For 20 years on Saturday afternoons, NBC Sports has brought major league baseball into our living rooms. And for just as long, many baseball fans have flicked on their television sets, sat back comfortably and screamed, "What? The Yankees are on again?"

As NBC likes to say, the traditions continue and the memories are waiting. What much of America wants to know is why all those traditions and memories seem to originate out of New York and Los Angeles.

According to Rich Hussey, NBC Sports' director of program planning, the network's research indicates there are three "national" teams -- the New York Yankees, Los Angeles Dodgers and Atlanta Braves (largely because of Ted Turner's superstation). "And the Cubs are knocking on the door," Hussey said.

Those teams, and the New York Mets, Detroit Tigers, California Angels and Chicago White Sox, will appear eight times each -- the maximum allowed by the network's agreement with major league baseball -- this season. The Baltimore Orioles will appear five times.

NBC's scheduling is tricky business. For one thing, its schedule for 26 Saturdays (including four doubleheaders) and two Friday nights had to be completed by Nov. 1. (The last two weeks of the season are "wild-card games," which NBC will select then.) In addition, some teams, such as Baltimore, have been reluctant to switch night games to earlier starts.

In the past, NBC had to black out games in both the home and visiting team's cities. Now, it can have the blackout lifted four times a season in both, a crucial ratings factor when it is televising a game involving teams from such markets as New York, Los Angeles or Chicago.

After NBC's baseball ratings peaked at 7.6 in 1978, they began a steady decline, dipping to 6.3 in 1981 and 1982 and to 5.8 in 1983. (The numbers represent the percentage of television homes tuning in.) But last year, the ratings jumped to 6.4, helped in part because there was no local televised baseball allowed between 1 p.m. and 4 p.m. (EDT) Saturdays under the terms of NBC's new six-year agreement.

When NBC started its new season yesterday -- Detroit at Kansas City was seen in most of the country and San Diego at Atlanta in the rest -- it included a new approach for the network. There used to be a national game and a backup game, which was seen in the blacked-out cities only. Not any more.

"Because we're now satellite-delivered," Hussey said, "you can carve up the country any way you want. So now we can choose which regions of the country we want each game to go to."

The technology benefits the viewer in more ways than one. Now we have a better chance of getting the entertaining "backup team" of Bob Costas and Tony Kubek instead of the No. 1 team of Vin Scully and Joe Garagiola. Scully is a pleasure, but Garagiola is a pain.

"Never in the history of network sports has one network had the depth of announcers that we have," said NBC Sports Executive Producer Michael Weisman, sending an unprovoked salvo toward the CBS and ABC camps. "You used to dread a rainout because you'd have to go to Ron Luciano."

NBC took a large step forward with its "less-is-more" approach during last year's World Series, a philosophy that should carry over to its 1985 telecasts.

"Although we have a reputation for being the best at baseball, we said, 'Let us start from the beginning and discuss it all,' " Weisman said. "Our producers had started to make the mistake of putting more elements in, and what bothered me with our regular-season baseball is that we got away from the story line.

"We made it simpler in the Series, with less full-paneled graphics, where you had to be Evelyn Wood to read all of it."

With less talk and graphics, the bottom line was a more restrained, reasonable presentation. Weisman now can say: "As a matter of practice, in moments of greatness, our guys shut up."

Which brings us to a brief footnote about ABC, the louder competition that will televise the World Series this year: the network's scant regular-season package of eight Monday night and three Sunday afternoon games does not begin until June 3. Al Michaels and Jim Palmer provide a capable No. 1 team (perhaps joined by Howard Cosell); Don Drysdale and Tim McCarver are the backup team.

Things We Saw Last Weekend That We're Asking the Networks To Never Do Again Dept.:

* On CBS' "Sports Saturday," its "News Update" at the program's end with Pat O'Brien contained only a promo/preview of the network's Masters golf coverage. The only news reported was that Ben Crenshaw won the 1984 Masters, a fact most people had known for nearly a year.

* ABC's attempt to follow the Olympic boxers is proving the foolishness of televising a fighter's first few professional bouts. While the network showed gold medalist Mark Breland in a lackluster affair against club fighter Steve Little, CBS was showing an engaging lightweight duel between Harry Arroyo and Jimmy Paul.

* NBC, striking a blow for the long-dormant trash sports movement, presented the Pizza Hut All-Star Softball Game featuring top major league players. This was the third recent broadcasting black eye for the temporarily misguided Costas, who also must live with the tragedies of hosting pro wrestling and sports music video shows in the same year.