I made a dft matrix using W = dftmtx(128) and plotted it's real and imaginary parts using imagesc. What should the axis labels be? I thought they would be x = frequency and y = amplitude, but I'm not sure if this is right.
The 'dftmtx' function returns an n*n matrix, where 'n' is the parameter passed to the function. The matrix returned by the 'dftmtx' function is the complex discrete fourier transform Matrix. We can get the discrete fourier transform 'y' of a signal 'x' by multiplying 'x' with the matrix that is returned by the 'dftmtx' function. Hence using the 'imagesc' function on this matrix would only show the value of the real/imaginary part of the complex coefficients in the matrix by the corresponding colour.