Swing Hacks

380

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Chapter 10, Audio

#75 Build an Audio Waveform Display

HACK

Frame

A frame is a cross section of samples across all channels in the audio file.

So, a 16-bit stereo (two channel) audio file will have 32-bit frames (16

bits per sample * 2 channels per frame = 32 bits per frame).

Load the Raw Data

Java reads raw audio data in 8-bit bytes, but most audio has a higher sam-

ple size. So, in order to represent the audio, you’ll have to combine multiple

bytes to create samples in the audio format. But first, you’ll need to load all

of the audio into a

buffer

before you combine the bytes into samples.

Start by getting an audio stream from a file:

File file = new File(filename);

AudioInputStream audioInputStream = AudioSystem.getAudioInputStream(new

BufferedInputStream (new FileInputStream (file)));

Now that you have the AudioInputStream, you can read in the audio data.

AudioInputStream has a read( ) method that takes an unpopulated byte[]

and reads in data the length of the byte[]. To read in the entire audio file in

one shot, create a

byte[] the length of the entire audio file. The complete

length of the file in bytes is:

total number of bytes = bytes per frame * total number of frames

You can get the number of frames for the whole file (frameLength) and the

size of the frame (

frameSize) from the AudioInputStream:

int frameLength = (int) audioInputStream.getFrameLength( );

int frameSize = (int) audioInputStream.getFormat().getFrameSize( );

You can create the byte[] with the length set to frameLength * frameSize:

byte[] bytes = new byte[frameLength * frameSize];

Finally, you can read in the audio, passing the AudioInputStream the empty

byte[] and catching the appropriate exceptions:

int result = 0;

try {

result = audioInputStream.read(bytes);

} catch (Exception e) {

e.printStackTrace( );

}

Convert to Samples and Channels

The raw audio data isn’t very useful. It needs to be broken up into channels

and samples. From there, it’s easy to paint the samples.

Build an Audio Waveform Display #75

Chapter 10, Audio

|

381

HACK

The bytes will be converted to samples and represented as ints. You’ll need

a container to store the samples across all channels. So, create a two dimen-

sional

int[][]

referencing the channel and samples per channel. You’ve

already seen how to get the frame length from the

AuduioInputStream

, and

you can get the number of channels the same way. Here is the code to ini-

tialize the

int[][]

:

int numChannels = audioInputStream.getFormat().getChannels( );

int frameLength = (int) audioInputStream.getFrameLength( );

int[][] toReturn = new int[numChannels][frameLength];

Now, you need to iterate through the byte[], convert the bytes to samples,

and place the sample in the appropriate channel in the

int[][]

. The

byte[]

is organized by frames, meaning that you’ll read in a sample for every chan-

nel rather than all of the samples for a specific channel in a row. So, the flow

is to loop through the channels and add samples until the

byte[] has been

iterated completely:

int sampleIndex = 0;

for (int t = 0; t < eightBitByteArray.length;) {

for (int channel = 0; channel < numChannels; channel++) {

int low = (int) eightBitByteArray[t];

t++;

int high = (int) eightBitByteArray[t];

t++;

int sample = getSixteenBitSample(high, low);

toReturn[channel][sampleIndex] = sample;

}

sampleIndex++;

}

This hack is going to deal exclusively with 16-bit samples.

They are by far the most common. Plus, you can get an idea

for how sample conversion works while still keeping things

pretty straightforward. This code gets much trickier with

multiple dynamic sample sizes.

Now for the getSixteenBitSample( ) method. You can’t simply add the bytes

together using regular addition because the bits are displaced—in a 16-bit

sample the high byte represents bits 0 through 7, and the low byte repre-

sents bits 8 through 15. It’s more like concatenation, so the type of math

shown here won’t work:

1010 1101 (high byte)

+ 0011 0010 (low byte)

-----------

1101 1111