Fairfax County awarded its cable television franchise yesterday to a Media General Inc. subsidiary that has promised to build an $85 million system offering 63 TV channels for as little as $2.95 per month.

Officials of the company, an affiliate of a Richmond-based communication firm, said they will begin work immediately. Some Fairfax residents should have cable service by next summer, they said, and county- wide service should be completed within three to four years.

The county board voted 8-to-1 for Media General Cable of Fairfax County Inc. against its only competitor, Fairfax Telecommunications Co., ignoring scores of local investors in the losing company, many of whom wield considerable political clout. Fairfax Telecommunications officials, who had threatened to sue the county if Media General won, declined to comment on the vote.

The supervisors showed surprising unity after almost four years of fractious debate and intensive lobbying over the franchise. The award, which a county consultant estimated could be worth $260 million in 15 years, was marked in recent months by bitter charges by Fairfax Telecommunications and an expensive advertising campaign by Media General. The maneuvering was capped late Sunday night by a last-minute telegram appeal from the losing applicant's parent company to board Chairman John F. Herrity.

In the end, only supervisor Marie Travesky of Springfield voted against Media General. The other supervisors said they favored the firm because of its willingness to guarantee construction of the system, the financial strength of the parent company and the lower prices in its proposal. Several also said they believed Fairfax Telecommunications' aggressive lobbying and stock sales to dozens of politicians and supervisors' friends may have backfired.

"I think their strategy was not good on political lines, especially for a board that wanted to divorce itself from the kind of political pressures that have existed in other areas, like Prince George's County," said Supervisor Joseph Alexander. The all-Democratic County Council there awarded a franchise for half the county to a firm headed by former county executive Winfield Kelly, a Democrat, raising charges of cronyism.

Fairfax now joins Alexandria and Arlington, where cable television is already available, and Prince George's, where construction is under way, in approving cable systems. Montgomery County is scheduled to award a franchise later this summer, while the District has not yet solicited proposals from cable firms.

"Cable will bring us benefits that we can only imagine today, technological advancements and information systems sophisticated beyond what I can either describe or fully understand," Herrity said.

Media General has promised to offer up to 126 television channels to subscribers, eventually including two-way cable for shopping and banking services and public access stations for telecasting civic association meetings, Little League games and anything else of interest to Fairfax's 600,000 residents. Media General vice president David L. Jordan said yesterday the local cable operation will spend $85 million in the first five years, creating 300 jobs, and $160 million during the 15-year life of the franchise.

In choosing Media General, which owns 90 percent of the cable affiliate, the board chose a firm with a reputation for conservative politics and fiscal management, but with little cable experience. Media General owns daily newspapers in Richmond, Tampa and Winston-Salem, N.C., several television stations and a small cable system that serves the Fredericksburg, Va., area.

Several supervisors said they were impressed with Media General's proposed programming. In addition to the basic $2.95 per month service, the system will offer two entertainment channels, a children's programming channel developed by the Walt Disney Studios and Front Row movie channel, for an extra $7.95 each.

For $8.95 per month, subscribers can receive 93 channels, including the sports network ESPN, with access to Home Box Office and other pay channels, also costing an additional $7.95 per month. For 96 channels, three pay services and a program guide, subscribers would pay a total of $36.75 a month.

Media General is bound under yesterday's award to maintain those rates for at least four years, after which the county would have to decide whether to continue regulating prices. Pay services such as Home Box Office cannot be regulated, county officials said.

According to Media General's application, communities scheduled to receive cable within the next two years include: Baileys Crossroads, Franconia, West Springfield, Falls Church and Merrifield, parts of Hybla Valley, Groveton and Fort Hunt, Greenbriar, Clifton, Herndon, Vienna, McLean and parts of Lorton and Centreville.

Fairfax City chose not to join the county franchise and has not yet solicited proposals on its own.

Media General also must contend with an ensconced cable company in the planned community of Reston, where Warner Amex Cable Communications Inc. already offers service under an agreement with Reston's developer. Although Fairfax officials have said Warner Amex has no right to operate without a franchise, Media General officials said they will try to negotiate purchase of the Reston system rather than decide the issue in court.

A more serious legal question may concern the actual cable award. Fairfax Telecommunications has accused Media General of illegally amending and improving its proposal after a Feb. 18 deadline. Fairfax state Sen. Adelard L. Brault, a lawyer for Fairfax Telecommunications, called one such amendment "legally indefensible and certainly morally unfair" in a letter to the county last week.

More than 215 shareholders in Fairfax Telecommunications invested a total of about $870,000, which they stand to lose with the board's decision. The company's communications law firm, headed by Washington cable expert Lee G. Lovett, also loses a $75,000 bonus it would have received from Fairfax Telecommunications and a bonus of about $1 million it would have received from Telecommunications Inc. of Denver if FTC had won.