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0.10 Lab 4: lab

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Implement a spectrogram application in the Native Android environment. Signal processing includes windowing, zero-padding, overlapping, FFT, and working with complex numbers.

Lab overview

In this lab, you will create an Android application that plots the spectrogram of streaming audio, and deploy it on the Google Nexus 7 tablet.

Similar to the previous labs, you will be provided with an existing project with all of the required peripherals already set up. Unlike the previous labs where data was processed sample-by-sample, this lab requires block-based processing.

What is the minimum latency for processing a block of length 256, with an audio sampling rate of 8 kHz?

[link] shows a block diagram of the system you will be implementing. The project is already configured to stream audio and display the processed output to the screen.

System-level block diagram

Audio capture -> Audio buffer -> Preprocessing -> FFT -> Energy calculation -> Display

You will be focusing on the signal processing tasks, which will be implemented in native C code. For those that are interested in the Android Java specifics, we will provide an optional tutorial that shows you how to build the project from scratch, explains the different Android classes that were used, and along the way, provides supplemental links to references and other useful Android tutorials available on the web.

Part 1: getting started with android and eclipse

In this section, you first setup your Android device in development mode, import a skeleton project into Eclipse, and familiarize yourself with the Android project structure and build process.

Setting up the google nexus for development

On the tablet, you must enable the Developer options under Settings :

  1. Go to Settings ,
  2. Click on About tablet ,
  3. Click Build number seven times (yes, 7).

Under Settings>Developer options , enable the Stay awake and USB debugging options.

Setting up eclipse

The development environment you will be using is the Nvidia Tegra Android Development Pack 2.0 . To get started:

  1. Start Cygwin by double-clicking on C:\NvPack\cygwin\cygwin.bat
  2. In the prompt, navigate to the Eclipse folder by typing cd /cygdrive/c/NvPack/eclipse
  3. Launch Eclipse with ./eclipse
Cygwin provides a Linux-like environment for Windows, and launching Eclipse from within Cygwin is required for native C debugging to work correctly.

Choose a new workspace on your U: drive, similar to what you did in Lab 0. This step feels familiar because CCS is actually based on the Eclipse framework.

Importing the project

Once Eclipse opens, select File>Import...

  1. General>Existing Projects into Workspace
  2. V:\ece420\nexus\Lab4\
  3. Check "Copy into Workspace"
Close out of the "Welcome" screen.

Once the project is imported, it will try to build automatically. If you see a build error, you will need to define the $NDKROOT variable in your workspace:

  1. Go to Project>Properties
  2. Expand C/C++ Build>Environment
  3. Add...
  4. Name: NDKROOT, Value: C:\NvPack\android-ndk-r8

Understanding the android project structure

An Android project with Native code support has 4 main components:

  • .\AndroidManifest.xml - contains app-related information, such as project name, activities, and required peripherals (e.g., microphone).
  • .\res\layout\main.xml - describes the layout of the user interface. In our project, we define an ImageView , which is used to display the spectrogram.
  • .\src\ - contains the Java source files.
  • .\jni\ - contains the native C source and Make files.

Questions & Answers

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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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