CHANGES IN RATINGS for areas like daytime and nightly news occur at such a glacial pace that when movement does occur, a “CSI”-like search for culprits immediately begins.

So in the analysis of “NBC Nightly News” slipping behind ABC’s “World News” during the February sweeps, theories abound — from the appeal of Charles Gibson’s fatherly mien to NBC’s alleged liberal skew, a charge vigorously promoted by Fox News’ Bill O’Reilly, apparently as retaliation against nightly taunts from MSNBC’s Keith Olbermann.

Yet nobody has pointed any fingers where the blame might truly reside — namely, the rest of NBC News, and the general softening of broadcast news in general.

In the deftly orchestrated transition from Tom Brokaw to Brian Williams, “Nightly News” mostly remained a stately, straightforward recap of the day’s events. Last week, the program originated from Iraq, opening March 5 with seven minutes of reporting and discussion — a lifetime in news terms — from the war-torn country.

Widen the lens, though, and NBC has made clear at virtually every level that its news brand rests on a muddy foundation. The news division figures prominently in the net’s sweeping budget cuts. The still-expanding morning show “Today” has become giddier since Meredith Vieira’s arrival, and promises to grow more so with the addition of a fourth feature-filled hour. Finally, there’s primetime newsmag “Dateline’s” preoccupation with sexual predators — cozying up to the sting-staging group Perverted Justice to deliver visceral broadcasts that are more “Gotcha!” train wreck than journalism.

Against that backdrop, the efforts of “NBC Nightly News” to remain a serious newscast stand out like a sore thumb. And if “World News” has made inroads through “more family- and women-oriented” story selection, as news analyst Andrew Tyndall told Variety‘s Michael Learmonth, then in essence, NBC is simply being hoisted on its own soft-news petard.

NBC’s shift is part of a powerful overall tide pulling away from thoughtful, in-depth reporting, as news outlets are irresistibly drawn to those twin towers of titillation, Anna Nicole Smith and Britney Spears — a kind of crack cocaine for cable news. When John McCain recently announced his presidential bid on David Letterman’s latenight comedy show, the maneuver seemed rational, even logical. Where’s he supposed to make that announcement, Wolf Blitzer? The GOP senator had a better chance of intelligent policy discourse with Dave.

This isn’t to say that tough-minded, important work isn’t being done, only that finding it requires a level of commitment, as well as surfing to TV’s lesser-seen platforms. Consider, for example, Ted Koppel’s new “Koppel on Discovery” special for the Discovery networks premiering March 11, “Our Children’s Children’s War,” a sobering documentary reconsidering what is too-blithely referred to as “the war on terror.”

In his methodical manner, Koppel takes a step back to provide a long view of a “generational struggle” against Islamic radicalism — one that won’t be solved during this administration or the next three, with all the implications that entails, particularly for the U.S. military. Going where the story leads, the project visits Africa and Afghanistan, while conducting tough interviews with mercenaries (a term, not surprisingly, that they don’t much like) being trained to augment the U.S. war effort.

This is not reassuring, feel-good news — the kind that can be neatly close a nightly newscast, in the same way that PBS’ “Frontline” or HBO’s Oscar-nominated documentary “Iraq in Fragments,” which debuts on the channel this month, won’t send anybody into the night air whistling. The prevailing sense is rather of a lingering, complicated mess with deep cultural roots, destined to haunt U.S. foreign policy planners for years to come.

Alex Wallace, the newly anointed exec producer of “Nightly News,” has stated that the network isn’t panicking about its recent ratings setbacks. There are no plans for Draconian changes, only minor tinkering to push the broadcast back on top.

Ultimately, though, the task is bigger than that — beginning with the nettlesome question of how a network with a flagging commitment to hard news can promote and sell a serious product. In that sense, NBC appears to be discovering that whatever the commercial benefits of its association with Perverted Justice, there are unintended consequences and a price to be paid for perverting the news.

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