Google has started testing its radio advertising system with a selected group of advertisers.

Google Audio Ads is an online ad-buying system that was developed by dMarc Broadcasting, a company purchased by Google for around $100 million in 2006.

The radio industry's advertising is worth around $20 billion annually, so it makes sense that Google would want in on that market. The company's reach is such that it's working with more than 700 stations across 200 US markets.

The AdWords system allows users to track how much is spent on the ads, as well as when it's aired. Analysts hope this simplified, web-based system shakes up the industry.

Google wants to expand beyond the web search advertising industry, and just this week announced a deal with Sky broadband where Sky will Google's video content tools and search and advertising tools across its websites.

The trial is going on in the US at the moment, but Google has not said when the system will be made available to all stations.