-lib" example project that compiles SoundTouch source codes into Android native library, and gives an example of JNI interface for invoking the native SoundTouch routines from an Android application written in Java.

Software prerequisites:

- Android SDK environment for developing your own Android application. Visit the Android developers' site for more information about the Android SDK and developing Android applications.
- Android NDK compiler kit for compiling native library binaries. The Android NDK is available for download at the Android developer tools site.
- In case you're working in Windows environment, install Cygwin to run the Android NDK/SDK compiler scripts
- Latest SoundTouch source code package available at soundtouch.surina.net.

Hint: As installing and configuring all the components for an Android SDK/NDK environment requires fair effort, it's good idea to create a dedicated Virtual Machine environment for the Android development environment installation. Having the Android developer environment setup in dedicated Virtual Machine allows keeping all these settings isolated from your other PC operations, and eases taking backup snapshots of your full development environment.

Compiling

To compile the SoundTouch library source codes into an Android native library, open Cygwin/bash shell, go to directory "soundtouch/source/Android-lib/jni" and invoke the NDK compiler with following command:

        $NDK/ndk-build

This will build the ARMv5 and ARMv7 versions of SoundTouch library (including also the example JNI interface, see below) into the "libs" subdirectory.

Notice that to allow Cygwin/bash to locate the NDK compile scripts, you need to define the location of the NDK installation defined in environment variable "NDK". That's easiest done by adding the NDK path definition at end of your ~/.bash_profile file, for instance as follows:

        NDK=/cygdrive/d/Android/android-ndk-r6

## Android floating-point performance considerations

Default build target for Android NDK is ARMv5 CPU generation, as that works in all ARM-based Android devices.

This has a pitfall though: For ideal sound quality SoundTouch should be compiled to use floating-point algorithms, however, all low-end Android devices do not have floating-point hardware in their CPUs, and hence the default ARMv5 compilation uses software-emulation for floating-point calculations instead of hardware floating-point to allow running the binary executables also in low-end devices.

The floating point software-emulation is however several tens of times slower than real hardware-level floating-point calculations, making floating-point-intensive applications such as SoundTouch infeasible with low-end devices.

As workaround, the SoundTouch Android compilation builds two separate versions of the library:

- ARMv5 version that compiles SoundTouch using integer algorithm version. The integer algorithm version compromises the sound quality but provides good performance also with low-end devices without hardware floating-point support in the CPU level.
- ARMv7 version that compiles SoundTouch using hardware floating-point algorithms. These algorithms provide ideal sound quality yet do not work in simpler CPU models.

These two library compilations are already defined in file "jni/Application.mk" so that these two separate library targets are automatically built under the "libs" directory. As far as you include both these compiled library versions into your application delivery, the Android devices can automatically select the right library version based on the available device's capabilities.

Please yet be aware that depending on capabilities of the Android devices you will need to provide the SoundTouch routines with samples in either integer or floating-point format, so build your interface routines to take this into account.

## Calling SoundTouch native routines from Android application

The NDK tools build the SoundTouch c++ routines into a native binary library, while Android applications are written in Java language. To call the SoundTouch and other c/c++ routines from an Android java application code, you'll need to use Java Native Interface (JNI).

The SoundTouch source code package provides source code example how to use JNI to call native c++ routines from a Java class through the following source code file pair:

- Android-lib/jni/soundtouch-jni.cpp: This file contains c/c++ routine that calls SoundTouch library routine to return the library version string to the main Android application. The NDK compiles this file along with the SoundTouch routines into the native binary library.
- Android-lib/src/net/surina/soundtouch/SoundTouch.java: This file provides a Java interface class to load the native library and to invoke the native routine implemented in the file soundtouch-jni.cpp

Feel free to examine and extend the provided cpp/java source code example file pair to implement and integrate the desired SoundTouch library capabilities into your Android application.

Copyright © Olli Parviainen