How can I add a low pass filter after each delay line in my multi channel feedback delay network

Question

Im about to program this structure in Matlab in a for loop   So far so good. And I have already done the for loop. I also implemented a Hadamard Mixing Matrix. Here is the code:     in = [ 1; 0 ]; % Dirac Impulse Fs = 44100; in = [in; zeros(3*Fs,1)]; % Space for reverb % Define delay parameters %Hadamard Matrix as feedback matrix H = 0.75*hadamard(16); feedbackGain =[0.1,-0.15,-0.2,0.25,-0.3,0.35,0.4,-0.45,-0.5,0.55,0.6,-0.65,-0.7,-0.75,-0.8,0.85]; %0.1,0.15,0.2,0.25,0.3,0.35,0.4,0.45,0.5,0.55,0.6,0.65,0.7,0.75,0.8,0.85 wetGain = [0.7,0.7,0.7,0.7,0.7,0.7,0.7,0.7,0.7,0.7,0.7,0.7,0.7,0.7,0.7,0.7]; % adjust to control wet signal level % Convert delay time to samples delaySamples = [101,233, 439,613, 853,1163, 1453,1667 , 1801,2053, 2269,2647, 3001,3607, 4153,4591]; %0.02s, 0.03, 0.04, 0.05 881, 1009, 1321,1511, 1777, 1999, 2203, 2351 %101, 439, 853, 1453 , 1801, 2269, 3001, 4153 %Summe delays = 30644 % delaySamples = [1200, 1800, 2200,2500]; % Create an empty array to store delayed audio delayedAudio = zeros(size(in)); delayedAudio2 = zeros(size(in)); delayedAudio3 = zeros(size(in)); delayedAudio4 = zeros(size(in)); delayedAudio5 = zeros(size(in)); delayedAudio6 = zeros(size(in)); delayedAudio7 = zeros(size(in)); delayedAudio8 = zeros(size(in)); delayedAudio9 = zeros(size(in)); delayedAudio10 = zeros(size(in)); delayedAudio11 = zeros(size(in)); delayedAudio12 = zeros(size(in)); delayedAudio13 = zeros(size(in)); delayedAudio14 = zeros(size(in)); delayedAudio15 = zeros(size(in)); delayedAudio16 = zeros(size(in)); delayedAudios =[delayedAudio,delayedAudio2,delayedAudio3,delayedAudio4,delayedAudio5,delayedAudio6,delayedAudio7,delayedAudio8,delayedAudio9,delayedAudio10,delayedAudio11,delayedAudio12,delayedAudio13,delayedAudio14,delayedAudio15,delayedAudio16]; delayedAudios(1,:) = 1; % Apply delay effect for i = 1:length(in)-2*max(delaySamples) for k = 1:16 if delayedAudios(i+delaySamples(k),k) == 0 delayedAudios(i+delaySamples(k),k) = feedbackGain(k)*delayedAudios(i,k); end end for k = 1:16 for j = 1:16 if delayedAudios(i+delaySamples(j)+delaySamples(k),k) == 0 && k~= j delayedAudios(i+delaySamples(j)+delaySamples(k),k) = H(j,k)*delayedAudios(i+delaySamples(j),j); end end end end %Original and Mix together for k = 1:16 delayedAudios(:,k) = wetGain(k)*delayedAudios(:,k); end %outputAudioM = delayedAudios(:,16) + delayedAudios(:,15) + delayedAudios(:,14) + delayedAudios(:,13) + delayedAudios(:,12) +delayedAudios(:,11) +delayedAudios(:,10) +delayedAudios(:,9)+delayedAudios(:,8) +delayedAudios(:,7) +delayedAudios(:,6) +delayedAudios(:,5) +delayedAudios(:,4) +delayedAudios(:,3) +delayedAudios(:,2 )+delayedAudios(:,1); outputAudioM = sum(delayedAudios,2); outputAudioM(1) = outputAudioM(1)*0.02; %Low pass filtering + phase adjusting d = designfilt('lowpassfir','FilterOrder',4 ,'CutoffFrequency',2000,'SampleRate', Fs); y = filtfilt(d,outputAudioM); % %Write the output audio to a new file and plot audiowrite('output_audio_with_delay.wav', y, Fs); plot(y); % Play the output audio sound(y,Fs); As you can see in the code I implemented a lpf AFTER the sound was computed. That of course is no problem but I have no idea how I can do that in the for loop like in the picture I showed. Can someone explain how it is to be done or give me hints?

Kshitij Singh · Accepted Answer

Hi Muhsin,
 
In general, in order to perform filtering and listen to the output inside your for loop, you use filter System objects to perform filtering, and audioDeviceWriter to play the output.
 
The application you are working with is somewhat similar to a reverberation object that ships with Audio Toolbox:
 
audioexample.FreeverbReverberator. That objects internally uses a bank of allpass filters implemented using dsp.IIRFilter objects.
 
Here is a simple sample code of how to use the object to filter input data and listen ot the output in the loop.
 
 
% Process samples from an audio file
afr = dsp.AudioFileReader("FunkyDrums-44p1-stereo-25secs.mp3",PlayCount=Inf);
% The filter
r =  audioexample.FreeverbReverberator;
% Play the result
adw = audioDeviceWriter(SampleRate=afr.SampleRate);

while ~isDone(afr)
    x = afr(); % read an input frame
    y = r(x);  % filter the frame. the filter retains states between consecutive calls
    adw(y);    % listen ot the output frame
end

How can I add a low pass filter after each delay line in my multi channel feedback delay network

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