convert char matrix to float 32

hello
i am looking in all kinds of forums, but can't find the solution:
I've red that [jit.slide] is more efficient with float 32 matrix, but i don't know how to convert my matrix (4 char 648 480) to a float 32 one.
I've red it is with the [jit.expr] object, but i can't understand anything... Math are far away from me :)
Anybody knows ?

Thanks