More than a dozen startup broadcast networks continue to vie for carriage on TV stations' subchannels, offering broadcasters a way to leverage their digital signals and generate a new source of revenue.

But the business has not been easy, with some of the so-called diginets falling by the wayside and others struggling to get enough coverage and audience to produce measurable ratings and attract advertising.

It may be years before any of these networks and their affiliates turn a profit, say diginet executives, station general managers and analysts.

"Viewer response has been very, very favorable," says Joanne Nasby, GM of KYMA, Sunbelt Communications' NBC affiliate in Yuma, Ariz., which carries MGM and Weigel Broadcasting's classic movie channel This TV on its multicast channel. "Advertisers have been a little slower coming on board. It's a little difficult because we don't have ratings."

She says it will likely take a few years for ad revenue just to offset the cost for the equipment added to broadcast multicast channels.

The weak economy isn't helping, of course.

"As far as being a revenue generator, it's been slow," says Charles Williams, GM of KAIL, the MNT affiliate in Fresno-Visalia, Calif., which carries Luken Communications' RTV on its multicast channel. "In our market, I don't think anyone's selling them. We're having enough problems with the primary channel."

For broadcasters, adding a diginet involves more than flipping switch.

"There are startup costs associated with anything like this," says Steve Ridge, president of the media strategy group at Frank N. Magid Associates. "When you do financial modeling around it, you have to look out several years before this becomes a viable, standalone business."

Although overshadowed of late by mobile DTV, multicasting is one of the great promises of digital broadcasting, which was fully implemented just last June after a long transition.

In analog, stations could broadcast just one channel of SD programming. But in digital, they can broadcast multiple channels, typically their main service in HD and secondary ones in SD. They can also add a mobile DTV channel or two. Just how many ancillary channels they broadcast depends on how much video quality they want to trade off.

Some stations have used their multicasting capability to add CW or MNT or to experiment with local programming services, often news or weather offerings. But others have been persuaded to try one of the nationally distributed diginets.

Mark Fratrik, VP of BIA Advisory Services, estimates that the 1,700 full-power TV stations are now airing around 1,400 multicast channels of some sort.

The commercial diginets have much in common, generally offering a split of the ad inventory (often 50-50). They have started with plenty of direct-response advertising, but have ambitions to sell more lucrative, conventional spots.

They also usually require affiliates to negotiate for local cable carriage since most viewers get their broadcast signal via cable and the law does not require cable systems to carry multicast channels.

"Cable carriage is important," says Fratrik. "A lot of people don't have antennas, so the only real way to get it is through cable carriage."

Some diginets are hybrids, reaching audiences directly via cable networks in some markets and via TV stations in others, but most depend solely on TV stations just as CBS does.

Some of the programming tends toward underserved markets.

Universal Sports and Olympusat's Untamed Sports air sports that aren't often seen on major networks, including those associated with the Olympics.

Several diginets target Spanish-speaking or bilingual Hispanics, including Liberman Broadcasting's Estrella TV, LATV Networks' LATV, public TV's V-me, Spanish Broadcasting System's Mega TV and Castalia Communications' Mexicanal.

"As far as the programming, we try to complement what already exists, which is primarily Univision and Telemundo," says Luis Torres-Bohl, president of Castalia, whose Mexicanal targets Mexican-Americans.

Ion Media Network's Qubo veers toward wholesome and educational. Create TV, a noncommercial entry from American Public Television, WNET.org and WGBH Boston, offers help with cooking and arts and crafts.

Ion Life and Disney-ABC's Live Well, which is carried as a multicast channel by 10 ABC O&Os, focus on healthy lifestyles.

Of the general interest diginets, This TV seems to have gotten the most traction so far.

The joint venture of MGM and Weigel, which launched in November 2008, is now carried on 110 stations in markets covering 80% of the country.

The network airs classic movies from MGM's vault and old TV shows like Stargate SG-1 and Cagney & Lacey and makes accommodation for affiliates to air local content.

"This is a national network," says John Bryan, EVP of broadcast strategy at MGM. "But we try to localize it, which is why we call it This | More …