BOSTON — BEGINNING this fall, an estimated 40 percent of United States television viewers may suddenly have trouble finding ``Seinfeld,'' ``NYPD Blue,'' ``Northern Exposure,'' and other favorite network shows.
A loosening of federal regulations and increased competition is spurring the biggest shake-up in television history, and ABC, CBS, NBC, and Fox will switch affiliates in 15 cities this fall.
Observers say the affiliate shuffling reflects the renewed strength of networks and structural changes in the television industry that could make network television dramatically better, or dramatically worse, in the future.
``There are a lot of different directions it can go in,'' says Matthew McAllister, a communications professor at Virginia Polytechnic Institute and State University in Blacksburg. ``Personally, I don't see anything good coming out of it.''
In a long-awaited April ruling, the Federal Communications Commission eased regulations that kept the networks from monopolizing the airwaves. Networks, which were barred from large-scale program production in the past, are now free to enter the lucrative fields of production and syndication.
The rules increase the likelihood that film studios, which produce most network programming, will merge with a network or launch their own networks. Ob-servers cite the failed CBS-QVC merger and rumors of a CBS-Walt Disney Co. or Turner Broadcasting merger as examples.
Networks are also being bolstered by a slowdown in the growth of cable television, a technology that observers once predicted would put the networks out of business. After steadily losing viewers to cable since the early 1980s, network ratings appear to have stabilized at roughly 60 percent of the TV audience in 1993.
Networks are the only way for advertisers to reach mass audiences, and network executives predict viewers will be attracted to familiar networks in a future 500-channel television universe.
New network ventures are rapidly being announced. Last October, Paramount, the maker of ``Cheers,'' and Chris-Craft Broadcasting announced the launch of the Paramount Network in January 1995. Last November, Warner Brothers, the maker of ``Murphy Brown,'' and Tribune Broadcasting announced that the WB network will premiere this fall.
``For somebody like Warner Brothers, they're already in the business of producing television shows,'' says Eric Rothenbuhler, a communications professor at the University of Iowa in Des Moines. ``Now they can get into the business of delivering those goods and making money from that.''
THE increasing number of networks has led to a bidding war for local affiliates and the current affiliate shake-up. Rupert Murdoch's Fox Broadcasting Company, established in 1987, stunned the traditional networks in May by announcing a new $500 million alliance with New World Communications and its 12 local VHF television stations. Fox picked up affiliates from CBS in Dallas; Detroit; Atlanta; Cleveland; Tampa, Fla.; Phoenix; Milwaukee; and Austin, Texas; from ABC in St. Louis; Greensboro, N.C.; and Birmingham, Ala.; and from NBC in Kansas City, Mo.
Higher-powered, lower-numbered (channels 2-13), VHF (very-high-frequency) stations can be received by more potential viewers than lower-powered, higher-numbered UHF (ultra-high-frequency) stations. FCC regulations allow only three VHF stations in many cities, meaning that one of the four major networks will lose out on a valuable VHF affiliate.
The Fox deal forced ABC, CBS, and NBC to find new affiliates. In June, ABC announced a 10-year deal with Scripps-Howard Broadcasting, bringing it NBC's VHF affiliate in Baltimore, former Fox UHF affiliates in Phoenix and Tampa, and locking up its Detroit and Cleveland affiliates.
Earlier this month, CBS announced a 10-year agreement with Group W Communications, bringing it five VHF affiliates. NBC will lose its Boston and Philadelphia affiliates; a former ABC affiliate in Baltimore was picked up; and CBS's San Francisco and Pittsburgh affiliates switched over.
Amid all the changes, some observers worry that large, corporate networks directly controlling nearly all aspects of television will stifle creativity and independent producers. But other observers say the changes will increase competition and improve quality.
The networks ``are just looking for the best product'' from any source, says David Waterman, a communications professor at the University of Indiana in Bloomington. ``The overriding concern for them is getting programs they think can sell.''