CODEWITHSUNDEEP
androidaudioandroid-music-player
Kunal Aggarwal
Kunal Aggarwal

Reputation: 301

How to convert audio from stereo to mono in Android?

I use the android.provider.MediaStore.Audio.Media.EXTERNAL_CONTENT_URI intent to load music files from the SD Card.

Intent tmpIntent1 = new Intent(Intent.ACTION_PICK, android.provider.MediaStore.Audio.Media.EXTERNAL_CONTENT_URI);
startActivityForResult(tmpIntent1, 0);

and in onActivityResult

Uri mediaPath = Uri.parse(data.getData().toString());
MediaPlayer mp = MediaPlayer.create(this, mediaPath);
mp.start();

Now MediaPlayer plays the audio in stereo. Is there any way to convert the selected music/audio file or the output from stereo to mono in the app itself?

I looked up API for SoundPool and AudioTrack, but didn't find how to convert mp3 files audio to mono.

Apps like PowerAMP have those Stereo <-> Mono switches that when pressed immediately convert the output audio to mono signal and back again, how do they do it?

Upvotes: 1

Views: 9569

Answers (2)

Ismail Osunlana
Ismail Osunlana

Reputation: 434

First of all, get an encoder to convert your audio file to short array (short[]).

Then convert the short array to mono by splitting the streams.

Then average the split streams.

short[] stereo  = new short[BUF_SIZE];
short[] monoCh1 = new short[BUF_SIZE/2];
short[] monoCh2 = new short[BUF_SIZE/2];
short[] monoAvg = new short[BUF_SIZE/2];

stereo = sampleBuffer.getBuffer(); //get the raw S16 PCM buffer here

 for( int i=0 ; i < stereo.length ; i+=2 )
 {
     monoCh1[i/2] = (short) stereo[i];
     monoCh2[i/2] = (short) stereo[i+1];
     monoAvg[i/2] = (short) ( ( stereo[i] + stereo[i+1] ) / 2 );
 }

Now you have:

PCM mono stream from channel 1 (Left channel) in monoCh1

PCM mono stream from channel 2 (Right channel) in monoCh2

PCM mono stream from averaging both channels in monoAvg

Upvotes: 1

G_J
G_J

Reputation: 314

Do you load .wav- files respectively PCM- data? If so then you could easily read each sample of each channel, superpose them and divide them by the amount of channels to get a mono signal.

If you store your stereo signal in form of interleaved signed shorts, the code to calculate the resulting mono signal might look like this: 

    short[] stereoSamples;//get them from somewhere

    //output array, which will contain the mono signal
    short[] monoSamples= new short[stereoSamples.length/2];
    //length of the .wav-file header-> 44 bytes
    final int HEADER_LENGTH=22;

    //additional counter
    int k=0;


    for(int i=0; i< monoSamples.length;i++){
        //skip the header andsuperpose the samples of the left and right channel
        if(k>HEADER_LENGTH){
        monoSamples[i]= (short) ((stereoSamples[i*2]+ stereoSamples[(i*2)+1])/2);
        }
        k++;
    }

I hope, I was able to help you.

Best regards, G_J

Upvotes: 3

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