It's said that time speeds up as we get older. The same is now apparently true for old TV shows.

Actress Courtney Cox, who starred in the series "Friends," noticed it while watching a rerun of one her old shows.

"My voice sounds so different because I think they speed it up a little bit, they speed it up a few frames to get another commercial in there," Cox told Conan O'Brien.

She's right. With ad growth slowing and ratings slipping at some cable channels, some broadcasters are shortening their shows to have room for more ads.

A viewer noticed it while watching "Seinfeld" and posted to YouTube a side-by side comparison of the same episode played on different channels. One version is sped up, so it's 15 seconds shorter than the original.

John Pellicano, CEO of Duplication Services, speeds up shows for some cable and broadcast networks.

"We can take a 30 minute 'I Love Lucy' show and make it 28 minutes without editing out any of the content at all," Pellicano says.

With a 30-second ad for the top cable channels selling for more than $17,000, adding two minutes of commercials can add another $68,000 in revenue.

"If there's a lot of movement in the program, you'll see stuttering, or stepping, people's voices speed up, so there are limitations," Pellicano says.

Pellicano showed how it works with an old story of mine. Sped up by seven percent, you barely notice the difference -- and a 22-second clip is shortened by a full second.

Of course, running more ads may drive more viewers away.