CS173: Intro to Computer Science - Guitar String Synthesier with the Karplus-Strong Algorithm (100 Points)

Assignment Goals

The goals of this assignment are:
  1. To implement an arithmetic expression into executable code
  2. To map variables to expression parameters
  3. To write code that interfaces with the computer audio interface
  4. To manipulate arrays with audio data

Background Reading and References

Please refer to the following readings and examples offering templates to help get you started:

The Assignment

In this assignment [[1], [2]], you will practice with arrays and loops in a fun application that involves digital audio.

Download the skeleton code for this assignment. In NetBeans, you can use the File menu, and select Import Project -> From ZIP File to select this file and open it.

You will be editing src/guitarstring/GuitarString.java.

We will be using the StdAudio library from the Princeton algs4cs repository. View the documentation for this library here. These files are already imported into the project directly for you.

Background: Digital Audio / Musical Notes

Sound is the result of pressure waves traveling through the air. Just as our ears pick up these pressure variations and send a signal to our brain, digital microphones/ADCs are designed to turn these variations into an array of pressure samples over time (in the discussion below, we often refer to “sample” and “array index” interchangeably). Digital audio is often sampled at 44100 samples per second, which we refer to as the sample rate (FS). This means that if we want to represent 2 seconds of audio, for instance, we need an array with 88200 samples (good thing we’re using arrays and don’t have to define 88200 individual variables!). The reason for this is that we need a sampling frequency that’s at least twice the highest frequency we want to represent. Since the highest frequency humans can hear is around 20,000hz, 44100hz is adequate. (Another fun fact about this number is it is \(2^{2} 3^{2} 5^{2} 7^{2}\).

We’ll define FS as a constant, as follows:

public static final int FS = 44100; // sample rate of 44.1 kHz

Square Waves And Pitches

One property of our perception of audio is that if a sound repeats a pattern over and over again quickly enough, we hear it as a pitch, or musical note. For example, let’s consider a “square wave.” The code below is a snippet from SquareWave.java in the homework 5 package which generates this pattern. The pattern will repeat itself every T elements in the array. This is referred to as the period of the audio.

* @param T The period of the square wave
* @param a The max/min value of the square wave
* @param duration The length in seconds of the wave
* @return An array containing a square wave
public static double[] getSquareWave(int T, double a, double duration) {
    // Create enough samples to hold "duration" seconds of audio
    int N = (int)Math.round(duration*FS);
    double[] audio = new double[N];
    // Loop through and fill in the array with the 
    // square wave pattern that repeats every T samples
    for (int i = 0; i < N; i++) {
        if (i/(T/2) %2 == 0) {
            // The first half of the square pattern
            // is positive
            audio[i] = a;
        else {
            // The second half of the square pattern
            // is negative
            audio[i] = -a;
    return audio;

For example, let’s consider the audio we get from the following call to the method, where the period is 100 and the audio goes on for one second:

double[] x = getSquareWave(100, 0.3, 1);
StdAudio.save("SquareWave100.wav", x);

The first 2000 samples of the array look like this:

First 2000 samples of the audio file array with a period of 100

and sounds like this:

If we shorten the period to 60, we obtain this instead:

double[] y = getSquareWave(60, 0.3, 1);
StdAudio.save("SquareWave60.wav", y);

Notice how there are more repetitions packed together over the same same number of samples. This makes the audio goes up in pitch to a higher note (the F# above concert A). Now, the first 2000 samples of the array look like this:

First 2000 samples of the audio file array with a period of 60

and sounds like this:

We can mix these two sounds together into one composite sound by using the sumArrays method we wrote in class (which I’ve placed in a class called ArrayUtils in the skeleton code):

double[] z = ArrayUtils.sumArrays(x, y);
StdAudio.save("2SquareWaves.wav", z);

which looks and sounds like this

First 2000 samples of the audio file array with the previous two arrays summed

Pulse Tone Sinusoids

It’s also possible to build sounds from sine waves, which we refer to as “pure tones.” Given a sample rate FS, a period T, and an amplitude (loudness) a, the formula for the value in the array at index i is:

\(a \times sin(\frac{2 \pi i}{T})\)

For those who know trigonometry, you’ll notice that this does indeed go through one period over T samples, since that will bring it from 0 to 2 \(\pi\). The code to do this, which can be found in Sinusoid.java in the skeleton code, looks like this:

 * @param T The period of the sinusoid
 * @param a The amplitude of the sinusoid
 * @param duration The length in seconds of the wave
 * @return An array containing a sinusoid
public static double[] getSinusoid(int T, double a, double duration) {
    int N = (int)Math.round(duration*FS);
    double[] audio = new double[N];
    for (int i = 0; i < N; i++) {
        audio[i] = a*Math.sin(2*Math.PI*(double)i/T);
    return audio;

