copy a matrix into another one, with offset

hi there,
a final output matrix, displaying blank screen at the beginning
another matrix with content changing each x seconds.
both same width, but the second one is 1/8 height of the first one.

The big one has to be filled progressively, each x seconds, by the content of the small one, using offset
in a cycle, after x second, the first 1/8 is filled.
then after x, the second 1/8 (the first 1/8 still contains the previous stuff)

etc till the end where all become blank again

struggling with @usedstdim and the other companions in tutorial 16 and the content of the smallest is always copied at the bottom
strangely.

Here is the machine:

any leads would totally unstick me :)