Joe Flint

When Stephen Cox was watching “The Wizard of Oz” on TBS last November, something didn’t sound quite right to him about the Munchkins, who are near and dear to his heart.

“Their voices were raised a notch,” said Mr. Cox, the author of several pop- culture books including one about the classic 1939 film. “It was astounding to me.”

He wasn’t imagining things. Time Warner Inc. ’s TBS used compression technology to speed up the movie. The purpose: stuffing in more TV commercials.

As they contend with steep ratings declines, many top cable networks are jamming more ads into programming to meet audience guarantees made to advertisers and prop up revenue despite falling ad prices.

Tinkering with shows to squeeze more advertising dollars out of them has been done before. Cable networks have long made room for ads by shortening the opening credits. Reruns of “Law & Order” on TNT have a 24-second opening, in contrast to the original 1 minute, 45-second opening when it aired on NBC.