For example, let’s consider the audio we get from the following call to the method, where the period is 100 and the audio goes on for one second:

double[] x = getSinusoid(100, 0.8, 1);
StdAudio.save("Sinusoid100.wav", x);

The first 2000 samples of the array look like this

First 2000 samples of the audio file array of a sinusoid with a period of 100

and the audio sounds like this (a concert A)

If we shorten the period to 60

double[] y = getSinusoid(60, 0.8, 1);
StdAudio.save("Sinusoid60.wav", y);

then the first 2000 samples look like this

First 2000 samples of the audio file array of a sinusoid with a period of 60

As with the square wave, there are more repetitions packed together over the same same number of samples, which makes the audio go to a higher pitch

Main Programming Task: The Karplus-Strong Algorithm

The square waves and sinusoids sound a bit dull and unrealistic (although a real clarinet does sound a bit like a square wave). But it is possible to make a very realistic plucked guitar sound with a surprisingly simple algorithm known as the Karplus-Strong Algorithm, which is the starting point for all of the digital synthesizers used in the 80s and beyond. Your job in this assignment will be to implement the Karplus-Strong algorithm in several stages by filling in three methods in GuitarString.java.

Part 1: Getting the Period (25%)

Most Western string orchestras tune to a “concert A,” which goes through 440 periods in one second. If our audio is sampled at 44100 samples per second, this means that each period takes up about 100 samples (which we saw in the examples above). In each octave in the Western chromatic scale, there are 12 notes total. Going from one note to its adjacent note in order is called a halfstep. For example, a B is 2 halfsteps above A. The formula for determining the period for a particular note h halfsteps away from a concert A is:

\(T = \frac{FS}{440 \times 2^{\frac{h}{12}}}\)

For example, at a sample rate of 44100, a B which is 2 halfsteps above concert A has a period of:

\(T = \frac{44100}{440 \times 2^{\frac{2}{12}}} \approx 89.29\)

Notice how the period has gotten shorter, which indeed corresponds to an increase in pitch, as we saw in the square/sinusoid examples.

It is also possible for the halfstep to be negative, in which case the formula yields a period longer than 100, for a pitch lower than 440. For example, for a G two halfsteps below concert A, the formula yields:

\(T = \frac{44100}{440 \times 2^{\frac{-2}{12}}} \approx 112.5\)

Your first task is to implement this equation in the getPeriod(int halfStep) method in GuitarString.java. You should round the period you return to the nearest integer.


  • Be sure you’re using the correct types here, and cast if you have to! In particular, h/12 should be a decimal number.
  • Use the Math.pow() function to raise a number to a power.
  • Use the Math.round() function to round to the nearest integer.

Part 2: Random Samples (25%)

The next step of the algorithm consists of coming up with some “random noise,” which sounds like static. In the context of the Karplus Strong algorithm, this can be thought of as randomly stimulating the string with a pluck. To do this, you will complete the method:

public static void fillRandomSamples(double[] arr, int numSamples)

This method should fill in the first numSamples indices of the array arr with random samples between -1.0 and 1.0. As a corner case, if the user asks for more samples than the capacity of the array, you should fill at most the length of the array.

For example, if you make an array that’s one second long (FS samples long) and fill the entire array with noise, the code would look like this:

double[] samples = new double[FS];
fillRandomSamples(samples, FS);

And the audio would sound like “static”:

If you were to plot the first 2000 samples of the array, they would look something like this (though the answer would be slightly different every time since the numbers are random)

First 2000 samples of the audio file array of random noise


  • You can make use of Java’s function Math.random(), which returns a random number between 0.0 and 1.0.
  • How can you manipulate this random number to become a value between 0.0 and 2.0? Then, you can further manipulate the value to be a number between -1.0 and 1.0.

Part 3: The Final Plucked Sound (50%)

Now it’s time to tie everything together to get our wonderful plucked sound. You will be filling in the method:

public static double[] getPluckedSound(int note, double duration, double decay)

The steps are as follows:

  1. Given a note, find its period T to the nearest integer by using your method that computes the period. You’ve made a function do to this already, so you should call that function!
  2. Create an array with enough samples to hold duration seconds audio. How many elements does this array need? It depends on the duration and the number of samples per second (also known as the sample rate or frequency): you have both of these!
  3. Fill in the first T samples with random noise using your noise method.
  4. Process all of the rest of the samples one by one in order, starting at index T. Each value should be the average of the two adjacent samples T indices back multiplied by a factor of decay. For example, if you’re at index 150 and your period is 100, you should average samples at indices 50 and 51. This simulates a traveling wave over a string of length T that decays and dampens “low frequencies” first, and is referred to as a “digital waveguide.”

