CS 1713 Section 3, Spring 1997
Assignment 5: Arrays: Signal Processing with the Fourier Transform

Your assignment is to write a program that will determine, given a sample of sound from a piano key, what key (1-88) and note (A-G#) it is.


We often encounter data collected over time at specific intervals. For instance, the temperature each day of this year, or the number of bacteria in a dish each minute. Sometimes this data is periodic, i.e., its behavior repeats at specific intervals. An everyday example of periodic data is sound. Sound is just a signal propogating through the air in compression waves. Think of a piano key being struck; this causes the piano string to move back and forth very rapidly, causing compression in the surrounding air which moves outward as a sound wave. Your eardrum is vibrated by this back-and-forth force, and you hear the sound. If we plot the strength (or amplitude) of the compression against time, we'll get this:

Wave Plot

The x axis is the number of seconds from the time the key was struck; as you can see, we are considering only 1/20 of a second of time. This piano key happens to be the A above middle C, tuned at 440 Hertz (abbreviated Hz). That means the string vibrates 440 times per second. The y axis plots the amplitude of the compression of air around a microphone connected to my computer; you can think of it as a voltage level. As you can see from the plot, the voltage level, or signal, spikes once every 1/440 of a second.

Sound is a continous signal, so how do we reprent it inside the computer? One way is to take a sample of the amplitude of the signal once every, say, 1/8000 of a second. We can store these samples in a large array. Whenever we want to look at the signal at time t, where t is in seconds, we just multiply t by 8000, find the closest integer, and look in the array at that point. In fact, this is exactly the way sound is stored on an ordinary CD, with samples taken every 1/44000 of a second.

How do you determine the frequency of a sampled sound? This situation is complicated by the fact that in most situations, including the piano key strike, there are many frequencies superimposed on each other. For example, the A above also vibrates at 880 Hz, 1320 Hz, and other frequencies, although these show up at a lower amplitude. These harmonic frequencies are what give the piano, other instruments, your voice, and pretty much every other sound its characteristic texture. However, we are now only interested in determining the fundamental frequency.

The data are given to us in the time domain, i.e., we know, for any given time, what the amplitude will be. It would be nicer if the data were in the frequency domain, where we would know, for any given frequency, what the amplitude will be. Then we can just look for the frequency with the highest amplitude, and that's the fundamental frequency (usually; it works for the middle part of the piano, at least). The Fourier Transform of a function takes a function y(t) in the time domain and ``transforms'' it into a function Y(f) in the frequency domain. It works for any periodic continuous function. The Fourier Transform, for your amusement, looks like this:

Equation for Fourier Transform

If you don't know what this means, don't worry; you don't have to.

How to do the assignment

On runner in /home/staff3/djimenez/cs1713, I have placed ten data files called 1.dat through 10.dat. These files contain piano sounds samples at 8000Hz. Each file contains 8192 samples; you can read them as float values using scanf in C. Note: Your program should get the name of the file to process from the command line, i.e., argc and argv; it should process only one file at a time. Your program should read the number into an array of complex numbers represented in C as an array of floats where the even elements are the real parts and the odd elements are the imaginary parts. Initially, of course, you will just set the imaginary parts equal to 0, placing the values read in into the real parts.

You can use a special version of the Fourier Transform called the Discrete Fourier Transform, or DFT, on the sampled data inside the computer. You give the DFT an array of n time-domain data, sampled at m samples per second, and it gives you back an array of n complex-valued frequency-domain data, where the magnitude of each element represents the amplitude of the sound at each frequency. After you take the DFT of the sample, you search the array for the point with the maximum magnitude and convert the corresponding array index to Hz by multiplying by m/n. You should only search up to about the 2000th position (i.e., 4000th index in the real array, 2000th in the complex array); anything past that is probably noise.

Once you have determined the fundamental frequency of the piano sound, you need to convert it to a number from 1-88, representing the number of the key on the piano. A simple formula for the conversion from a frequency f to a piano key is:

key = the closest integer to (the log base 2^(1/12) of (f / 27.5)) + 1

where 2^(1/12) means "two to the one twelfth power." Remember, to find the logarithm base b of x, you can just divide the natural logarithm of x by the natural log of b. Also, you don't necessarily get the closest integer to a float by casting to int, e.g., 3.6 is closer to 4 than it is to 3.

Once you have the key, the final task is to decide which note it is. There are twelve symbols for notes, repeating every twelve notes. Your program should print the notes as A A# B C C# D D# E F F# G G#. (The '#' is read ``sharp''). The first note on the keyboard is an A; so is the 13th, the 25th, and so forth. Use the % (modulus) operator to help decide which key is which; don't use 88 if statements. The files cfft.o and cfft.h in the same directory are object code and a header file for a fast version of the DFT, called the complex Fast Fourier Transform, or FFT, as a C void function. You can apply this FFT to your float array of length 16384 (representing 8192 complex numbers) using the following C function call:
        cfft (complex_array, 8192)
where complex_array is the name of your array. Your time-domain data will be replaced with the frequency domain data from the Fourier transform. Copy cfft.o and cfft.h to your directory. Call your program file piano.c and write a Makefile for it; remember to link the file cfft.o with a line like:
	$(CC) -o piano piano.c cfft.o -lm
in your Makefile. You may also wish to use the complex array routines discussed in class; they are available as complex.c and complex.h on this web site. I have done so for my program; my Makefile looks something like this:
CFLAGS  =       -g -Wall
LDLIBS  =       -lm
CC      =       gcc
all:            piano
piano:          piano.o cfft.o complex.o
                $(CC) -o piano piano.o complex.o cfft.o $(LDLIBS)
piano.o:        piano.c cfft.h complex.h
complex.o:      complex.h complex.c
You should do all your work in a subdirectory called assign5. The output of your program should be give the maximum amplitude, the fundamental frequency, the key number, and the symbol for the note. So, your program should: Here is the output of my program run on the first three data files, 1.dat, 2.dat, and 3.dat:
runner% piano 1.dat
maximum amplitude is    55.04 at 155.27 Hz, key = 31
key letter is D#
runner% piano 2.dat 
maximum amplitude is    33.75 at 330.08 Hz, key = 44
key letter is E
runner% piano 3.dat 
maximum amplitude is   143.59 at 310.55 Hz, key = 43
key letter is D#
To turn in this program, do exactly this from the prompt on runner, after having compiled your program:
% cat piano.c > output
% piano ~djimenez/cs1713/1.dat >> output
% piano ~djimenez/cs1713/2.dat >> output
% piano ~djimenez/cs1713/3.dat >> output
% piano ~djimenez/cs1713/4.dat >> output
% piano ~djimenez/cs1713/5.dat >> output
% piano ~djimenez/cs1713/6.dat >> output
% piano ~djimenez/cs1713/7.dat >> output
% piano ~djimenez/cs1713/8.dat >> output
% piano ~djimenez/cs1713/9.dat >> output
% piano ~djimenez/cs1713/10.dat >> output
% Mail -s "piano program" djimenez@ringer.cs.utsa.edu < output
This will run your program on all the files and send me the output. (If you have written other files beside piano.c, e-mail me them, too, separately.) Expect the run time of this program to be rather long, on the order of a few seconds; an FFT is a computationally expensive operation, especially when there are 30 other students on runner running the same program.

This program can be done in about 70 lines of C code, not including comments. It is not as difficult as it sounds, but it is by no means easy, so you should get started right away.

This assignment is due Monday, March 3, 1997 at midnight.