Complex sentence: Petrucci later developed a process which required only two passes through the press, but was still taxing since each pass required very precise alignment for the result to be legible (i.e., so that the note heads would be correctly lined up with the staff lines).
Split into simpler sentences: Petrucci later developed a process which required only two passes through the press. But it was still taxing since each pass required very precise alignment for the result to be legible (i.e., so that the note heads would be correctly lined up with the staff lines).