confused about dimensions of detailed cofficients when apply db3 of level 6

I have a cell array comprises of three cell with dimension of 868 by 16 of each cell. When i apply db3 with level 6 on each cell and safe detailed coefficient and approximate coefficient in an array corresponding to original data cells by using Matlab code shown below:

dwt_coeffs_struct = struct('A', cell(size(data)), 'D', cell(size(data)));

% Loop over each cell in the data array

for i = 1:numel(data)

% Convert the cell data to double and reshape it to a column vector

signal = reshape(cell2mat(data{i}), [], 1);

% Initialize arrays to store approximation coefficients and detail coefficients

A_coeffs = cell(1, 6);

D_coeffs = cell(1, 6);

% Perform the DWT for each level

for j = 1:6

[C, L] = wavedec(signal, j, 'db3');

% Store the approximation coefficients and detail coefficients separately

if j == 1

A_coeffs{j} = appcoef(C, L, 'db3', j);

end

D_coeffs{j} = detcoef(C, L, j);

end

% Store the DWT coefficients for this signal in the structure array

dwt_coeffs_struct(i).A = A_coeffs;

dwt_coeffs_struct(i).D = D_coeffs;

end

Than i got detiled cofficents D1=41666 by 1

D2=20835 by 1

D3=10420 by 1

D4=5212 by 1

D5=2608 by 1

D6=1306 by 1

Am i getting correct dimensions of these detailed coffinites ....Please guide so that i may further extract features from these detailed coefficients

Answer 1

[Prathamesh]

Hi @Saima Raza,

I understand that you have a cell array consisting of three cells, each with dimensions of 868 by 16. After applying a level 6 discrete wavelet transform (DWT) using the 'db3' wavelet on each cell, are the dimensions of the resulting detail and approximation coefficients correctly corresponding to the original data cells?

Here's a corrected version of your code with comments explaining the changes:

dwt_coeffs_struct = struct('A', cell(size(data)), 'D', cell(size(data)));

for i = 1:numel(data)
    % Convert the cell data to double and reshape it to a column vector
    signal = reshape(cell2mat(data{i}), [], 1);
    
    % Perform the DWT for 6 levels using 'db3' wavelet
    [C, L] = wavedec(signal, 6, 'db3');
    
    % Initialize arrays to store approximation coefficients and detail coefficients
    A_coeffs = cell(1, 6);
    D_coeffs = cell(1, 6);
    
    % Extract approximation coefficients at level 6
    A_coeffs{6} = appcoef(C, L, 'db3', 6);
    
    % Extract detail coefficients for each level
    for j = 1:6
        D_coeffs{j} = detcoef(C, L, j);
    end
    
    % Store the DWT coefficients for this signal in the structure array
    dwt_coeffs_struct(i).A = A_coeffs;
    dwt_coeffs_struct(i).D = D_coeffs;
end

for i = 1:numel(data)
    fprintf('Detail Coefficients for signal %d:\n', i);
    for j = 1:6
        fprintf('D%d: %d by 1\n', j, length(dwt_coeffs_struct(i).D{j}));
    end
end

Key Corrections:

Signal Reshaping : Ensure that each cell's data is reshaped to a column vector before applying the DWT. This step is crucial to maintain the integrity of the signal.

Coefficient Storage: Store the approximation coefficients only at the highest level (level 6). Store detail coefficients for each level from 1 to 6.

Correct Usage of wavedec: The 'wavedec' function is correctly used to decompose the signal up to the specified level.