Echo Filter

An Audio Echo Filter is an FIR filter that repeats a sound after a given Delay attenuating the repetitions. 

You may test the filter using any input signal (i.e. Matlab "splat" signal).  In the command window,

>> load splat;
>> [yecho,h]=echo_filter(y,[0.05 0.1 0.14],[1 0.9 0.75],Fs);
>> sound(y,Fs)
>> sound(yecho,Fs)

% function echo_filter:
%     1) computes and returns the impulse response h of the LTI system (echo filter)
%     2) Filter the input signal.  The output signal is yecho. 
%
% INPUTS:
% signal = input signal
% times = vector containing the time delays (in seconds) of the repetitions(echoes)
% attenuations = vector containing the respective attenuations of the echoes
% fs = sampling frequency of the input signal.
%
% OUTPUTS:
% yecho = output signal (passed through the echo filter)
% h = impulse response of the echo filter.

function [yecho, h] = echo_filter(signal, times, attenuations, fs)

h(1)=1; % This is the first coefficient of the echo filter

% Computing the impulse response h
for i=1:length(times),
    samples(i) = times(i)*fs; % Calculating the sample-times (N = t*fs)
    h(floor(samples(i))) = attenuations(i); % Impulse response coeficients
end

% #########################################################################
% You may use the following implementation instead of the illustrated
% above:

%       samples = times*fs;
%       h(floor(samples)) = attenuations;

% In this case, the implementation is done without a for-loop.
% #########################################################################

yecho = filter(h,1,signal(:,1)); % filtering of the input signal results in a signal with echoes

% You may test the filter using the Matlab signal "splat" (write load splat
% in the Matlab command window).  Use function sound() in order to
% listening both, the input and the output, signals.