Two of the four major US broadcast networks have precipitated a seismic shift in the way television will be viewed in future.

Viacom's CBS and General Electric's NBC have inked separate deals that will enable digital pay-TV subscribers on other platforms to download some of their most popular programs shortly after they air.

CBS has signed an agreement with cable giant Comcast that will provide the latter's viewers with on-demand access to top-rated shows for just 99 cents (€0.83; £0.56) an episode.

A similar accord has been struck between NBC network and News Corporation-controlled satellite broadcaster DirecTV.

The CBS on-demand shows will include all the commercials that appear during the initial broadcast, although customers will retain the ability to fast-forward through them.

The NBC selection, which will be available via a personal video recorder, will not include advertising.

The deals make on-demand TV available to a broader audience. They are also a significant pointer to the way technology is changing the traditional network business and how TV executives are attempting to retain some control over content and revenues.

Says David Zaslav, president of NBC Universal's cable group: "For 50 years TV has been a passive medium. But consumers want more choice and more convenience. All the signals are there of meaningful change in how people watch television."

And that change is disquieting to advertisers and their agencies.

Says Jon Mandel, co-ceo of Grey Global Group's MediaCom: "Advertisers are upset because there's an immediate run to no commercials. We automatically get left out."

Data sourced from Wall Street Journal Online and; additional content by WARC staff