Programming Assignment Checklist: Digital Signal Processing

Pair programming. On THIS assignment, you are encouraged (not required) to work with a partner provided you practice pair programming. Pair programming "is a practice in which two programmers work side-by-side at one computer, continuously collaborating on the same design, algorithm, code, or test." One partner is driving (designing and typing the code) while the other is navigating (reviewing the work, identifying bugs, and asking questions). The two partners switch roles every 30-40 minutes, and on demand, brainstorm. Before pair programming, you must read the article All I really need to know about pair programming I learned in kindergarten.

You may choose a partner from the same or a different precept. You and your partner will each turn in a separate individually written readme.txt for the assignment, stating who your partner is and detailing your experience. However, only one of you should turn in the assignment (the .java files).

Frequently Asked Questions

I'm having trouble with trigonometry. What could be wrong? It's fine to just use the formula provided. Note that a sine wave repeats every 2π radians. Use Math.PI to get the mathematical constant π. Recall that in Java, the argument to Math.sin() is in radians, not degrees.

I am getting a runtime error when I run EchoFilter with felten.mp3 or pearlharbor.mp3. What should I do? Download stdplayer.jar again.

I am getting a warning when I run EchoFilter with felten.mp3 or pearlharbor.mp3. What should I do? This may be ok. Check that bushism.mp3 works without warnings, and that felten.mp3 works until sample 705, and then gets a warning. Warnings before then or in during the playing of bushism.mp3 are a sign that something is wrong with your code.

I can play CDs and MP3s using my favorite media player, but I don't get any sound out of Java. What could be wrong? First, be sure to try the JavaLayer Player javazoom.jl.player.jlp as suggested in the assignment - this will ensure that the problem is not in any of your code. If you don't get any sound, it may be because your operating system has turned off the sound device that Java uses. On my Windows XP machine, I had to go to Start -> Programs -> Accessories -> Multimedia -> Volume Control and unmute the Wave Out slider. The 017 labs in the Friend center may need a similar adjustment.

How do I convert from a double to an int? Use an explicit cast: int n = (int) x; - this will truncate the fractional part of x. A slightly better solution is to round to the nearest integer using: int n = (int) Math.round(x);. The cast is still needed since Math.round() returns a long.

How do I determine how many elements are in an array? If you're asking this, you probably need to review Section 1.4.

I get an ArrayOutOfBounds or NullPointerException error. What could cause this? Do your constructors initialize all of the data members (left, right, and N)? Did you allocate memory for each of your arrays with new? Did you inadvertently redeclare int N or double[] left in a method or constructor, thereby hiding the instance variable with the same name?

Do I need to worry about adding two waves that have a different number of samples? Or a wave whose left and right channels have a different number of samples? No. It's fine to assume all waves within a given program will have the same length (but that number need not be 44,100 or 1,152), but it can't hurt to do some error checking.

Should I play the echo of the last 10 Waves, after all the Waves have been read in? Yes, the last Wave read in is played with an echo, but there are 9 remaining echos in your array that have not been played. They should be played without adding them to a Wave from the mp3 file.

Why do I need to include System.exit(0) at the end of my program? There is a documented bug in the Java VM that prevents the program from exiting cleanly when you interact with a sound card. This is the temporary work-around.

Why can't I just put in my working directory and avoid the classpath stuff as with You can, but relies on 78 other user-defined files, so you'd have to put all of them in the same place too.

The first wave I plot is incredibly slow. How can I fix this? Be sure to turn animation mode on before you plot the first wave using Then, after plotting each wave, redisplay using

Java complains that it can't find javazoom.j1.player.j1p even though stdplayer.jar is in the current directory. What am I doing wrong? It should be javazoom.jl.player.jlp with the letter l and not the number 1. There is a lesson to be learned here about selecting good variable names.

Input, Output, and Testing

Input.   Here are some sample input files.

Compilation.   Your programs must be named,, and Don't forget to hit the "Run Script" button on the submission system to test that it compiles cleanly.


Use the following readme file template.

Possible Progress Steps

These are purely suggestions for how you might make progress. You do not have to follow these steps.


Music instruments. A music instrument sounds more vibrant than the pure sine waves that we synthesize in Part 1. The reason for this is that music instruments generate not only the fundamental frequency f (as we did in Part 1), but also the higher harmonics 2f, 3f, 4f and so on. In addition, the note rises in intensity very quickly (but not instantaneously) and then gradually fades away. A simple way to model this is to multiply the amplitude at time t by e^-kt for some constant k.

felten.mp3. The song All My Trials was performed and recorded by Princeton professor Perry Cook after another Princeton professor Ed Felten was threatened with a lawsuit by the RIAA (Recording Industry Association of America) for presenting an academic paper that explained the inherent flaws in several digital watermarking schemes. Lifted by the spirits of Perry Cook's song, and with support from the EFF, Ed Felten was able to defeat the mighty RIAA.

MP3 file format. The MP3 file format stores digital audio in a compressed format. It compresses digital audio by eliminating (i) sounds that the human ear cannot hear, (ii) sounds that go unheard because there is another sound that masks it, and (iii) sounds that are not heard well at all. The net effect is a file that is 10-12 times smaller than the original, without sacrificing too much in fidelity. The mp3 file consists of frames. Each frame, when decoded, contains an array of 2,304 signed 16-bit integers that represent instantaneous amplitude. mp3 files interleave the left and right stereo channels: the first element of the array is the amplitude of the first sampling of the left stereo channel, and the second element is the amplitude of the first sampling of the right stereo channel, and so on.

Ripple echo effect. To get a ripple echo effect (where the sound echoes forever, but decays over time), take the average of the current Wave and the Wave you played 10 time steps ago (instead of the one you read in 10 steps ago).

Wave interference. Here's a fun and easy suggestion from a former student: set the right channel to be the opposite (negative) of the left channel; point your computer speakers at each other; and move them toward and away from each other. You should hear the effects of the waves cancelling each other out.

Ssllooww ddoowwnn ppllaayybbaacckk. Anothe fun thing to try is to play the wave in slow motion. Specifically create a Wave method expand() that returns a new Wave that is twice as long, and has each sample of the invoking Wave repeated twice, consecutively.

Frequencies of musical notes. Middle C is 240Hz. Each octave has a frequency that is twice the previous one. In Western culture, each octave is divided into twelve steps. Each note has a frequency that is the twelfth root of two times the previous one. This logarithmic scale correlates with human hearing.

Music theory. For a massive overload in information regarding Music Theory, check out

More data files. Here are some famous speeches and soundbytes.

Java sound. Java Sound Resources contains code samples ranging from ripping CDs to MIDI synthesis.

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