Notwithstanding sections
541
(c) and
543
(a) of this title, at such time as cable systems with 36 or more activated channels are available to 70 percent of households within the United States and are subscribed to by 70 percent of the households to which such systems are available, the Commission may promulgate any additional rules necessary to provide diversity of information sources. Any rules promulgated by the Commission pursuant to this subsection shall not preempt authority expressly granted to franchising authorities under this subchapter.