CBS, Fox and the other TV broadcasters are delivering more shows on demand through pay-TV services to wean audiences off digital video recorders, which let viewers skip through commercials and, thereby, cost the TV industry money.

Comcast, owner of NBC and the largest U.S. pay-TV service, is offering the 100 most popular shows on demand in the new TV season that's just begun.

"This platform has reached critical mass and has become deeply integrated into how customers watch TV," said Matthew Strauss, a senior vice president at Comcast. "On demand has always been growing, and it's growing faster now than we've ever seen it."

As many as a third of viewers today watch shows after their original air date, and each one who hits the fast-forward button on the digital video recorder (DVR) represents a loss for the TV industry, which generates about $70 billion in annual ad sales. Video-on-demand libraries, which free consumers from the task of setting up recordings in advance, are available in about 60 percent of households, according to Nielsen.

The growth in VOD is a major factor in the TV industry's push for a new advertising formula. With viewers unable to skip commercials, networks now want to get paid for spots seen as many as seven days after a live telecast.

The cost of a 15- or 30-second commercial is based on the size and demographics of a show's audience. Advertisers already pay for ads seen in the first three days after the first telecast of a show — provided no one hits the fast-forward button. Broadcasters and cable networks could generate an additional $400 million in ad sales if 70 to 80 percent of DVR users convert to VOD, according to Laura Martin, an analyst with Needham & Co.

"It recaptures money that has been lost to the DVR," Martin said. The DVR "has been slowing growth for 10 years."

The majority of VOD viewing takes place after three days. If the big four networks can persuade marketers to pay for viewing up to seven days later, they could add 3 to 5 percent to their combined prime-time ad sales of $10 billion to $11 billion, according to Martin. That's on top of the $400 million figure the industry stands to gain from retiring DVRs.

The impact of delayed viewing varies by show, with some, such as ABC's "Modern Family" and Fox's "Sleepy Hollow," increasing their audiences by 6 percent or more in days four to seven.

"You see these massive gains," said Will Somers, Fox's senior vice president of research. "What we learned about 'Sleepy Hollow' is that the sampling tail will be extremely long. It continues to rack up viewership as the show moves forward."