The nation's largest Spanish-language broadcaster yesterday sued to stop Nielsen Media Research Inc. from rolling out a controversial television ratings technology in Los Angeles that opponents say undercounts minority viewers.

Univision Communications Inc. asked the California Superior Court for an injunction preventing Nielsen from launching its "local people meter" ratings devices, which the company introduced in New York last week with plans to expand to Los Angeles, the nation's largest Spanish-speaking media market, on July 8.

Nielsen is trying to switch from a diary-based ratings system -- which requires viewers to remember and write down what they watch on television -- to the people-meter system, which automatically keeps track of what appears on viewers' screens.

"The sample and the weighting technique currently employed in Nielsen's proposed Los Angeles [local people-meter] system -- just as in its New York [local people-meter] system -- will result in a seriously flawed measuring tool and, as a result, Hispanic media and the Hispanic community in general will be unfairly prejudiced," said Ray Rodriguez, president of the Univision television networks, in a statement.

Univision owns more than 35 television stations, Galavision cable network, 66 radio stations and music and online groups. Its programs reach 98 percent of Spanish-speaking U.S. households. When Nielsen tested the people meter on Univision's KMEX, the top-rated Spanish-speaking station in Los Angeles, the station experienced "dramatic declines" in ratings, according to Univision's suit.

In side-by-side Nielsen tests pitting the people meters against the diaries in New York, some broadcast programs featuring predominantly black casts -- such as "Girlfriends," "Eve" and "The Parkers" -- experienced large one-month ratings drops when compared with ratings they had received based on diaries.

Nielsen, which contends the new system is much more accurate, said it would fight the lawsuit.

"We stand firmly behind our Los Angeles sample and our proven methodology," the company said in a statement yesterday. "People meters do a better job of representing what people are watching on TV and they in no way prejudice any viewer group."

The people meter is designed to record what channels are being watched at any time. It allows individual family members to identify who was watching a particular show by punching a button on a remote control. Nielsen already uses the system to tally national network viewing habits. It wants to extend the system to determine how shows fare in local markets.

Nationally, people meters are used to show quick, overnight ratings of shows, but generally not to determine ad rates. The local people meters would replace the diaries as the sole tool for determining a show's popularity and therefore its value to advertisers. The company introduced local people meters in Boston in 2002 and has been using them nationally since 1987.

In response to concerns about minority counting, Nielsen says it has met with more than 100 lawmakers and community leaders in New York and Los Angeles. Further, the company planned to kill its diaries in New York on June 3, replacing them with people meters, but decided to let its clients continue using diary ratings data when selling to advertisers until Sept. 3. After that, Nielsen plans to use only people meters in New York.

The company has put together an 11-member task force of community leaders and television executives to study minority viewing measurement.

Many of the shows that took a ratings hit in the New York people-meter test appear on UPN affiliates, many of which are owned by News Corp., including New York's WWOR and WNYW. If the shows lose ratings, their value to advertisers drops and the News Corp.-owned stations would have to reduce rates charged for ads accordingly. Lachlan Murdoch, chairman of News Corp.'s 35-station group, earlier vowed to "shut down" the people-meter project.

Nielsen -- and other ratings groups, such as Arbitron Inc. -- traditionally have had lower response rates among minority and young male viewers. To compensate, the company says that it over-counts these viewers to ensure that a suitable sample size is attained. Premiums, frequently cash payments, that Arbitron and Nielsen participants receive are increased for minority viewers to entice their cooperation.

The people meters have created rifts even within the same company. CBS and BET, both owned by Viacom Inc., have issued opposite statements on the devices. CBS opposes their rollout but BET sees the meters as a way to document increasing viewer numbers for its cable television programs.

"We have always been concerned that the paper diary system for gathering local ratings was vulnerable to substantial inaccuracies and that African American households were underrepresented in Nielsen's paper diary sampling," BET President Debra Lee said in a May statement. "Quite frankly, we don't understand why some broadcasters would oppose any method of truthfully tracking what viewers are and are not watching, regardless of their ethnic background."