WE ARE PLEASED that the Supreme, Court has affirmed the three-year-old decision of the Federal Communications Commission restricting ownership of radio and television stations by newspapers. That decision rested on the presumption that the public is best served when its information comes from diverse and antagonistic sources. The presumption is sound. It lies behind the First Amendment as well as some earlier FCC decisions. But putting it into operation, after years in which it was disregarded, is not easy. The balance between theory and practicality struck by the FCC - and now upheld by the Supreme Court - seems to us about right.

The ruling means that no newspaper can now acquire a radio or television station (and no station can acquire an newspaper) in the community in which it is published. Existing newspaper-radio and newspaper-television combinations may continue, with two exceptions. In 16 small cities where the sole daily newspaper and the sole broadcast outlet have the same owner, the combination must be ended by 1980. In other places (more than 100) where similar combinations exist, a split is required only if the ownership of the combination changes.

A few newspapers and broadcast stations will go on the block as a result. But the disturbance will be minimal compared with what had been ordered by the Court of Appeals after it reviewed the FCC decision. The court had directed a breakup of all existing combinations because it believed the presumption of diverse sources of information must outweigh all other considerations. But the Supreme Court said the FCC, in drawing up exceptions, was justified in considering other factors, such as longtime and local ownership of stations and the existence of other sources of information.

When radio and television were young, and licenses for new stations were not eagerly sought, the government encouraged newspapers to get into electronic journalism. Many did, and some have come to depend upon TV revenue for their own economic health. In some cities, newspapers were the only local companies seeking licenses, and the FCC prefered local to absentee ownership. In others, newspapaers promised more news and information for viewers and listeners than did other would-be owners who regarded the new media as sources of entertainment. Thus was the goal of diverse sources of information submerged.

The FCC is right in resurrecting that goal, but it is also right in pushing toward it gradually. An abrupt shift, like that ordered by the Court of Appeals, would have created a situation in which broadcast licenses were traded or sold with little regard to any factor other than the need to meet a deadline.

In this city, there was little objection until recent years to the ownership of two major television stations by two major newspapers. Twenty-five years ago, fascination with what television was and might become outweighed concern about diverse sources of information. Television was not then a major source of news, and many other newspapers existed.

That has changed, and the links between newspapers and television stations here are disappearing. The Washington Star's tie with Channel 7 had to be broken under the FCC's rule when the company was sold. This newspaper's corporated tie with Channel 9 is in the process of being broken, although that is not required under the rule. That the Supreme Court might have required The Washington Post Company to sever its link with WTOP encouraged the swap of stations now under way, but the transaction stands on its own as good public policy. If the presumption about diverse sources of information has any validity - and we believe it does - the trade of stations willserve both Detroit and Washington. That, in turn, will be good for those who own the stations.

It is this kind of gradual move toward diverse sources that the FCC should encourage. While its rules quite properly limit the number of stations one owner can control, its principal concern should be not the size of one owner's total audience but the number of different sources of information available to a community.