Steve Jobs, chief executive of Apple Computer Inc., is expected to launch at least one revolutionary product Tuesday at the Macworld Conference & Expo in San Francisco. Speculation has focused mainly on an Apple-branded cellular phone and a set-top box that allows people to send video from their computers to their televisions.

Although Apple spokeswoman Natalie Kerris declined comment on ''rumors and speculation,'' bloggers, enthusiasts and investors have been guessing whether Jobs will unveil a ''smart phone'' (which some are calling iPhone), iTV or both.

Apple's iPod music player wasn't the first on the market when it launched in 2001. But its sleek design and intuitive user interface made it a hit.

Industry analysts believe another well-designed product from Apple could seriously threaten major tech companies such as Motorola Inc., Nokia Corp., Samsung Electronics Co., Verizon Wireless and TiVo Inc.

Expectations for Macworld are so lofty that a failure to launch an earth-shattering product this week could dent Apple's already volatile stock price, investors say. The stock has traded in a 52-week range of $50.16 to $93.16. It closed at about $85 on Friday.

Wall Street's confidence in Apple has wavered slightly because of the possibility that improper handling of employee stock options would erase some of Apple's record profits. The scandal threatened to plunge Jobs into a legal morass, if not cost him his job.

Still, anticipation is so breathless in the blogosphere that some pundits have already credited Apple with changing the global telecommunications sector - even though the iPhone is technically still ''vaporware,'' or nonexistent.

The mere suggestion that Apple might make a simple cell phone that downloads, plays and shares digital music has intimidated competitors, said Avi Greengart, an analyst with Current Analysis.

''Even if Apple does not announce a phone, just the threat of Apple's entry could spur innovation,'' Greengart wrote in a research note Thursday. ''If Apple builds a phone that is easy and genuinely pleasurable to use, the company will have a winner no matter how it is priced or sold.''

Apple also is expected to unveil a set-top box designed to bridge computers and television sets so users can more easily watch their downloaded movies on a big screen. The move would complement Apple's entry into online movies - it began selling titles through its iTunes store in September.

As Apple launched online movies, Jobs showed off a gadget that records and plays high-definition videos, selling for $299 starting sometime in the first quarter.