CODEWITHSUNDEEP
javaaudiojavasound
Kevin Johnson
Kevin Johnson

Reputation: 935

What is the format of the data you can write to a SourceDataLine?

In java you can create a SourceDataLine like so:

AudioFormat af = new AudioFormat(AudioFormat.Encoding.PCM_SIGNED, 44100.0, 16, 1, 2, 44100.0, false);       
SourceDataLine sdl = AudioSystem.getSourceDataLine(af);

After which you can open and then write data to it:

byte[] data = new byte[1024];
fillwithsounds(data);

sdl.open()
sdl.start()
sdl.write(data, 0, 1024);

This all works fine for mono data.

What I'd like to do is to be able to write stereo data, and I can't find any documentation online on how I need to change my byte array to be able to write stereo data.

It seems like I need to increase the amount of channels when I create the AudioFormat - to make it stereo - and then I need to half the framerate (otherwise Java throws an IllegalArgumentException)

I don't understand why this is though, or what the new format should be for the data that I feed to the DataSourceLine.

Perhaps somebody with a little more experience in audio formats than I could shed some light on this problem. Thanks in advance!

Upvotes: 1

Views: 1331

Answers (2)

Phil Freihofner
Phil Freihofner

Reputation: 7910

The format I use for stereo is as follows:

        audioFmt = new AudioFormat(AudioFormat.Encoding.PCM_SIGNED, 
            44100, 16, 2, 4, 44100, false);

You probably want to double the bytes per frame instead of halving your bits-encoding. I'm not sure what 8-bit encoding sounds like, but it is definitely going to be noisier than 16-bit encoding!

The resulting file is twice as long. You can then take the two-byte pairs that make the 16-bit sample and copy them into the next two positions, for "mono" playback (both stereo channels identical).

Given:

frame = F
little end byte = A
big end byte = B
AB = 16-bit little-endian encoding
left channel = L
right channel = R

Your original mono:

F1A, F1B, F2A, F2B, F3A, F3B ...

Stereo using the above format:

F1AL, F1BL, F1AR, F1BR, F2AL, F2BL, F2AR, F2BR, F3AL, F3BL, F3AR, F3BR ...

I could very well have the order of left and right mixed up. But I hope you get the idea!

Upvotes: 2

Kevin Johnson
Kevin Johnson

Reputation: 935

I found out the solution just now, and found Andrew Thompson's comment to explain exactly what I needed.

I figured that I'd have to write each frame twice, what caught me up was the fact that Java wouldn't let me just use the frame size I had for my mono channel. (It threw an IllegalArgumentException)

So I halved the framerate to satisfy Java, but I didn't remember to modify the byte array.

I've implemented code that takes the "2 bytes per frame, 1 channel" byte[] and converts it into a "1 byte per frame, 2 channel" byte[].

private static final float LEFT   = -1;
private static final float CENTER = 0;
private static final float RIGHT  = 1;

private byte[] monoToStereo(byte[] data, float pan){
    byte[] output = new byte[data.length];
    for (int i = 0; i < (data.length - 2); i+=2){
        int currentvalue = (data[i+1]*256 + data[i])/(256*2);
        if (pan == LEFT || pan == CENTER){
            output[i] = (byte) currentvalue;
        } else {
            output[i] = 0;
        }
        if (pan == RIGHT || pan == CENTER){
            output[i+1] = (byte) currentvalue;
        } else {
            output[i+1] = 0;
        }
    }
    return output;
}

Using this, I can get stereo audio to playback (although there is soft static, I can clearly hear the original track)

Upvotes: 0

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