COS 233

Digital Signal Processing 		Programming Assignment

Due: Monday, Feb. 15 at 11:59pm

Editor's note. This assignment was adapted from the COS126 assignment of the same name. The assignment checklist is also available, with the usual hints, guidelines, irrelevant extra material, and suggestions for overly-complicated extra credit projects. Note that the checklist's progress steps provide a concrete way to get started if you are confused.

Write a program to generate sound waves, apply an echo filter to an MP3 file, and plot the waves.

Provided code. The project template is available as a ZIP archive. The sample MP3 file is: pearlharbor.mp3

A crash course in sound. A music note can be characterized by its frequency of oscillation. For example, the music note concert A is a sine wave repeated 440 times per second (440 Hz); the note C is a sine wave repeated approximately 523.25 times per second. We amplify the signal to an audible level by multiplying it by a constant, say 8,000. Below are pictures of an A and a C with duration 15 milliseconds and maximum amplitude of 8,000. The picture for A consists of 0.015 × 440 = 6.6 sine waves.

AC

Below is a table containing the frequencies in Hertz of all twelve musical notes in the fifth octave. The ratio between the frequency of successive notes is the 12th root of two (1.05946). An octave is a doubling or halving of the frequency. For reference, the normal range of human hearing is between 20 and 20,000 Hz.

A A# B C C# D D# E F F# G G# A
440.00 466.16 493.88 523.25 554.37 587.33 622.25 659.26 698.46 739.99 783.99 830.61 880.00

Digital audio. Digital audio is produced by sampling the instantaneous amplitude of the continuous sound wave many times a second. Each sample is a signed 16-bit integer (a short in Java) that represents the amplitude of the sound wave at a particular instant in time. The sampling rate is the number of samples taken per second. Audio from CDs typically uses a sampling rate of 44,100 and two channels (left and right). Below is a picture of an A using a sampling rate of 44,100 and a duration of approximately 1/440th of a second. The figure below displays every 10th sample.

A zoomed

The following arrays contain the short integer representing the height of the wave above, at a sampling rate of 44,100: 

left  = { 0, 501, 1000, 1495, 1985, 2466, 2938, 3399, 3846, 4277, 4693, ..., -113 }
right = { 0, 501, 1000, 1495, 1985, 2466, 2938, 3399, 3846, 4277, 4693, ..., -113 }
Sample i is given by 8000 sin(2 π * 440 * i / 44,100), and we've rounded the numbers towards zero.

Sound synthesis. Your first task is to create a data type Wave to store and manipulate sound wave samples so that the program A.java below plays concert A for 2 seconds with maximum amplitude 8,000. Similarly, FurElise.java plays the first nine notes of Fur Elise. 

  import javazoom.jl.player.Player;

  public class A {
      public static void main(String[] args) {
          Player.open();
          Wave A = new Wave(440.0, 2.0, 8000);  
          A.play();
          Player.close();
          System.exit(0);
      }
  }

For this part of the assignment, you need to write the data type Wave that A.java uses to play sound. Each Wave object will consist of two arrays to hold the samples for the left and right channels; these should be implemented as two arrays of type short. First, write the constructor 

public Wave(double Hz, double seconds, double amplitude).
It should create a new Wave object that represents a sine wave of the specified number of Hz that is sampled 44,100 times per second over the specified number of seconds. Use Math.sin and Math.PI to initialize the left and right channels. Initialize both arrays with the same values so that the note will play in both speakers. Next, implement a method 

public void play()
that sends the Wave data to the sound card. Use the static library method Player.playWave(left, right), which takes as input two short arrays representing the left and the right channel.

Compilation and execution.  In order to play music on your computer, you will need to use a specialized library called player.jar, which is included in the assignment template. A .jar file is an archive like a .zip file that contains Java classes. This library is a modified version of the JavaLayer 0.2 MP3 Player. (As per the GPL license, the jar file contains the original JavaLayer library and our modified source code.)

In order to use the library in your Java program, you must put the following import statement at the beginning of any code that needs to use the Player class: 

import javazoom.jl.player.Player;

You must also tell Java where to find the library.

