Morgenstern proof for FFT lower bound

Question

I looked at my notes from a class about fast forier transform , and the professor proved in class theorem thanks to Morgenstern , first he defined linear algorithm as a algorithm that inly uses multiplications by scalar and addtions, that he stated Morgenstern's theorem the following way

Every linear algorithm that computes DFT (Discrete Fourier Transform) , in which the scalar are bounded in their absolute value by $C$ , takes at least $\Omega_C(n\log{n})$ addition operations.

I haven't understood his proof , if someone can give me guidlines to the proof I'll be more than happy :) [right now my proof of the theorem looks like a mess]

p.s.: In class he said we will write in each step the coeffiecents of the linear function that was just computed , then he defined a "computation advancment function" , I didn't understand his definition of this function , it was definied something like $\phi(k)=$ maximum determinant of n$\times$n matrices that were computed untill the k'th step [I probably copied wrong]