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== 1.Introduction ==
In this section, you will learn how to use the signal processing technique in Matlab to add several sound effects to music. I will introduce three basic sound effects, namely, echo, flanger and chorus.


== 2.Echo effect ==
The fundamental theory behind the echo effect is that the original music signal combines with its repeated signal with a fixed delay period.  This effect can be achieved by implementing a comb filter. The difference equation for this simple comb filter can be written as follows:
   			Y[n] = X[n] + a * X[n – D]
Where:
X[n] : input signal
Y[n]: output signal
D :  number of samples during delay
a :   attenuation coefficient.  |a| < 1

===== % Matlab code =====
[x,fs] = wavread(‘singsing.wav’);   % load the music and get the sampling frequency
length = size(x);   % get the length of the music file 
a = 0.3;   % set the attenuation factor
delay = 0.38;   
D = delay*fs;       %  set the delay time in s

y = zeros(length);   % initialize the output music signal
for i = D + 1 : 1 : length;
 	y(i) = x(i) + a*x(i-D);
end;
sound(y, fs);   % play the echo

Example:


== 3.Flanger effect ==
The flanging effect is produced by mixing two identical music signals with a varying delay function. Unlike the fixed delay D in the comb filter, the flanger filter has a non-constant delay D, which changes periodically. The difference equation for this simple comb filter can be written as follows:
                                   Y[n] = X[n] + a * X[ n – D[n] ]

===== % Matlab code ===== 

[y, fs, nbits] = wavread(file);  %Reading the file

             low_n = round(0.0*fs);  %Creating the vector according to which delay is varied
high_n = round(0.0057*fs);

             delay_vary_p = 8;

delay_step = (delay_vary_p/4)/(1/fs);

delay_1 = round(linspace(low_n,high_n,delay_step));

delay_2 = round(linspace(high_n,low_n,delay_step));

delay = [delay_1 delay_2];

no_points = length(y(:,1));

n_rep = round(no_points/length(delay));

delay = repmat(delay,1,n_rep);

delay = [delay delay(1:no_points-length(delay))];

out_wav(:,1) = zeros(1,no_points);

out_wav(:,2) = zeros(1,no_points);

for i=1:no_points

    n = i-delay(i);

    if n>0

        out_wav(i,1) = y(i,1)+y(n,1);

        out_wav(i,2) = y(i,2)+y(n,2);

    else

        out_wav(i,1) = y(i,1);

        out_wav(i,2) = y(i,2);

    end

end