Hey, I just started with the book and I have a question on formula 2.68. Here the constraint

D^TD=I_l

is stated. But this would mean that *D* has to be a orthogonal matrix with *n=l*. And this would miss the whole point of the compression. At the beginning of section 2.12 it is explained that *l<n*, columns of *D* are orthogonal and

||D_{:,i}||_2 = 1

Is the constraint above a mistake?