Due

Objectives

• to read and write binary files

Reading

Introduction
Sound is created by air (or whatever) pressure changing over time. If you graph pressure versus time for a given sound, you may come up with a graph that looks something like this:

To represent a continuous curve digitally, we can sample it at regular intervals. At each sample point, we measure the pressure to the desired resolution and record the result as an integer. As we increase the sample frequency and resolution, we will digitize the sound more accurately. In picture above, the blue curve represents the original sound. The tick marks along the bottom show the times at which we take samples. The tick marks along the side reflect the resolution of our samples; the value recorded will be that represented by the closest tick to the actual curve. The red dots represent the samples and the white curve represents the approximation of the original curve that our samples give us.

If we want to create an audio file without an original sound source, we can synthesize sounds by coming up with what the pressure vs. time graph would be if the sound were actually played. For example, a pure tone has a sine curve for a graph. The shorter the period of the sine curve, the higher the tone. Higher amplitudes represent louder pure tones. For example, if the range of our samples is -128 to 127, then

0 61 108 127 115 76 18 -45 -96 -124 -122 -90 -35 27 83 119 126

The .au file format
An audio file must contain information about the sample range and frequency in order to be played back correctly. Different formats specify different ways of recording this information. For this lab, we will create .au files. .au files begin with 6 pieces of information, each written as a 32-bit integer. Common values are given below.

Assignment

You must complete the part of the AudioClip class that reads and writes the files. Your code to read .au files will go in the AudioClip constructor and the code to write .au files will go in the write method. Both methods will make use of the RandomAccessFile class. In particular, you will need to use the following methods and constructor.

• RandomAccessFile(String name, String mode), which opens a file for reading or writing depending on the mode string. mode should be "r" for an input file and "rw" for an output file.
• int readByte() and int readInt(), which read 8-bit and 32-bit integers. 8-bit integers are used when the data to be stored is in the range -128 to 127 (as will be the case with the audio sample data). 32-bit integers are used when the data is in the range -2147483648 to 2147483647 (as for the data in the .au header).
• void writeByte(int v) and void writeInt(int v), which write 8- and 32-bit integers.
• long length(), which returns the size, in bytes, of the file. Note that each 8-bit sample takes up 1 byte, so the size of the file will be approximately the same as the number of samples (the size of the file will be a little larger because length counts the header too).
• void seek(long pos), which sets the current position in the file to the given position, so that the next value read (or written) will be read from that position. This is what makes RandomAccessFiles random access -- you don't have to read or write data in sequential order. If you want to read data later in the file, or reread data you have already passed, you can seek to it and then read it.

In the constructor, you should

• open the file for reading by creating an instance of RandomAccessFile,
• read and save the 6 32-bit (4-byte) values in the .au header with the readInt method (there is a field for the sample rate; all the others can be read into local variables),
• use seek to skip ahead to the beginning of the sample data,
• use length to figure out how many samples there are in the file (remember to subtract anything that comes before the sample data),
• create an array to hold the samples, and
• use a loop that invokes readByte to read the samples into the array.

In write, you should

• open the output file by creating an instance of RandomAccessFile,
• write the 6 values in the header with writeInt (the only one that needs to be different for each file is the 5th entry for the sample rate; all the other values can be as in the table above),
• use a loop that invoked writeByte to write the data in the sample array to the file, and
• close the file.

Extra credit

• writes a message using writeBytes,
• saves the current position in the file as reported by getFilePointer,
• seeks to the 4th byte in the file to position the file pointer just before the data offset field,
• write the saved file pointer using writeInt, and
• seek back to that position using seek.

Files

Exercises

1. Coming later?

Submissions