The broadcast biz, in need of an immediate injection of hits, has gotten just that this fall.

Thanks to a sitcom surge, some unique new dramas and the return of Simon Cowell, the Big Four have combined for year-over-year ratings gains — something that had happened only twice in the previous 10 years.

Nielsen estimates that ABC, CBS, NBC and Fox are watched on average by 12.3% of adults under 50 on a nightly basis, up from 11.9% a year ago.

Fox and CBS are the early-season leaders in demos, with Fox up more than 15% across the board, and CBS slightly improved, and remaining dominant in total viewers. ABC has held steady, while fourth-place NBC has slid further.

Overall, comedy is hotter than at any time in recent memory, and that was apparent on opening night — when Ashton Kutcher’s debut on CBS vet “Two and a Half Men” drew a whopping 28.7 million viewers.

Two months into the season, the Nielsen rankers show comedies comprising 17 of the top 30 non-sports programs among adults 18-49, including four newbies.

The genre is especially dominant at the tip-top of the charts, as half-hour laffers claim the top five 18-49 positions among entertainment series — four on CBS (“Two and a Half Men,” “The Big Bang Theory,” “How I Met Your Mother” and rookie “2 Broke Girls”) and one on ABC (“Modern Family,” perhaps the hottest of them all). This hasn’t happened since “Home Improvement,” “Seinfeld,” “Roseanne,” “Grace Under Fire” and “Coach” in 1993.

Of course, primetime has proven to be cyclical in nature, and comedy — which has historically been more popular during tough economic times — was overdue to shine. But there’s also a sense that auds, more restless than ever, are also looking for something they can watch while taking part in social media.

If the half-hour format is more conducive to this, the two-hour reality show would seem to be cutting against the grain. ABC’s two-hour “Dancing With the Stars” is down 22% year-to-year, while NBC’s “The Biggest Loser” has shed 17% of its demo audience. Another two-hour competition series, Fox’s “The X Factor,” has emerged a solid performer this fall, but came in lower than most guesstimates.

Even the hourlong drama seems to be under siege in some respects, with young auds clearly looking for something fresh.

In a fall that has seen cabler AMC’s zombie series “The Walking Dead” rival the ratings of some broadcast dramas, the only new drama to crack the top 20 is ABC’s “Once Upon a Time,” a fairy tale fantasy from some of the people behind “Lost.”

Meanwhile, the crime drama appears to have cooled — or at least auds seem to have enough faves already.

While CBS remains potent (and actually is up year-to-year) with the likes of “NCIS,” “Criminal Minds” and “Blue Bloods,” its new crime offerings, “Person of Interest” and “Unforgettable,” haven’t yet broken out in the early going this fall.

In recent years, a well-executed crime show was about the safest drama bet for the broadcast nets, but 2011 has seen failed spinoffs of “Criminal Minds” and NBC’s “Law and Order.” ABC’s “Body of Proof,” which bowed last spring, has proven to be nothing special in its second season, and the Peacock canceled cop remake “Prime Suspect” after seven low-rated episodes this fall.

From a network standpoint, Fox is in the rare position of leading the 18-49 pack even before “American Idol” hits the air, thanks largely to two shows: the Cowell-fronted reality contest “The X Factor,” which ranks 12th among non-sports series this fall and has boosted both Wednesday and Thursday nights; and Tuesday comedy “New Girl,” a top-10 performer (though it has slipped some after a hot start).

Among its other new shows, Fox’s dinosaur drama “Terra Nova” has underwhelmed, as has Sunday animated comedy “Allen Gregory.”

And the net must have some concern about the slide of third-year skein “Glee,” which is down about 25% year-to-year.

The best news for CBS is its Monday lineup, where 9 p.m. anchor “Two and a Half Men” is up more than 30% year to year, even if you throw out Kutcher’s debut, and 8 p.m. stalwart “How I Met Your Mother” is up 24%. These growth spurts have helped the hot newcomer airing between them at 8:30, “2 Broke Girls,” to break out as the season’s top new show.

The Eye’s midweek vets are looking really good, including “NCIS” (the season’s No. 1 drama in 25-54) and “The Big Bang Theory,” which is up 15% year-to-year (perhaps due to increased exposure in syndication) and is the fall’s No. 3 series in 18-49.

Two CBS hours that could use improvment are Fridays at 8, where “A Gifted Man” hasn’t done much, and Sundays at 9, where “The Good Wife” has performed modestly on its new night.

In addition to “Once Upon a Time,” ABC has been boosted by three other new shows, including comedies “Last Man Standing” and “Suburgatory.” The latter has fit in nicely between returning laffers “The Middle” (up 17% vs. last year) and “Modern Family” (up 20% and a close second behind “Two and a Half Men” among entertainment series this fall).

And after struggling for years to find something that would work Wednesdays at 10 (think “Cashmere Mafia,” “Dirty Sexy Money” and “The Whole Truth”), the net has hit on something with “Revenge,” a modernized tweak on the soap format that hasn’t really thrived on the Big Three in about 20 years; the show is among the most buzzed about newbies of the fall, and has been winning or challenging for its timeslot’s lead every week.

The ABC misfire of the fall was its reboot of “Charlie’s Angels,” while new drama “Pan Am” is doing OK but hasn’t taken flight like the net would have liked.

Unfortunately for NBC, the less said about its performance this fall, the better. Its “Sunday Night Football” is the season’s No. 1 show, up a bit vs. last year, but the only Peacock program among the top 40 non-sports shows this fall is “The Office,” down 17% without Steve Carell.

“The Playboy Club” was axed after three airings, and the net’s biggest sparks among new shows have come from Friday drama “Grimm” and Wednesday comedy “Up All Night,” with the latter getting a midseason timeslot switch to the 9:30 p.m. post-“Office” position.

Still, even with a Super Bowl boost come February, the net will have to work some midseason magic to avoid another fourth-place finish. n Fall TV season at a glance

Adults 18-49, excluding sports Top new shows

Rating/share1.2 Broke Girls (CBS)5.5/142.New Girl (Fox)5.0/133.Once Upon a Time (ABC)4.5/114.The X Factor (Fox)4.4/125.Suburgatory (ABC)3.7/10 Top returning shows

Rating/share1.Two and a Half Men (CBS)7.6/182.Modern Family (ABC)7.3/193.The Big Bang Theory (CBS)6.0/184.How I Met Your Mother (CBS)5.1/145.Grey’s Anatomy (ABC)5.0/13 Number of top-30 shows by network

CBS11ABC10FOX8NBC1 * Includes “Glee” and

“Desperate Housewives” – (Sept. 19 – Nov. 13, 2011) Source: Nielsen