Creating a tune. Now that you can play single notes, your next challenge is to play several notes at once, for example, to play a chord. To accomplish this, first add a new constructor 

public Wave (short[] left, short[] right)
to Wave.java that takes two short integer arrays as arguments and initializes a new Wave object with the given data. Now, to create a chord, you can create individual notes and combine them together by writing a method: 
public Wave plus(Wave b)
so that a.plus(b) returns the sum of a and b. To add two Waves, add the corresponding array entries for the left and right channels, element-by-element. Test your data type by using the program StairwayToHeaven.java, which is the beginning of the famous Led Zeppelin tune. Note the fifth wave played is created by the following sequence of statements: 
  Wave B6  = new Wave(493.88 * 2, 0.4, 8000.0);
  Wave Gs4 = new Wave(830.61 / 2, 0.4, 8000.0);  
  Wave GsB = B6.plus(Gs4);

Score to Stairway to Heaven

Playing an MP3 file. The program MP3Player.java decodes the MP3 file specified by the command line input and plays it. Compile and execute the program, passing pearlharbor.mp3 as argument.

Assuming that you implemented all of the Wave.java methods and constructors above, there is no need to write any code for this part (but you should test it and enjoy).

The key part of the code is the following loop. It will be useful for the remaining parts of the assignment. 

  Player.open(args[0]);
  while (!Player.isEmpty()) {
      short[] left  = Player.getLeftChannel();
      short[] right = Player.getRightChannel();  
      Wave w = new Wave(left, right);
      w.play();
  }
This program uses three new methods from the MP3 Player library to decode data from an MP3 file. The String argument to the method Player.open specifies which MP3 file to use. The method Player.getLeftChannel returns an array of 1,152 short integers which are the samples of the music intended for the left speaker. The method Player.getRightChannel is analogous.

Echo filter. Now that you can decode and play MP3 files, you're ready to modify the data and change the characteristics of the sound waves. An analog filter accomplishes this by manipulating the electrical signals that represent the sound wave; a digital filter does this by manipulating the digital data that represents that Wave.

Your task is to write a program EchoFilter.java that implements a digital echo filter. Use MP3Player.java as a starting point. An echo filter of delay 10 is a filter that adds an echo to the sound by adding the sound wave at time t - 10 to the one at time t. To create this effect, maintain an array of the past 10 Wave objects and add the Wave that was originally read n waves ago to the current Wave.

The echo filter is a client program and should be written entirely in EchoFilter.java. It should take in the name of the file to play. For example: java EchoFilter pearlharbor.mp3

Extra Credit. (Optional) Write a program MP3Viewer.java that takes the name of an MP3 file as a command line argument and animates both stereo channels by plotting the waves. This program is not supposed to play the MP3 file, only to animate the sound waves. Add the following method to Wave.java to plot the left channel on the top half of the screen, and the right channel on the bottom half: 

public void draw()
Use a 576-by-512 window and scale the data to fit snugly in the window. We use 576 because it divides evenly into 1,152, the number of samples in one channel of an MP3 wave. When rescaling, recall that each sample is a short integer between -32,768 and 32,767. If you can't remember how to draw things to the string take a look back at the description of the StdDraw library in Section 1.5 of the book.

Challenge for the bored. (More Extra Credit) Write a program MP3Visualizer.java that plays the MP3 file and simultaneously displays a cool effect based on the raw data. To keep the music and animation smooth, you may need to tweak a few parameters. For example, adjust the delay in the StdDraw.pause method to keep the wave from scrolling by too fast. Also, try plotting every other wave if your computer is too slow.

Deliverables. Submit Wave.java, EchoFilter.java, and any extra credit code along with the readme.txt file.

You should not change, or submit, any of the other provided code: A.java, FurElize.java, TestMP3.java, or MP3Player.java. Your Wave.java should work against fresh, unmodified versions of these files.

Do not forget to comment your code. No comments = Points off.

This assignment was created by Bradley Zankel and Kevin Wayne.