A partnership of Zenith Electronics Corp. and American Telephone and Telegraph Co. yesterday became the latest contender in the high-definition television sweepstakes to propose an advanced technology that would leapfrog systems in Japan and Europe.

The partnership became the second U.S. team within a month to opt for an "all-digital" system, a technology that would turn a television set into a special-purpose computer that could potentially share information with other types of equipment, including videocassette recorders, medical devices or cameras. With a digital system, consumers might also store a segment of their favorite TV show and change the colors or details of that picture later.

As recently as a year ago, many researchers dismissed the notion of digital HDTV, saying they had little hope of overcoming soon the technical hurdles to transmitting the enormous amount of information needed for the crisp, wide images promised by HDTV.

The AT&T-Zenith venture is one of six competitors submitting proposals to the Federal Communications Commission, which will set the U.S. standard for advanced television. Three of the six have now proposed digital over analog systems, an older technology that limits the ways consumers can use their television sets. One of the analog systems is not considered full HDTV.

In Europe and in Japan, HDTV systems are expected to be analog, at least for the foreseeable future.

"They have anchored themselves in the past," said Dale Cripps, publisher of the HDTV Newsletter in Portland, Ore. Cripps cautioned that much research remains before digital HDTV is a proven technology, but, he said, "Anything that offers more in processing at a lower cost is certainly a leapfrog."

Digital technology "is the wave of the future," said D. Joseph Donahue, senior vice president for Thomson Consumer Electronics Inc.

With a digital TV, consumers would be able to store and manipulate images, for example, and enjoy compact disc-quality audio. "You can take a {broadcast} HDTV picture of Maui and take a digital camcorder and put yourself on the beach. It's all done with digital processing," said Ralph Cerbone, advanced-television director for AT&T Microelectronics.

AT&T officials said they had made breakthroughs within the past year that allow them to squeeze an HDTV picture into the limited slice of broadcast spectrum provided by the FCC. Digital HDTV images are formed from 1 billion bits of data switching on or off each second. But to fit all that into a single TV channel, AT&T said it needed to compress the data by a factor of about 50 to 1.

The trick to compression is to transmit the minimum amount of data needed to form a picture. For example, the HDTV transmitter would analyze parts of a picture that don't change from frame to frame -- such as the background behind a newscaster. It would simply send an instruction to the home TV set, where the signal is decompressed at blinding speeds, that essentially says: "Keep the same color on the upper left and right."

Similarly, formulas have been developed to predict motion. If a car were moving across the screen, the system would anticipate where it would be next and could avoid sending the complete picture over and over. In addition, compression takes advantage of the frailties of the human eye in order to avoid sending details of color, or texture or motion that would be imperceptible.

The FCC intends to begin testing the HDTV systems next spring and to select one by mid-1993. Zenith, which is the official applicant to the FCC, said that, if chosen, it would expect to be able to sell HDTV sets by 1994 costing about $700 more than traditional sets. Traditional television signals would continue to be broadcast to homes.