Cable company Comcast Corp is teaming up with its NBC Universal unit to help programmers dynamically place commercials around on-demand TV shows in a bid to drive new advertising revenue.

Until now, cable companies have manually inserted advertising into on-demand programing, where it remains as long as the TV show is available to subscribers. This has meant the ads could soon be out of date.

Using new technology from Comcast and partner Black Arrow, the ads can now easily be changed at any time. The new system is being tried out with TV shows from NBCU's networks including Bravo, USA Network and Oxygen Media.

"On-demand is a huge platform and it's going to grow significantly along with on-demand advertising," said Marcien Jenckes, Comcast senior vice president of video services. He said Comcast has more than 350 million on-demand video streams.

Other cable operators are expected to soon be able to deliver a similar service to advertisers in partnership with Canoe Ventures -- the cable industry-owned advertising company.

Philadelphia-based Comcast took control of NBC Universal in January.

Wall Street analysts had been critical of the $30 billion deal, saying there are few major synergies between Comcast's cable distribution business and NBC Universal's array of programing, movies and theme parks.

"We've not been huge believers in the synergies between Comcast and NBC but they seem to have found some nominal savings so far," said Thomas Eagan analyst at Collins Stewart.