The Federal Communications Commission, under the gun to keep the United States in the race with Japan and Europe to develop the next generation of television, announced an "ambitious but realistic" test schedule yesterday to select a U.S. high-definition TV standard by mid-1993.

The FCC said five companies, including Japan Broadcasting Corp. and Europe's major TV manufacturers, will be in the running for the coveted HDTV standard, which will be the sole format to be used to provide Americans with HDTV's crystal-clear pictures and compact disc-quality sound by cable, satellite and the major commercial networks.

Backers of HDTV say it could revitalize a lagging U.S. electronics industry and -- through military, educational and medical spinoffs -- be the linchpin for U.S. economic growth in the next century.

Testing will begin April 8, 1991, at an Advanced Television Test Center in Alexandria. Testing will run for about two months for each proposal, in conjunction with Cable Labs, the cable industry's research laboratories. Viewer-testing of the proposals will follow in Canada, along with field testing in Washington.

The FCC's Advisory Committee on Advanced Television Service is to recommend a winning format to the FCC by September 1992. The commission is scheduled to announce a standard by June 1993.

To prevent making millions of TVs obsolete, the commission already has said the winning HDTV system must use a "simulcast" technique in which a TV station will broadcast two signals -- one on its regularly assigned channel in today's format, known as NTSC, the other in high-definition on a separate channel.

Viewers would have to buy new, and probably costly, TV sets to watch HDTV programs, which could become available by about 1995. The commission also said it will test a sixth proposal, a partial HDTV system being proposed by a David Sarnoff Research Center consortium that includes Thomson Consumer Electronics and NBC.

Testing originally was to have begun this year, but the schedule slipped because of changing proposals from nearly a dozen companies that had expressed interest in competing and unexpected difficulties in developing proper testing equipment.

FCC Chairman Alfred Sikes said the test schedule was "ambitious but realistic."

Responding to assertions that the United States lagged far behind other nations in developing HDTV, which could be a multibillion-dollar industry by the year 2000, Sikes and other officials emphasized the extraordinary difficulties in building the test equipment and in balancing the needs of the competing applicants.

On Tuesday, one of the applicants had announced it was radically changing the system it would offer for testing, going from traditional "analog" technology to the computer-precise technique of digital electronics.

Five of the six applicants are proposing to go completely or partially digital -- a move that could give the United States major advantages over systems being developed elsewhere.

The six applicants are the David Sarnoff Research Center; Japan Broadcasting Corp.; General Instrument Corp.; Zenith Electronics Corp.; N.A. Philips Consumer Electronics Co.; and the Massachusetts Institute of Technology.

Sikes said other applicants could be considered in the testing if they come up with different technology "that appears to offer important benefits to the public."

But he said the United States cannot afford to wait too long to launch HDTV. Already, HDTV is being delivered an hour a day to Japanese viewers on sets costing $17,800 and up, but only by satellite. Full-time broadcasting is expected to begin in Japan by 1992, and Europeans are expected to be able to watch the 1992 Olympics on a European HDTV system.

Although Congress has kicked in some money for U.S. HDTV research through the Defense Department and other agencies, the American electronics industry is basically going it alone against foreign competitors that are being heavily subsidized by their governments.