MATLAB: Multiply sound by pulse

Question

How do I make a pulse and make the time of the pulse match the audio file time? For the pulse I would like the period to be 2 seconds, and amp of 1, and a duty cycle of 50%. I would then like to multiply the pulse by the audio file.

Answer 1

Is your signal stereo instead of monochannel? You need to take that into account.

See this little demo for mono, and adapt as needed:

% Initialization steps.
clc;    % Clear the command window.
fprintf('Beginning to run %s.m ...\n', mfilename);
close all;  % Close all figures (except those of imtool.)
clear;  % Erase all existing variables. Or clearvars if you want.
workspace;  % Make sure the workspace panel is showing.
format short g;
format compact;
fontSize = 15;
% Read in the sound.
[y, Fs] = audioread('guitartune.wav');
% load('handel.mat');	% Another sound you can load.
% Play the sound.

soundsc(y, Fs);
% Compute the time axis
t = (1 : length(y)) / Fs;
subplot(3, 1, 1);
plot(t, y, 'b-', 'LineWidth', 2);
title('Original Audio Waveform', 'FontSize', fontSize);
xlabel('Seconds', 'FontSize', fontSize);
ylabel('Amplitude', 'FontSize', fontSize);
xticks(1 : round(t(end)));
grid on;
drawnow;
audioTime = length(y) / Fs; % In seconds.
fprintf('Audio length = %.3f seconds.\n', audioTime);
% Compute how many seconds 2 seconds is.
pulseWidth = Fs; % Whatever it needs to get 1 (half of 2) of whatever units you're using.
onePulse = [ones(1, pulseWidth), zeros(1, pulseWidth)];
% Replicate as many times as we need (more actually)
numReps = ceil(audioTime/2);
pulseTrain = repmat(onePulse, [1, numReps]);
% Now crop down to the actual signal length that y is
pulseTrain = pulseTrain(1 : length(y));
% Reshape to be a column vector like y
pulseTrain = pulseTrain(:);
% Plot the pulse train.

subplot(3, 1, 2);
plot(t, pulseTrain, 'b-', 'LineWidth', 2);
title('Pulse Train', 'FontSize', fontSize);
xlabel('Seconds', 'FontSize', fontSize);
ylabel('Amplitude', 'FontSize', fontSize);
grid on;
xticks(1 : round(t(end)));
% Multiple the pulse train by the audio signal and plot.
yPulsed = y .* pulseTrain;
% Plot the pulse train.
subplot(3, 1, 3);
plot(t, yPulsed, 'b-', 'LineWidth', 2);
title('Pulsed Waveform', 'FontSize', fontSize);
xlabel('Seconds', 'FontSize', fontSize);
ylabel('Amplitude', 'FontSize', fontSize);
grid on;
xticks(1 : round(t(end)));
pause(audioTime);
% Play the sound.
soundsc(yPulsed, Fs);
fprintf('Done running %s.m ...\n', mfilename);