function [signal] = project_01_part01(f0, fs, secs, velocity)

%comment out f0, fs, secs, and velocity to specify values from command line
c = 340;        %speed of sound
velocity = 10;  %velocity of sound source
f0 = 440;       %center frequency of sound
fs = 8000;      %sampling frequency
secs = 10;      %duration of sound

time = -secs/2:1/fs:secs/2;
dist = velocity*time;
env = 1./abs(dist);

%since 1/x has vertical asymptotes, limit envelope y-values to 1
for i = 1:length(env)
    if env(i) > 1
        env(i) = 1;
    end
end

v = zeros(1,length(time));
for i = 1:length(v)
    if dist(i) < 0
        v(i) = -velocity;
    else
        v(i) = velocity;
    end
end

f = zeros(1,length(time));
for i = 1:length(time)
    f(i) = f0*(c/(c+v(i)));
end

signal = env.*sin(2*pi*f.*time);

subplot(411), plot(time,v,'Color',[1,0.12,0.12]), axis([min(time),max(time),-1.5*velocity,1.5*velocity]), grid on;
title('Velocity of sound source'), xlabel('time (s)'), ylabel('velocity (m/s)');
subplot(412), plot(time,f,'g'), grid on;
title('Frequency shift'), xlabel('time (s)'), ylabel('frequency (Hz)');
subplot(413), plot(time,env,'b'), grid on;
title('Amplitude envelope'), xlabel('time (s)'), ylabel('amplitude');
subplot(414), plot(time,signal,'Color',[0.98,.68,0]), grid on;
title('Doppler effect on signal'), xlabel('time (s)'), ylabel('amplitude');

signal = signal/max(abs(signal)); %normalizes input

%print project_01_part01 -dpng -r100;
%wavwrite(signal,fs,'project_01_part01');

soundsc(signal,fs);