Apple has postponed the release of the Apple TV software update by a week or two, saying it needed more time to finish the upgrade that would let people rent movies directly from their televisions.

The company also said on Wednesday it had started shipping the MacBook Air, the ultra-thin notebook introduced at Macworld in San Francisco this month. The Air, which costs $1,799, weighs about 3 pounds, has a 13.3-inch display, and is 0.16 of an inch thick at its thinnest point.

In delaying the Apple TV upgrade, the company said the software wasn't quite finished. "Apple now plans to make the free software download available to existing Apple TV customers in another week or two," the company said in a statement. Apple chief executive Steve Jobs had said during his Macworld keynote that the software would be ready by the end of January.

The software update will allow people to rent movies from Apple's iTunes online music store directly from their TVs. Apple expects to have more than 1,000 movie titles available by the end of February, including more than 100 high-definition titles.

Apple's catalog represents only a tiny portion of the movie libraries of the six major studios that have agreed to make films available on iTunes. The studios include 20th Century Fox, Warner Bros., Walt Disney, Paramount, Universal, and Sony. New films will not be available on iTunes until 30 days after they ship to retailers on DVD.

DVD-quality movies will be available to rent for $2.99 for older titles, and $3.99 for new ones. High-definition versions will cost a dollar more.