A federal law enacted long before the advent of cable television doesn't empower the government to order cable TV systems to set aside a channel for public access or adopt certain other rules intended to achieve "a definite societal good," the Supreme Cout ruled yesterday by 6 to 3.

The decision dealt a blow to efforts by the Federal Communications Commission, in its words, to "integrate new media into the national communications structure."

Under rules adopted by the FCC in 1976, cable TV systems with 3,500 or more subscribers were directed to dericted to develop a capacity of at least 20 channels by 1986; to designate one channel each for access by the public, educational authorities, local governments and renters; and to furnish the equipment and facilities needed for the access.

As of January 1978, more than 4,000 cable systems - including at least 500 with 20-channel capacity-were serving 12.9 million subscribers, or 17.6 percent of the homes equipped with TV sets.

Mildwest Video Corp. field a legal challenge to the rules. The 8th U.S. Circuit Court of Appeals ruled not only that the agency had gone beyong the bounds of its jurisdiction under the Federal Communications Act of 1934, but also that the rule might violate the rights of cable operators under the First Amendment to the Constitution.

The Supreme Court Affirmed solely on the basis of the law without reaching the constitutional issues. Using language from a 1968 cable TV decision, the majority, in an opinion by Justice Byron R. White, struck down the rules because they are not "reasonably ancillary to the effective performance of the commission's various responsibilities for the regulation of television broadcasting."

In New York City, the American Civil Liberties Union, which had challenged the rules as too weak, said it will ask Congress to enable the commission specifically "to provide public access on a first-come, first-served non-discriminatory basis."

The central issue in the case was whether cable TV systems are broad-casters. If they are, the law gives them editorial control of programming. But if they are common carriers, such as telephone companies, their facilities must be open to all members of the public to communicate generally what they care to.

The court held that the commission rules plainly imposed the obligations of common carriers on cable TV system, but that they are broadcasters despite their different technology.

The court took other action.

Rent Control

Starting in 1972, Fort Lee, N.J., has enacted a series of rent-control ordinances. Similar laws in effect in more than 200 communities, including the District of Columbia.

Realtors, particularly concerned about a Fort Lee limit of 2.5 percent on rent increases for the period 1974-1976, filed a constitutional challenge. They alleged that they were being deprived of their property without due process of law, even thought there was no national emergency, no emergency affecting the health, safety and welfare of residents, and, in fact, no "rational basis" for such a deprivation.

Last October, New Jersey's highest tribunal rejected the challenge. The Supreme Court let the ruling stand.

Union Label

The court preserved a rulling by the 2d U.S. Circuit Court of Appeals that upheld the constitutionality of a New York City policy of denying contracts to non-union printing firms.