Week 9 – Audio Processing CS 177 Recitation Week 9 – Audio Processing

Announcements Project 3 (final version) is due on Thursday (Oct 29) Any questions about the project?

Questions?

Sound Like light, sound is a wave We can’t record an (analog) wave directly on a (digital) computer, so we have to do sampling Sampling converts the analog wave into a digital approximation Break the wave down into a lot of samples (44,100 samples per second = 44,100 Hz) Each sample is a double value in the range [-1, 1] Each sample records the amplitude of the wave at a certain place In the computer, the sound wave is just an array of doubles

Sound Amplitude of the wave determines the sound’s volume Frequency of the wave determines how high or low the sound is is louder than low-pitched sound high-pitched sound

StdAudio Written by the book’s author, like StdIn and StdDraw As usual, remember to download the file and put it in the same directory if you want to use the StdAudio methods Allows you to manipulate sound as a single array of samples Allows you to read and write WAV files

StdAudio Methods

Simple Audio Example Reads a sound from file and plays it public class PlaySound { public static void main(String[] args){ System.out.print("Enter filename: "); String file = StdIn.readString(); double[] sound = StdAudio.read(file); StdAudio.play(sound); }

Slowing Down a Sound The example in lecture just doubled the length of the sound by using each sample twice Example: slow[0] = sound[0] slow[1] = sound[0] slow[2] = sound[1] slow[3] = sound[1] This works, but doesn’t sound so great Using each sample twice in a row makes the wave kind of blocky instead of

Slow.java To smooth the slowed sound, use the average of adjacent samples instead of copies public class Slow{ public static void main(String[] args){ double[] sound = StdAudio.read("world.wav"); double[] slow = new double[sound.length*2]; for( int i = 0; i < slow.length-1; i++ ) //slow[i] = sound[i/2]; //old way slow[i] = (sound[i/2] + sound[(i+1)/2]) / 2.0; //better way StdAudio.play(slow); }

Adding Noise We can add noise to a sound by adding or subtracting a small random number from each sample For simplicity, we’ll only add Remember, the samples can only be in the range -1 to 1 Whenever you’re changing the samples, be sure that they stay in that range!

AddNoise.java public class AddNoise { public static void main( String [] args ) { String file = “world.wav”; double samples [] = StdAudio.read(file); for ( int i = 0; i < samples.length; i++ ) { double noise = Math.random(); samples[i] += noise; if ( samples[i] > 1 ) samples[i] = 1; } StdAudio.play(samples);

Playing Two Sounds Together Add the samples at each position in the array First sound: .5 .2 -.1 1 … Second sound: .1 -.2 .3 -.6 .2 … Combined sound: .6 -.1 1.3 -.4 .4 … Don’t forget to check for samples that become >1 or <-1!

Playing Two Sounds Together If one sound is longer than the other: mix the two until the end of the shorter sound copy the rest of the longer sound unchanged onto the end of the mixed sound First sound: .5 .2 -.1 1 Second sound: .1 -.2 .3 -.6 .2 … Combined sound: .6 -.1 1.3 -.6 .2 … Mix of both sounds Copy of the longer sound’s data

public class PlayTwo { public static void main( String [] args ) { String file1 = ”world.wav"; String file2 = "breakbeato.wav"; double samples1 [] = StdAudio.read(file1); double samples2 [] = StdAudio.read(file2); int longer = samples1.length; int shorter = samples2.length; if ( samples2.length > longer ) { longer = samples2.length; shorter = samples1.length; } double mix [] = new double[longer]; for ( int i = 0; i < shorter; i++ ) { mix[i] = samples1[i] + samples2[i]; for ( int j = shorter; j < longer; j++ ) { if ( samples1.length > samples2.length ) mix[j] = samples1[j]; else mix[j] = samples2[j]; PlayTwo.java

Improving PlayThatTune.java The book’s PlayThatTune program works fine, but produces vaguely creepy robot music We want to make it a little less creepily robotic Add harmonic tones one octave above and below each note to produce a more realistic sound To run PlayThatTune (has to be done from the command line): java PlayThatTune < elise.txt

Improving PlayThatTune.java Originally the wave for a note looks like: With the harmonics, it looks like:

PlayThatTune2.java public class PlayThatTune2 { public static double[] sum(double[]a, double [] b, double awt, double bwt ) { double [] c = new double[a.length]; for ( int i = 0; i < a.length; i++) c[i] = a[i] * awt + b[i] * bwt; return c; } public static double[] tone(double hz, double t) { int SAMPLE_RATE = 44100; int N = (int) (SAMPLE_RATE * t); double[] a = new double[N+1]; for (int i = 0; i <= N; i++) { a[i] = Math.sin(2 * Math.PI * i * hz / SAMPLE_RATE); return a;

PlayThatTune2.java con. double hz = 440.0 * Math.pow( 2, p / 12.0); public static double[]note ( int p, double t) { double hz = 440.0 * Math.pow( 2, p / 12.0); double[] a = tone( hz, t); double[] hi = tone(2 * hz, t); double[]lo = tone(hz/2, t); double[] h = sum(hi, lo, .5, .5); return sum(a,h,.5,.5); } public static void main(String [] args) { while ( !StdIn.isEmpty()) { int pitch = StdIn.readInt(); double duration = StdIn.readDouble(); double[]a = note(pitch, duration); StdAudio.play(a);