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Echo generator problem - Coursera Introduction to Matlab Programming

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ey21
ey21 on 18 Apr 2020
Hey all,
I am trying to program the solution to the Echo Blur problem (I have attached a screenshot below describing the task):
My code is written below, could anyone give me any pointers as to where I am going wrong?
function[output] = echo_gen(input, fs, delay, amp)
input = input'; %storing the column vector input as a row vector
samplesinthesrecording = length(input); %finding the number of samples in the input vector
numberofdelaysamples = round(delay)*fs; %finding the number of samples in the delay
sampleatwhichechostarts = numberofdelaysamples + 1; %finding the sample number at which the echo begins
soundbeforecho = input(input < input(sampleatwhichechostarts)); % the sound before the echo is playing during the samples which appear before the echo
amplifiedecho = input*amp; %the echo is amplified by an amount specified by amp
originalsoundplusecho = [soundbeforecho, amplifiedecho]; %I am binding the vector of samples for both the original sound and the echo
if min(originalsoundplusecho) < -1 || max(originalsoundplusecho) > 1 %I am making sure the values within the vector 'originalsoundplusecho' are less than 1 and greater than -1
normalisedvector = rescale(originalsoundplusecho,-1,1); %I am normalising the vector if the condition above is not met, making sure it is in the boundaries -1 to 1
output = normalisedvector'; %the output is the normalised vector
else
output = originalsoundplusecho'; %if the values of the vector 'originalsoundplusecho' are within -1 to 1, then the output is the vector 'originalsoundplusecho'
end
end
Here is the code to call the function:
load splat
output = echo_gen(y, Fs, 0.25, 0.6); %calling the function
dt = 1/Fs; %the interval between values on the x-axis
t = 0:dt:dt*(length(output)-1);%the x-axis scale
plot(t, output) %plotting the graph
Thank you very much!

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Emre Yavuz
Emre Yavuz on 18 Apr 2020
@Walter Roberson, in the function:
soundbeforecho = input(input < input(sampleatwhichechostarts));
In am selecting the samples which come before the echo from the 'input' column vector.
I am then binding this sample to a vector with the samples within the echo:
originalsoundplusecho = [soundbeforecho, amplifiedecho];
So effectively, the output of the function should be the values of the y-axis which correspond to the sound being replayed and then the echo of the sound (with an amplification).
Walter Roberson
Walter Roberson on 19 Apr 2020
In am selecting the samples which come before the echo from the 'input' column vector.
No, you are not. Suppose for example
input = [3 1 4 1 5 9 2 6 5 3 5];
sampleatwhichechostarts = 9;
input(sampleatwhichechostarts)
input < input(sampleatwhichechostarts)
input(input < input(sampleatwhichechostarts))
ans =
5
ans =
1×11 logical array
1 1 1 1 0 0 1 0 0 1 0
ans =
3 1 4 1 2 3
When what you want is [3 1 4 1 5 9 2 6] which is input(1:8)
ey21
ey21 on 19 Apr 2020
@Walter Roberson, thank you very much!
The code below works!
function[output] = echo_gen(input, fs, delay, amp)
numberofdelaysamples = round(delay*fs);
soundvectorplusecho = zeros(numberofdelaysamples + length(input),1);
soundvectorbeforecho = soundvectorplusecho;
for i = 1:length(input)
soundvectorplusecho(i + numberofdelaysamples) = input(i)*amp;
soundvectorbeforecho(i) = input(i);
end
fullsoundvector = soundvectorbeforecho + soundvectorplusecho;
range = abs(fullsoundvector);
maxrange = max(range);
if maxrange>1
fullsoundvector = fullsoundvector/maxrange;
end
output = fullsoundvector;
end

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Answers (1)

Muhammad Qaisar Ali
Muhammad Qaisar Ali on 27 Jun 2020
function output = echo_gen(input, fs, delay, amp)
delay_mat=zeros(round(delay*fs),1); % a col vector.
echo_mat=([delay_mat;input])*amp; % make echo col vector,input is a column vector.
output=input+echo_mat(1:length(input),1); % super imposing echo with origional input sound track.
output=[output;echo_mat(length(input)+1:end,1)];
if max(abs(output))>1 % scaling b/w -1,+1 throught relative scaling.
output=output/max(abs(output));
end
end

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