Java - Convert 16bit Mono PCM raw data to stereo

Question

I am mixing two 16bit PCM samples into a short buffer.

// This is our buffer for PCM audio data
mp3Buffer = new short[minBufferSize];
wavBuffer = new short[minBufferSize];
mixedBuffer = new short[minBufferSize];

I am filling these buffers with samples from both the mp3 and wav files. I found out that the wav file will always be in mono and the mp3 will always be stereo.

I've read that if you "Just allocate a buffer twice the size of the original PCM data, and for every sample in the original buffer put it twice in the new buffer"

short[] stereoWavBuffer = new short[minBufferSize];
int k = 1;
for (int j = 0; j < minBufferSize / 2; j += 2)
{
    stereoWavBuffer[j] = wavBuffer[j];
    stereoWavBuffer[k] = wavBuffer[k];
     k += 2;

                }
                // TO DO -  Add the 2 buffers together

                for (int i = 0; i < minBufferSize; i++){
                    mixedBuffer[i] = (short)(mp3Buffer[i] + stereoWavBuffer[i]);
                }


                track.write(mixedBuffer, 0, minBufferSize); 
}

How can I accomplish this? I tried this but the wav audio now is at regular speed but sounds like chipmunk.

Answer 1

It looks to me as if your first for loop should be

j < minBufferSize - 1

/2 would mean you will never read all of of the wave buffer or write your entire stereo buffer, - even if you only read half the wave buffer because that's all the data since it's mono you still need to write the entire stereo buffer. Also you need to increment J by 1 not 2 so you read each mono sample.

The speed issue appears to be because you should set stereowavebuffer at j and k both equal to wavebuffer at j. it seems that you are in fact just duplicating half of the original mono file. and then playing it back as stereo (ie: double the byterate).

I would think the first loop should look something more like this

int k = 0;
for (int j = 0; j < minBufferSize / 2; j++) //Assuming you only have half a buffer since mono???
{
    stereoWavBuffer[k] = wavBuffer[j];
    stereoWavBuffer[k+1] = wavBuffer[j];
     k += 2;
}

-edited to fix bad ipad typing!