Jim Parsons, Kaley Cuoco-Sweeting and Johnny Galecki

Every advertising selling season, broadcast-network executives must privately ask themselves the same question: "How much longer can we defy gravity?"

Through the first quarter of 2004, ABC, CBS, NBC and Fox combined had a 48 percent share of viewers ages 18 to 49, the sweet spot for advertisers. In 2013, their share was down to 34 percent. The networks reportedly took in $9 billion in revenue during the 2004 upfronts. Last year, that total was closer to $8 billion — down, but not nearly commensurate with the decline in ratings. With the exception of 2009, the year after the economic meltdown, ad rates, measured by the cost per thousand viewers, have kept going up. In other words, the Keynesian law of supply and demand has been very good to the networks.

"If you go back over the past seven or eight years, most of the cost-per-thousand increases in broadcast have been driven by the fact that the supply is down from the loss of ratings," says Rino Scanzoni, chief investment officer for GroupM, the world's biggest media planning and buying company.

Ad-supported cable networks have a dominant share of the TV ratings pie. But the network broadcasters have held an advantage in pricing, as they still have the ability to reach every American home and have more high-rated shows that can quickly deliver a mass audience. Advertisers also feel they gain prestige by running their spots in a top broadcast program, even if the same series airs on a cable network. "A lot of the advertisers who buy The Big Bang Theory will say, 'I want my products to be on the highest-rated show on broadcast television [on CBS], not a rerun on TBS,'" says one veteran network executive.

But the jig could soon be up. Cable networks are airing more first-run shows every year, and a growing number of them, such as AMC's The Walking Dead and A&E's Duck Dynasty, are broadcast-size hits. The graphic content on The Walking Dead and other original cable dramas can scare off some advertisers, but overall, Scanzoni says, "the tolerance of adult themes has definitely broadened."

Even with more dollars moving to the cable networks, broadcast will survive thanks to the influx of revenue from retransmission fees paid by cable and satellite operators. Scanzoni also believes broadcast networks will benefit when Nielsen can accurately measure their audiences across video on demand, as well as tablets, smartphones and other devices. "We're probably a year away from that," he says.

Sure, advertisers have been paying more to the broadcast networks while getting less. But when mixed with the lower cost of time on cable — considering its hundreds of channels — national TV ad rates have actually declined. "The problem is, people are looking at the television business as components," says Scanzoni. "What you should be looking at is national TV in its entirety. That's the way the viewers look at it."

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