WHATS ON?. RADIO A Fascinating TV Play, 'The Cube,' Makes a Hit By BEN GROSS Although the Italian playwright Pirandello has often dealt with the theme of Experiment in Television Channel 4 yesterday, was the first original TV drama to concern itself with this topic. It proved to be a fascinating offering. In fact, this script by Jim Henson and Jerry Juhl, produced and directed with consummate skill by Henson, from the standpoint of novelty was by far the most interesting item on the Sunday schedules. As such, instead of being telecast from 4:30 to 5:30 in the afternoon, it should have been presented in prime time. But most sponsors are unwilling to spend money on offerings that depart from the norm. Let's face it: commercial television is -well, commercial-and, as a rule, does not lavish cash on the experimental. Trapped "The Cube" was about a young man trapped, in an apparently doorless and windowless chamber made of glowing, translucent plastic. Why is he there' ? He does not know. But despite his apparent isolation, he is not alone. For all sorts of bizarre visitors appear. These range from a handyman and the manager of the establishment to scientists, two comedians, a seductress and a monk. They come in through mysterious entrances and leave the same way. He attempts to follow them but fails each time. Are these people real or the figments of a nightmare? Is he sane or insane? Is he alive or dead? What's the Answer? There is no clear answer. Toward the end, it seems that he may be in an institution undergoing psychiatric treatment. He is cured and on the verge of being released. But that too is an illusion, for again he finds himself illusion versus reality, the NBC drama, "The Cube," seen on Phyllis Frankie Diller Howerd isolated in his plastic cube of a cell. Yes, all of these zany proceedings, an' intellectutal guessing game, contained echoes of Pirandello and also of a Sartre play, "No Exit." But even so, as far as TV is concerned, here was a truly original venture. Vivid Acting Dick Schaal, as the young man in the cube, gave a remarkably vivid performance a firstrate actor. Others of the cast, especially Hugh Webster, as the handyman, contributed engrossing characterizations. All in all, this play, taped at the studios of CFTO-TV. in Toronto, Canada, was a dramatic highlight of the season. It -should be repeated. Nixon Coverage: Some oddities marked the three-network coverage of President Nixon's arrival in Brussels, Belgium, yesterday. First, although the color was best on NBC, it really was of poor quality there as well as on CBS and ABC. The tinted signals as used by European stations are transmitted on 625 lines hile those in the U.S. employ only 525 per inch. The result: a poor picture when seen here. For that reason, NBC ran the European signal through a converter device to improve its quality. This caused the NBC transmission here to be some seconds behind that of CBS. An ABC spokesman said that his' network also suffered a delay of 10 seconds, "but I haven't the faintest idea why." Borough Presidents: The many problems besetting our city were highlighted during last night's premiere on WPIX of its informative series, New York Closeup--The Borough Presidents. With Lee Nelson as the able host-moderator, borough heads Abe Stark of Brooklyn (Kings), Herman Badillo of the Bronx, Sidney Leviss of Queens and Robert T. O'Connor of Staten Island (Richmond) took part in a lively discussion. Because of a death in his family, Manhattan's Percy E. Sutton was unable to attend. Program Comment: David Frost presented Frankie Howerd, a British comedian, in a review on Channel 5. The latter is a very funny fellow Although "The Going Thing" on Channel 7 was primarily an auto company plug, it had some amusing contributions by Donald O'- Connor, Phyllis Diller and others. Get your Driver's License the Easy Way! LEARN TO DRIVE WITH THE U.S. AUTO CLUB & CALL FOR FREE BROCHURE 855-1200 FREE DOOR TO DOOR PICK-UP IN ALL 5 BOROUGHS 404 Jay St.. Bk. Tonight!