Processing Sound

Reading: none

Code from class:

• soundLecture.m
• pureNote.m
• rampNote.m
• organ.m
• squareNote.m
• sawtoothNote.m
• painoNote.m
• fluteNote.m
• playSong.m (uses canon.txt)
• backmaskLecture.m
• reverb.m

We will be using the following file in lecture: canon.txt. This file contains the notes of Pachelbel's Canon represented in an unusual format. Each row is a separate note to be played: the first column is the note's frequency, the second column is the note's starting time (in seconds), and the third column is the note's duration.

Wav files:

• Beatles.wav (backmasking at 2:25 through 2:37)
• instr_flute.wav middle C flute tone at 261Hz
• instr_piano.wav middle C piano tone at 261Hz

MATLAB support for sound

• audioread
  Sample usage:

        [samples freq bitsPerSample aux] = audioread(filename)
        

• audiowrite
  Sample usages:

        audiowrite(samples, filename)
        audiowrite(samples, freq, filename)
        audiowrite(samples, freq, bitsPerSample, filename)
        

• audioplay
  Sample usages:

        audioplay(samples, freq)
        audioplay(samples, freq, 'async')    %  call returns even while sound is still playing
        audioplay(samples, freq, 'sync')     %  call waits until sound is complete before returning
        

• sound
  Sample usage:

  	sound(samples, freq)	%call returns while sound is playing
  	

Plotting audio waves

function plotStereo(y, fs)
  subplot(2,1,1);
  x = linspace(0, length(y)/fs, length(y));
  plot(x,y(:,1), 'b');      % plot in blue
  ylabel('Left');
  xlabel('seconds');
  axis([0 length(y)/fs -1 1]);
  grid on;
  subplot(2,1,2);
  plot(x,y(:,2), 'r');       % plot in red
  ylabel('Right');
  xlabel('seconds');
  axis([0 length(y)/fs -1 1]);
  grid on;

Synthesizing sounds

We examined how to develop pure tones of a given frequency as a cosine wave, how to create other waves such as square or sawtooth waves, and how to combine different harmonics of a note to create a more realistic sounding instrument. We then examined high-level approaches to representing songs as a series of notes, or as a composition of overlayed notes.