The NFL has quietly passed a rule during its fall meeting that will force one franchise per year to be filmed for HBO’s Hard Knocks. The news came the same day that the league was fending off some negative PR over PBS’ premiere of the football concussion docu League Of Denial and President Obama weighing in on the Washington Redskins nickname flap. The popular series, which follows a team through preseason, had a tough time finding a subject this year. Five teams declined to participate before the Cincinnati Bengals — a good-not-great small-market franchise with a negligible national following — agreed to do it for a second time. The NFL and HBO signed a multiyear extension for the program in July, so something had to be done. Under the new rules, teams can volunteer to be on the program, but now the league will select one if there are no takers. Teams that have a new head coach, have been in the playoffs at least once in the past two seasons, or have done one Hard Knocks in the past 10 years are exempt from appearing on the show.