As the number of high-definition television sets sold has risen during the past year, so has the amount of prime-time broadcast content produced in Dolby Digital 5.1, the audio counterpart to HD pictures. More than 50 prime-time shows are now in Dolby Digital 5.1, compared with 15 a year ago.

Last year broadcasters were primarily producing special events and awards shows with Dolby Digital 5.1 sound, the format for multichannel surround sound. ABC was the only broadcaster widely offering the higher sound quality, including Dolby Digital 5.1 surround sound, in all of its scripted series and movies in HD and required that all new scripted shows be produced in surround sound.

ABC has been joined by CBS, NBC and Fox, which have expanded the amount of prime-time HD content in surround sound, said Rocky Graham, manager of digital television applications at Dolby Laboratories.

NBC has added surround sound to about 17 shows, including the “Law & Order” franchise and “The West Wing,” Mr. Graham said. CBS now produces all of its “CSI” shows in surround sound, and Fox offers “24” and “American Idol,” among others, he said.

Dolby Digital 5.1 is available only with HD or digital TV, he said.

“It’s sort of a natural proliferation,” Mr. Graham said of the transition. “It’s not like you could point to one event or one particular product that came out and made it possible. It’s part of an evolution.”

After all, more cable systems are carrying the HD versions of local channels, making HD more widely available. The Consumer Electronics Association predicts that a total of 31 million digital TV sets will have been shipped to retail by the end of this year, up from a base of 16 million at the end of last year. Nearly 90 percent of those sets are HD sets.

The surround experience is “much more like being there than when you just listen to two channels,” Mr. Graham said. For instance, watching a football game in surround sound lets viewers hear the sounds from different directions, similar to being in the actual stadium, he said.