SAN JOSE, CALIF. -- The countdown for a portable Macintosh has begun. According to John Sculley, Apple Computer Inc.'s chief executive officer, the Cupertino personal computer maker is determined to introduce a Mac lap-top. The only question is when.

"If we could bring a product out that has the resolution and the power of our desk-top Macintoshes, there would obviously be a lot of interest," Sculley said. "The only thing that has held us up is that we won't compromise on quality. We avoided going to the low-end product. We said we'd wait until the technology is good enough to make the high-end."

Is the technology good enough now? "It's getting closer, but not close enough," Sculley said.

Despite Sculley's caution, some industry observers believe Apple, which already has completed more than a dozen prototype portables in its labs, will introduce a lap-top in mid-1988.

"There's no question that the time is ripe for Apple to bring out a lap-top," said Tim Bajarin, executive vice president at Creative Strategies Research.

Others aren't so sure. "I don't see the demand being large enough to interest Apple right now," said Stephen Hull, president of Dynamac Computer Products Inc. of Golden, Colo.

Dynamac has been selling a portable Mac for the past six months using two different flat-panel screens. Apple has given the company permission to buy Macintosh computers, rip them apart and outfit them with flat-panel screens and other components needed for a lap-top. Dynamac sells an 18-pound briefcase-size Mac with a 40-megabyte hard drive and 4 megabytes of internal memory for about $8,000.

Hull thinks that Apple will sell its own portable someday but that the biggest stumbling block is finding an adequate screen.

The Macintosh requires a higher-resolution screen than the many portables compatible with IBM's computers. PC-type portables use a liquid crystal display, or LCD, enhanced with backlighting and a technical trick that "supertwists" the molecules of the display to make characters easier to read.

"The weak link in the technology is the display," said Mark Eppley, president of Traveling Software of Bothell, Wash., which writes software for lap-top computers.

Proven approaches using liquid crystal or electroluminescence, called EL, are either too expensive or offer poor resolution.

Apple's probable choice is an active-matrix screen that offers a picture superior to liquid-crystal at a lower price than EL.