android PPM audio library

I need to implement audio PPM (Pulse Position Modulation) on android

Reference: http://ift.tt/PdoAGi

I want to output PPM from the audio output of the smartphone. The final scope is to create a joystick for radiocontrol. but this library may have many future purposes (follow me, lightbridge, etc.etc.). The radios commonly have a PPM output. Transmitters (and pc flight simulators) commonly have PPM input. My scope is to replace the radio with an android device. I wish to know if there is some piece of code ready to use or should i start from scratch?

EDIT: I found some points where to start

1) smartpropplus is a windows software that receives PPM audio and decodes it http://ift.tt/1OTf5vR

2) this is how PPM is structured: http://ift.tt/1DSNF5u

3) this is a easy image that explains how the signal is structured: http://ift.tt/1DSNF5w

I calculated that sampling the audio signal at 22000Hz will be sufficient to achieve a good resolution for each channel (22 steps for each channel)

EDIT: I started to write the class, cause no one of you wrote a solution. actually this test application outputs a sine waveform frame. now it misses the functions to set the channels inside frame. i ear strange noise over the sine waveform, and i'm not sure which is the reason why:

this is the class PPMencoder

  package com.tr3ma.PPMencoderProject;

  import android.content.Context;
  import android.media.AudioFormat;
  import android.media.AudioManager;
  import android.media.AudioRecord;
  import android.media.AudioTrack;
  import android.os.AsyncTask;

  public class PPMencoder  {

int samplingRate=44100;

//private int bufferSize ;
private int streamBufferSize = (int)(samplingRate* 0.0225); // 22KHz * 22,5ms 
public AudioTrack audioPlayer ;
AudioManager audioManager;
private boolean started;
private byte[] frame;

public PPMencoder(Context context) {

    //bufferSize= AudioRecord.getMinBufferSize(samplingRate, AudioFormat.CHANNEL_OUT_MONO, AudioFormat.ENCODING_PCM_16BIT);

    //set volume to max
    audioManager=(AudioManager) context.getSystemService(Context.AUDIO_SERVICE);
    int tmpVol = audioManager.getStreamMaxVolume(AudioManager.STREAM_MUSIC);    
    audioManager.setStreamVolume(AudioManager.STREAM_MUSIC, tmpVol, 0);
    frame=new byte[streamBufferSize*2]; //doubled cause each sample is 1 integer

}

public int setSamplingRate(int freq) {
    //we can change the sampling frequency in case it is not supported
    samplingRate=freq;
    //bufferSize = AudioRecord.getMinBufferSize(samplingRate, AudioFormat.CHANNEL_OUT_MONO, AudioFormat.ENCODING_PCM_16BIT);

    //if (!(bufferSize>0)){
    //  //the sampling frequency is not supported
    //  return -2;
    //}
    streamBufferSize = (int)(samplingRate* 0.0225); // 22KHz * 22,5ms
    frame=new byte[streamBufferSize*2];
    return 0;

}

public int startGeneration(){
    try {
        //if (!(bufferSize>0)){
        //  //the sampling frequency is not supported
        //  return -2;
        //}



        //start the loop that sends out the data continuously
        started=true;
        StreamAudio streamAudio;
        streamAudio=new StreamAudio();
        streamAudio.execute();

        return 0;
    } catch (Exception e) {
        e.printStackTrace();
        return -1;
    }
}

public int stopGeneration(){
    try {
        started=false;
        if (audioPlayer==null) return 0;
        audioPlayer.stop();
        audioPlayer.flush();
        audioPlayer.release();
        return 0;
    } catch (Exception e) {
        e.printStackTrace();
        return -1;
    }
}

public int setTestFrame() {
    int tmpIndex=0;
    float tmpVal;

    //let's generate a sine at 2kHz
    int sineFreq=2000;
    for (int i = 0;i<frame.length;i=i+2){
        tmpVal = (float) Math.sin( (float)tmpIndex * ((float)(2*Math.PI) * sineFreq / samplingRate));    //the part that makes this a sine wave....
        short tmpVal1 = (short) (tmpVal * 32767);
        frame[i]     = (byte) (tmpVal1 & 0x00FF); 
        frame[i+1] = (byte) ((tmpVal1 & 0xFF00) >> 8);
        tmpIndex=tmpIndex+1;
    }

    return 0;

}

public class StreamAudio extends AsyncTask<Void, Double, Void> {

    @Override
    protected Void doInBackground(Void... arg0) {
        AudioTrack audioPlayer = new AudioTrack(AudioManager.STREAM_MUSIC, samplingRate,  AudioFormat.CHANNEL_OUT_MONO,
                AudioFormat.ENCODING_PCM_16BIT, frame.length, AudioTrack.MODE_STREAM);


        //set volume of audioplayer to max
        audioPlayer.setStereoVolume((float)1.0,(float)1.0);

        if(audioPlayer.getPlayState() != AudioTrack.PLAYSTATE_PLAYING)
        audioPlayer.play();


        //feed the speakers with our audio, by continuously send the PPM frame
        while (started) {
            try {
                //if ( audioPlayer.getState() != AudioTrack.STATE_INITIALIZED){
                    audioPlayer.write(frame, 0, frame.length);
                //}

            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        return null;
    }
  }
 }

and this is the activity class used to test the library:

  package com.tr3ma.PPMencoderProject;

  import android.os.Bundle;
  import android.app.Activity;
  import com.tr3ma.PPMencoderProject.R;

  public class Test extends Activity {

PPMencoder ppmencoder;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_test);

    ppmencoder=new PPMencoder(this);

    //Just for test, let's feed the audio with a sine waveform
    ppmencoder.setTestFrame();

    //start the generation of the signal through the speakers
    int result=ppmencoder.startGeneration();
    if (result==-2){
        //sampling frequency not allowed, you can ask the user to change it
    }
    if (result==-1){
        //error occoured, something went wrong
    }




}

@Override
protected void onDestroy() {
    super.onDestroy();
    int result=ppmencoder.stopGeneration();
    if (result!=0){
        //error occoured, something went wrong
    }
}
  }

and this is the manifest file:

 <?xml version="1.0" encoding="utf-8"?>
 <manifest xmlns:android="http://ift.tt/nIICcg"
package="com.tr3ma.PPMencoderProject"
android:versionCode="1"
android:versionName="1.0" >
<uses-permission android:name="android.permission.MODIFY_AUDIO_SETTINGS"/>
<uses-permission android:name="android.permission.RECORD_AUDIO" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

<uses-sdk
    android:minSdkVersion="8"
    android:targetSdkVersion="17" />

<application
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name="com.tr3ma.PPMencoderProject.Test"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
</application>

 </manifest>