LOS GATOS, Calif. — Vance Ikezoye didn't set out to become one of the digital revolution's top copyright cops when he first began tinkering with the technology that launched Audible Magic Corp.

At first, all he was looking for was a better way to identify songs and advertisements broadcast on the radio.

But Ikezoye's ambitions have shifted with the media landscape, positioning Audible Magic to control what can and can't be watched on the Internet. Online video sites are adopting its filtering tools to prevent the kind of copyright trouble that provoked a legal battle between Viacom Inc. and Google Inc.'s www.YouTube.com.

"It's been an interesting ride," said Ikezoye, Audible Magic's 49-year-old chief executive. "We are kind of in the middle of everything, where we are part mediator and part battering ram."

Audible Magic's system scans online files for copyrighted material, checking against a vast database of audio and video content provided by recording, movie and TV studios. After analyzing the digital fingerprints, Audible Magic determines whether the material has been authorized to be shared on a site like YouTube.

While several other startups are developing their own weapons to combat the unauthorized distribution of copyright music and video on the Internet, Audible Magic has emerged as the early leader of the policing pack. Privately held, Audible Magic doesn't disclose its financial results. The company employs fewer than 50 people and expects to become profitable this year, Ikezoye said. Despite its Silicon Valley roots, Audible Magic is primarily backed by a Naples, Fla., venture capital firm, Tierra Del Oro, which has invested less than $30 million in the company so far.

Although YouTube and other sites revolve around homemade videos, copyrighted content has also helped attract millions of viewers to the Web.

In January alone, 123 million people in the United States watched 7.2 billion videos online — an average of nearly two videos per viewer each day, according to comScore Inc.'s Video Metrix.

But audiences could dwindle while legal bills rise for Web sites that have been showing unauthorized video clips.

That threat crystallized this month when Viacom sued YouTube and Google for more than $1 billion in a federal complaint alleging YouTube hasn't done enough to prevent its users from posting thousands of copyright clips to the site.

News Corp. and NBC Universal recently underscored their growing impatience with the unauthorized use of their video by forming a new joint venture that will distribute their TV shows and movies on the Internet.

Those actions make it more likely that sites hosting online video will have to buy copyright-checking tools from Audible Magic or one of its rivals — a group that includes Gracenote, Advestigo, Auditude and Vobile.

Another startup still in development, Attributor, is taking things a step further by developing software that is supposed to enable copyright owners to scan the entire Internet to uncover unauthorized use of their material.