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Use the Project Explorer (left pane in Eclipse) to open the different files and familiarize yourself with each component.

Building the project

Building the project actually consists of first building your Native C code as a library using ndk-build , and then building the Java code, which is responsible for loading the C library and executing your functions.

For this lab project, the build process in Eclipse has already been configured to build everything automatically, and so building your project is as simple as clicking Project>Build Project .

You may want to uncheck Build Automatically , and then build by first Clean ing the project and checking the "Immediately start build" option.

C syntax support

Open .\jni\process.c in the Eclipse editor. This file contains the processing function, which is where you will implement your signal processing. If you see notifications for syntax errors, then in the Project Explorer , right-click on the project name and go to Project Properties>C/C++ General>Paths and Symbols and add the following three files into the Include directories for GNU C : C:\NvPack\android-ndk-r8\platforms\android-9\arch-arm\usr\include C:\NvPack\android-ndk-r8\sources\cxx-stl\gnu-libstdc++\includeC:\NvPack\android-ndk-r8\sources\cxx-stl\gnu-libstdc++\libs\armeabi-v7a\include

Part 2: a development environment for signal-processing applications

Function declarations in java and c

In the Lab4Activity Java class, the following function declaration is made:

public static native void process(ShortBuffer inbuf, DoubleBuffer outbuf, int N);

Where is the Lab4Activity class defined?

According to the "Understanding the Android Project Structure" section in Part 1, you can find it in Lab4Activity.java in a sub-directory of .\src\

Here, the native keyword states that process() is a Native C function. The first argument points to the Java-equivalent of a short[N] array, which holds a block of the 16-bit audio data. The second argument points to the Java-equivalent of a double[N] array, which you have to fill with data to visualize on-screen.

In process.c , the C-equivalent function declaration for process() , along with pointers to the input and output buffers have already been written for you. Appropriate header files have also been included, including the FFTW library, a popular C library for implementing the FFT.

Debugging in java

To introduce the Java debugging environment, you will verify that the mechanism for using test vectors is working correctly. First, in the Lab4Activity class definition, set the variable FILE_INPUT to true . Set a breakpoint on the line where process() is called.

Click Run>Debug , and debug as an Android Application . The project will automatically rebuild if out-of-date, and should switch to the Debug Perspective . The application will automatically run and be halted at the set breakpoint.

Click Window>Show View>Expressions and add the following expression: sb.get(0) , which returns the first value in the buffer. Verify that the value is correct by comparing to the values in the lookup table defined in LOOKUP.java . Repeat this for other values in the table to convince yourself that the test vector values were copied correctly.

The Android Reference guide contains detailed information about the classes you will encounter in Java (e.g., the ShortBuffer class).

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Source:  OpenStax, Ece 420 fall 2013. OpenStax CNX. Sep 26, 2013 Download for free at http://cnx.org/content/col11560/1.3
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