CBS has become the first broadcast network to sign up to trials of an online video-on-demand service which has become the focus of the US cable industry’s efforts to preserve its business model in the digital era.

The broadcaster’s alliance with Comcast, the cable group which unveiled plans for its On Demand Online service last month, could help CBS build up an alternative to Hulu, the fast-growing online video site backed by three rival US networks – NBC, News Corp’s Fox and Walt Disney’s ABC.