Downmixing 7.1.2ch to 3.1.2ch using MATLAB

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It’s like trying to fit all your friends in a tiny car instead of a big bus!

Here’s how you can do this using MATLAB:

First, let’s say we have some audio data that looks like this:

% Load the audio file and convert it into a matrix with 7.1 channels (left front, center, right front, left surround, right surround, LFE, and back)
audio = audioread('example_sound.wav'); % Load the audio file and store it in the variable 'audio'
channels = size(audio,2); % Get the number of channels in our audio file and store it in the variable 'channels'
numChannels = 7; % Set the number of channels we want to keep (in this case, 7 for 7.1 surround)
downmixMatrix = zeros(size(audio)); % Create an empty matrix with the same size as our original audio data and store it in the variable 'downmixMatrix'

% Loop through each channel in our original audio file starting from the left front (channel 1) and ending at the back (channel 7)
for i=1:channels-numChannels+1 
    % Copy the selected channels into our downmixMatrix matrix
    for j=i:min(i+numChannels-1, channels) 
        downmixMatrix(:,j-i+1) = audio(:,i); % Add the current channel to the corresponding position in our new matrix (starting from 0 for left front and ending at numChannels-1 for back)
    end
end

% Convert our downmixed data into a stereo format with two channels (left and right) by selecting every other column of our downmixMatrix matrix
downmixedStereo = downmixMatrix(:,1:2); % This will give us the left channel in the first column and the right channel in the second column

% Save our new audio file as a stereo WAV format with 44.1 kHz sampling rate
audiowrite('downmixed_sound.wav', downmixedStereo, 44100); % This will save our data to a new file called 'downmixed_sound.wav' at the specified sampling rate of 44.1 kHz

So what we’re doing here is creating an empty matrix with the same size as our original audio data, and then looping through each channel in our original audio file starting from left front (channel 1) and ending at back (channel 7). For each selected channel, we copy it into our new downmixMatrix matrix by adding it to the corresponding position based on its index. Finally, we convert our downmixed data into a stereo format with two channels (left and right) by selecting every other column of our downmixMatrix matrix.

Your 7.1 surround sound is now squeezed into just two channels for your listening pleasure.

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