Canada's Rogers Telecommunications Ltd., the largest cable television company in North America, and the Tribune Co., parent company of The Chicago Tribune, yesterday announced plans to form a joint venture to seek the Montgomery County cable franchise.

Douglas Dittrick, president of Tribune Company Cable Inc., said his firm approached Rogers about the 50-50 partnership. The Tribune Co. owns the ony cable franchise in the county, a Gaithersburg system that serves about 2,000 subscribers.

In announcing the venture at a Bethesda press conference, Dittrick said the two companies had not yet determined how much they would spend seeking the franchise award. They also would not disclose the precise services they planned to offer in a franchise proposal but stressed that the system would have a heavy emhasis on local access and two-way programming and data-retrieval services.

"We feel that Montgomery County deserves the finest cable system that can be produced," said Edward Rogers, vice chairman and chief executive of Rogers Cablesystems Inc. "It will be the most advanced system in North America.

In an interview, Rogers said he did not believe that partial Canadian ownership of American cable systems should be an issue, noting that U.S. interests control 58 percent of Canadian manufacturing. Municipalities "want the best cable system they can get," Rogers said.

At least six of the nation's top 10 cable firms and many others are expected to seek the ocunty's franchise when proposals are submitted. Industry experts say that because of the wealth and influence of county residents, the franchise will be one of the most highly sought and potentially lucrative in the country.

So far, only Times Mirror Corp., owner of The Los Angles Times and other broadcasting and publishing interests, has formally announced plans to bid for the franchise. The county has yet to issue a request for bids, although major cable companies already have set up staffs to work on seeking the franchise. Construction of the system for nearly 200,000 homes could cost $100 million.

The new joint venture has no fulltime staff to pursue the county's franchise and does not plan to recruit local stockholders. The two companies have, however, hired a District consulting firm, Shooshan & Jackson Inc., founded recently by two members of the House communications subcommittee.

Both partners in Tribune-Rogers Cablesystems of Montgomery County are large compnaies. Tribune, with revenues of $1.2 billion last year, recently bought the Chicago Cubs, owns The New York Daily News, has extensive broadcast holdings and owns seven cable systems with 80,000 subscribers.