Iran's revolutionaries proved again last night that they have become masters of manipulating the best of 20th century technology to spread their message of 7th century Islam.

Their revolution has been fed by tape cassette, by direct-dial telephone systems and increasingly by the omnivorous demands of American television, which tonight broadcast an interview with Marine Cpl. William Gallegos, one of the 50 U.S. hostages inside the American Embassy here.

Excerpts of the interview appeared on NBC's evening news and a half-hour version scheduled for 9 p.m. was screened after a delay that the network said was the result of technical difficulties.

The militants holding the hostages have played off three American networks against each other from the beginning. NBC proposed the interview with the hostage, and quickly agreed to the counterdemand of the Iranian hostage-takers that they be allowed to argue to American viewers their case for Islamic fundamentalism, and for the return of the deposed shah to face trial.

The students insisted that an Iranian cameraman film the proceedings and arranged the satellite transmission so that the film could not be edited in New York. NBC executives said that although Iranian representatives were present when the tape was edited in Tehran, the Iranians had no role in editing.

A guard was stationed outside the Tehran transmission station to make sure the students' message went on the air without tampering from political foes.

NBC interviewers were reportedly ordered by the students not to ask the hostage political questions and not to ask about "security" matters.

The students' message was dull television by the standards of American networks.

But for the students, the broadcast represented their major effort to foreswear the more inflammatory language they have been using and to sell the line that the American public should force Washington to extradite the shah.

Children of Marshall McLuhan's global village, they have little time for those journalists who remember what the students and government officials said two weeks, five days, or three hours earlier and want them to justify transparent contradictions.

Moreover, their control is complete. Television coverage by satellite can be stopped by the flick of a switch.

These latter-day messengers of a higher truth have found a ready and willing television world with which to deal.

One British television company brought in three British Moslems to smooth their way and produce a sympathetic view of the students' case.

Other journalists have described themselves as anti-imperialists to achieve the access they sought.

This is impressive homage for a revolutionary movement that has achieved much of its success due to modern technology paradoxically left in place by the much reviled Shah Mohammad Reza Phalavi.

From January on, the direct-dial telephone, functioning for both domestic and international calls, allowed revolutionaries to communicate with each other without fear of effective secret police eavesdropping.

As soon as Ayatollah Ruhollah Khomeini left Iraqi exile for France in October 1978, his daily comments on events were recorded on cassettes. They were then phoned to the faithful of Iran thanks to the automatic dial system.

Once here, cassette copies were made and widely distributed throughout the country.

The media has been of crucial importance to this revolution. Within weeks of the revolution's triumph in February, the radio and television monopoly started censoring local news.

Azerbaijani rebels seized the radio and television stations in their capital of Tabriz to disseminate the communiques of their leader, Ayatollah Kazem Shariatmadari, who had been kept off the air recently by the central government.

The hard-pressed revolutionaries have only recently carried their uses of television to artful heights after seizing the embassy on Nov. 4

Intent on controlling every step of the television process, the student captors now have three cameras and a microwave dish inside the embassy compound so that the film can be transmitted directly to the main television station by satellite instead of being driven across town through Tehran's endless traffic jams.

The captors also keep up with the product of their labors. CBS has been in the doghouse of late, apparently because of a Carl Rowan commentary that touched some raw Iranian nerves.

With tens of thousands of Iranians living in the United States, the students receive nearly instantaneous telephone reports on how the three networks play the day's events.

Rightly or wrongly, Iranians think the embassy seizure has provoked more concentrated television coverage -- with the networks each maintaining five or six correspondents and crews -- than did the turmoil of the previous two years, during the fall of the monarchy and the revolution's early months.

The students' sophistication is evident. The other night, for example, they put on a professional performance in producing a faked Belgian passport allegedly used by a member of the U.S. embassy.

The faked passport was shown next to a real Belgian passport and the cemera showed the different stamps and how they were used.