If you're a Red Sox fan, you hear Joe Castiglione's familiar voice. If you're a Yankees fan, you're listening to John Sterling's unmistakable tone.

But amid the balls and strikes, the selling is intense. Before the Red Sox lineup is announced, you're advised to buy a lobster sandwich. After one pitch in a Yankees game, you're told about high-speed Internet service.

On and on it goes, every day.

"It's hawking, vending, prostitution," said Curt Smith, baseball historian and author of "Voices of Summer: Ranking Baseball's 101 All-Time Best Announcers." "It doesn't fit. It's terribly forced. ... To me, it's just obscene."

For Smith and like-minded baseball purists, the proliferation of in-game advertising is a leap from radio calls of the past. The slow pace of a baseball game has always allowed announcers to drop sponsors into their play-by-play — longtime Yankees voice Mel Allen called home runs a "Ballantine Blast" or a "White Owl Wallop."

But advertising during games has become more deliberate. Sponsors attach their name to the daily minutiae of baseball, whether it's a pitching change, the reading of the lineup or the reporting of the temperature.

More noticeable are the advertisements dropped between pitches. Clients fear the fleeing of listeners between innings, so it's like product placement when all ears are focused on the radio.

"The trend has really been there because that's when you have a captive audience," said Maury Brown, a sports business analyst and president of the Business of Sports Network. "The one time people are going to flip the dial is between innings. So when the game is going on is when you have the most captive audience."

While legendary broadcasters such as Allen and Curt Gowdy weaved ads into their play-by-play, in-game ads arrived in the 1970s. When WMEX in Boston paid a then-lofty price of $450,000 to broadcast Red Sox games after the 1975 season, the station tried to recoup the money by adding more advertising to the broadcasts.

Veteran announcers Ned Martin and Jim Woods balked, which is among the reasons the station — renamed WITS — bumped the popular team after the 1978 season.

"It just appalled them," said Smith, who knew both Martin and Woods. "They were reduced to basically becoming Monty Hall. They hated it."

Smith calls the WMEX story the precursor for what has become common. As rights fees have risen, stations have searched for ways to defray costs.

"It's been going on for years," said Castiglione, a Red Sox broadcaster since 1983. "There are a lot of high rights fees, so it's a necessity."

But Castiglione says he is not offended by the need to sell.

"Games are three hours," Castiglione said. "There's plenty of time between pitches. It's really no big deal. I don't think it gets in the way. There are enough foul balls and visits to the mound and pitching changes. The way the game is today, it's never a problem."

Dean Harris, chief marketing officer for text-based directory assistance company KGB, says a baseball radio broadcast offers unique marketing opportunities. KGB provides a service that answers questions through text messaging.

"And many of the questions are sports related," Harris said. "Baseball broadcasts specifically provide a sense of continuity. That sort of continuity of message and audience is very, very useful in building a brand. And it gives you the kind of programming that is original. It's all fresh, original programming every day."

KGB advertises on Red Sox, Yankees and Mets broadcasts. The company runs the pregame trivia questions, asking listeners to text their answers.

Brown said that sort of branding is also happening on television, but it may be more noticeable on radio. With rights fees rising — Entercom is reportedly paying $150 million over 10 years to broadcast Red Sox games through 2016, while CBS is reportedly giving the Yankees $70 million over five years to carry games through 2011 — the ads aren't going away.

It's unclear how much sponsors pay for in-game spots, but Harris describes baseball radio pricing as "fair."

"Clearly, in this economy there is a lot of pressure for clubs and for radio stations to garner as much revenue as they can," Brown said. "Advertisers and radio stations and clubs will continue to push until there is some kind of slap back by the public. Until there's some negative, huge wash of noise coming from people, we'll continue to see this trend."

All of this is bad news for Smith, a former speechwriter for President George H.W. Bush and a proponent of free-market economics.

"But this is the dark side of private enterprise," Smith said.

And while Smith is irked by the prevalence of commercials on radio broadcasts throughout the country, he is particularly disgusted by the Yankees. Smith argues that broadcasters dropping ads amid the flow of the game sends one message to listeners.

"Everything is for sale," Smith said. "It is reminiscent to Heidi Fleiss. ... The New York Yankees, arguably the most marquee team in the world, resorts to this? It's a shame."