Here’s the dirty little secret behind broadcasters’ campaign to change the way ads are sold — to include people who watch them up to seven days after they air (called C7), up from three (C3): It wouldn’t increase advertiser spending on TV. It would just change the proportion of sales that go to broadcast vs cable. That’s the main reason why Bernstein Research’s Todd Juenger says this morning that a new arrangement “would be largely a wash” for Big Media companies that have broadcast and cable networks. The exception is CBS, which collects relatively little from cable ads. “Broadcast programming, especially primetime, is timeshifted more than cable network programming,” he says. TiVo data show that broadcast network commercial ratings would rise 6% in DVR homes while cable would be +4% if the sales period is expanded to seven days. (That includes ad zapping: Overall program ratings would rise 11% for broadcast and 8% for cable.) Since about half of all households have a DVR, that might translate into an overall increase in counted ad viewing of 3% for broadcasters and 2% for cable. If you assume that advertisers wouldn’t increase their TV budgets, then a change that largely affects DVR homes would move about 0.5% of the $40B in national TV ad spending to broadcast from cable — about $115M. That’s “worth fighting for,” Juenger says. But not so much for companies that sell lots of ads on cable networks. No wonder “we will once again see CBS leading the charge in this fight on behalf of the broadcast networks.”