For example, if your code works properly, then the following call to your method will play a guitar string at a concert A:

StdAudio.play(getPluckedSound(0, 0.5, 0.98));

which sounds like this

If we plot the first 2000 values of the array, we can see it starts with totally random noise in the first 100 samples, but then it gradually evens out to a periodic sound that decays over time:

Karplus Strong Concert A

By default, the main function in GuitarString.java plays the first 24 halfsteps in sequence, which is two octaves of the chromatic scale. If your code is working properly and you run the main, you should hear the following sound:

Extra Credit (25 Points)

As extra credit, you should fill in the method playFile, which loads in and plays a bunch of notes in sequence from a text file. The text file contains a bunch of lines with comma separated values for halfstep,duration,decay. For example, the “Happy Birthday” song contains the following sequence of 25 notes:


If you copy and paste this into a file called HappyBirthday.txt, you can run:



  • If you have a String s of comma-separated values, then s.split(",") will return an array of Strings that are on either side of the commas. For instance, "11,12,13".split(",") will return the array {"11", "12", "13"}. You will then need to convert each to the correct type. The function Integer.parseInt(string) will convert a string to an integer, and the function Double.parseDouble(string) will convert a string to a double.
  • Then, from within playFile, you can call getPluckedSound and pass these three values as parameters to play each note of the song!

Exporting your Project for Submission

When you’re done, write a README for your project, and save all your files, before exporting your project to ZIP. In your README, answer any bolded questions presented on this page. Here is a video tutorial describing how to write a README for your project, and how to export it.

  1. Developed by Prof. Chris Tralie 

  2. Adapted from Princeton COS126 course 


In your submission, please include answers to any questions asked on the assignment page in your README file. If you wrote code as part of this assignment, please describe your design, approach, and implementation in your README file as well. Finally, include answers to the following questions:
  • Describe what you did, how you did it, what challenges you encountered, and how you solved them.
  • Please answer any questions found throughout the narrative of this assignment.
  • If collaboration with a buddy was permitted, did you work with a buddy on this assignment? If so, who? If not, do you certify that this submission represents your own original work?
  • Please identify any and all portions of your submission that were not originally written by you (for example, code originally written by your buddy, or anything taken or adapted from a non-classroom resource). It is always OK to use your textbook and instructor notes; however, you are certifying that any portions not designated as coming from an outside person or source are your own original work.
  • Approximately how many hours it took you to finish this assignment (I will not judge you for this at all...I am simply using it to gauge if the assignments are too easy or hard)?
  • Your overall impression of the assignment. Did you love it, hate it, or were you neutral? One word answers are fine, but if you have any suggestions for the future let me know.
  • Any other concerns that you have. For instance, if you have a bug that you were unable to solve but you made progress, write that here. The more you articulate the problem the more partial credit you will receive (it is fine to leave this blank).

Assignment Rubric

Description Pre-Emerging (< 50%) Beginning (50%) Progressing (85%) Proficient (100%)
Algorithm Implementation (60%) The algorithm fails on the test inputs due to major issues, or the program fails to compile and/or run The algorithm fails on the test inputs due to one or more minor issues The algorithm is implemented to solve the problem correctly according to given test inputs, but would fail if executed in a general case due to a minor issue or omission in the algorithm design or implementation A reasonable algorithm is implemented to solve the problem which correctly solves the problem according to the given test inputs, and would be reasonably expected to solve the problem in the general case
Code Quality and Documentation (30%) Code commenting and structure are absent, or code structure departs significantly from best practice, and/or the code departs significantly from the style guide Code commenting and structure is limited in ways that reduce the readability of the program, and/or there are minor departures from the style guide Code documentation is present that re-states the explicit code definitions, and/or code is written that mostly adheres to the style guide Code is documented at non-trivial points in a manner that enhances the readability of the program, and code is written according to the style guide, and each function contains relevant and appropriate Javadoc documentation
Writeup and Submission (10%) An incomplete submission is provided The program is submitted, but not according to the directions in one or more ways (for example, because it is lacking a readme writeup or missing answers to written questions) The program is submitted according to the directions with a minor omission or correction needed, including a readme writeup describing the solution and answering nearly all questions posed in the instructions The program is submitted according to the directions, including a readme writeup describing the solution and answering all questions posed in the instructions

Please refer to the Style Guide for code quality examples and guidelines.