LOS ANGELES (AP) _ Four of the nation's biggest cable television companies announced today the formation of a research and development consortium that will focus on developing high definition TV and other new technologies for the cable industry.

Cable Television Laboratories Inc. is expected to have an annual operating budget of $7.5 million, the companies said.

Their announcement followed the National Cable Television Association's three-day annual convention that ended here Tuesday and an association board meeting Thursday.

''This will allow us to develop our own technology more effectively and fully, and that will be good for consumers,'' who can benefit from research to improve reliability, capacity and picture quality of cable systems, said Trygve Myhren, chairman of American Television and Communications Corp., the nation's second-largest cable company.

''We want to make sure we're doing the best job for consumers and be competitive with others,'' he said.

Joining ATC in making an initial three-year financial commitment to participate are Tele-Communications Inc., the largest U.S. cable company, No. 3 Continental Cablevision and Comcast Cable Communications, the seventh largest.

Combined, these companies operate cable systems with more than 11 million subscribers, about a quarter of all the country's cable subscribers.

Other cable companies are expected to join the effort, said Myhren. Monthly dues will be 2 cents per subscriber.

The projected annual operating budget of $7.5 million is based on anticipated contributions from cable systems serving a total of about 31 million subscribers, he said.

Cable Labs' early research will include how best to use high definition TV technologies and fiber optics, he said.

High definition TV, still in a development phase, is considered the next generation in television. HDTV delivers a clearer picture with more brilliant color on a wider screen.

Fiber optics could significantly increase capacity of cable systems and would help the cable industry compete with telephone companies if laws restricting phone company participation in the cable TV business were relaxed, as many in the cable industry fear they might be.

Unlike most of the wire now used by phone and cable companies, fiber optic cable has capacity sufficient to bring voice, data and video into the home through a single wire.

''Why can't that wire be our wire'' rather than the phone company's, Myhren said.