It was just months ago that media buyers and sellers adopted a dramatically changed set of television ratings from Nielsen Media Research, based on commercials rather than programs and including ratings for shows played back on DVRs up to several days later.

Now yet another major overhaul of the system for rating and buying ad time on TV shows is in its early stages.

The goal: to be able to provide advertisers with second-by-second ratings that precisely measure viewing of their actual commercials when they air.

That would be a huge advance.

The best Nielsen can do under the existing system is to provide advertisers with averages for viewership of all commercials within a program, based on minute-by-minute ratings, which can also include snippets of shows. Nielsen doesn't have the technology in place to deliver second-by-second ratings.

So to develop its new rating system, Nielsen is pairing up with companies, such as Google, that have that capability. On Wednesday, Nielsen agreed to partner with Google TV Ads, an upstart online system for buying and selling TV inventory. This year, Google TV Ads began tracking viewing via set-top boxes on EchoStar Dish Network.

For Nielsen, the partnership means it can sidestep the enormous cost of overhauling its existing system and gain access to second-by-second data, which media buyers have been wanting for years.

“What Nielsen is trying to do, because their sample cannot get to the second-by-second level, is get set-top box data from other companies,” says John Spiropoulos, vice president and group research director at MediaVest.

The best Nielsen can do with its existing technology is to measure down to only 2.7 seconds. To get down to a second would mean a huge investment and might take years.

“Our meters collect data every 2.7 seconds and nationally round up to the average minute,” explains Anne Elliot, a spokesperson for Nielsen. “Theoretically, we could get to down to 2.7 seconds, but I’m not sure what type of timetable we’d have to get there. It’s not our chief priority now.”

For Google, the partnership gives it access to Nielsen’s demographic data, which it can’t currently provide its Google TV Ads subscribers.

“It’s still unclear from Google what they are doing with the data, unless it’s a step for them to start working with demographic data in addition to set-top box data,” says Spiropoulos.

Nielsen has been seeing competitors inch into its monopoly on TV ratings.

TNS Media Research has been quietly launching its own second-by-second system, including a partnership with Charter cable in Los Angeles. It’s branching out into more markets and signing up agencies that want to fill in holes in Nielsen’s system.

The DVR company TiVo this year launched its Stop||Watch system, which is tracking second-by-second viewing on some 20,000 DVRs. But the sample has serious limitations for buyers because it only reflects the viewing of people with TiVo.

The value of Nielsen’s TV sample to media buyers is that it is representative of the entire population. But for Nielsen to create a comparable sample on a digital platform with second—by-second ratings, it would need to partner with several companies like Google.

“If the industry wants to move to evaluating on post-buy to creative executions, we have to use second-by-second data because executions don’t start and end on the clock minute,” says Spiropoulos. “So you need to aggregate second-by-second viewing information.”

Big questions remain about the Google-Nielsen partnership, including where it goes after generating second-by-second demographic ratings.

One idea is that Nielsen is vying to become the ratings source for television content on the internet and other new media platforms.

Google acquired online video site YouTube last October for $1.65 billion. Moreover, the broadcast networks have recently been launching their own sites, including Hula, which NBC and Fox parent News Corp. will debut this Monday.