IT IS NOW almost 89 years since the sudden, and still-controversial, death in St. Petersburg of Peter ilyich Tchaikovsky. He was then at the peak of his powers; the "Pathetique"Symphony had premiered three weeks earlier. He was one of the world's most popular composers, and he has continued to be ever since.

So why is it such a public service that Mstislav Rostropovich and the National Symphony are opening a two-week festival of Tchaikovsky's music on Tuesday? Won't it just be more of the same?

Part of it will be -- this week we get the Fourth Symphony, the Violin Concerto and the B-flat Piano Concerto. But next week we get a strongly cast concert version of "Iolanta," a one-act opera from Tchaikovsky's latter days that is as good as unknown here--as are many Tchaikovsky works, including some very fine ones.

Western perceptions of Tchaikovsky's work were formed by what the conductors and virtuosos of the first half of the century chose to perform. The catalogue of what was left out was vast. The opinion-makers of the time apparently felt that a work like "The Nutcracker," especially the fragments Tchaikovsky arranged into a suite, exported better than a lengthy opera sung in Russian like "Iolanta." This bias is responsible for the early misrepresentation of Tchaikovsky's work, one that con-tinues to this very day.

"Iolanta" was just one of many such neglected works. So was "Manfred," a magnificent program symphony that was largely unknown in this country 30 years ago except for an occasional performance by its one champion, Arturo Toscanini. There were also the ardent, flowing and spontaneous early symphonies, and the orchestral suites--symphonies in all but name. Notable among these is the exquisite Suite No. 3, which be-came well known finally when Balanchine choreographed it and which Rostropovich will conduct this week.

Even more astonishing was the relegation of the evening-length ballets--"Swan Lake," "The Sleeping Beauty" and "The Nutcracker"--to mere excerpts as encores and pops. Eventually, the Sadler's Wells Ballet (now the Royal Ballet) started performing them uncut after the war, both in Europe and in this country.

And the Tchaikovsky operas were hardly known here at all. "Eugene Onegin" became a Metropolitan Opera staple only recently, and "The Queen of Spades" still barely has its foot in the door there. (Last year's new production was canceled because of the Met strike.) And with the exception of several fragments that have been popular from the start as salon music, the chamber music and the songs continue to rest in obscurity in this country.

Imagine if our overview of the works of Mozart--or even of the other great Russian composer, Stravinsky--were so spotty and misleading.

As a result of these biases, there is no other musical giant (and a carping few still would challenge that value judgment) about whose music opinions remain so cautious and ambivalent.

Even so dedicated a Tchaikovsky proponent as Rostropovich--with his more balanced, Russian view of the whole Tchaikovsky corpus--is influenced by this problem.

In an interview just before the National Symphony's recent tour of Europe, Rostropovich explained why he was omitting "Manfred" from the European program, even though he had scored a triumph with it in concerts here.

"There are still some who do not know 'Manfred' is very good," Rostropovich observed with regret. "And someone in one of the European cities might decide to write that it is a poor composition. They would write about that, instead of writing about the orchestra. It's our first visit, and I don't want to take any risk for our orchestra--anything that could overshadow the performance."

Such caution about so remarkable a work, even from a Rostropovich, reflects a feeling that it will take a while for the public to get to know the less-famous Tchaikovsky. But there is no reason to believe that the public appetite for Tchaikovsky is reaching limits. Consider this statistic: In a recent edition of that Bible of the recording industry, the Schwann catalogue, no fewer than 10 Tchaikovsky works had 25 or more entries. There were 39 stereo versions of the 1812 Overture; 35 recordings of "Romeo and Juliet"; 30 of the B-flat piano concerto; 29 of the "Pathetique," and so on.

Numbers, of course, aren't everything, but the message from these massive accumulations seems unmistakable. In the same catalogue, Bach had only five such entries; Mozart and Beethoven each had only three.

Today, a revisionist view of Tchaikovsky's work, a New Tchaikovsky, is gradually taking form--not unlike the reevaluation of the works of Verdi that started in the 1930s and now seems substantially complete. Just as with Tchaikovsky, many of the finest Verdi operas that were only barely known before that--"Don Carlo," for instance--were brought into the repertory, and our appreciation of Verdi's stature was greatly broadened and deepened.

Dare one suggest that the New Tchaikovsky may be quite unlike the Old Tchaikovsky? The man once accused by his German critics, in particular, of not adhering precisely to formalistic criteria seems to be emerging as one of the great originals in all music.

There is nothing new about acclaim of his remarkable facility for rapturous melody; in this respect, he has long ranked with Mozart (his musical idol), Schubert and Chopin. Nor has there been much question about Tchaikovsky's assurance as an orchestrator; he had an extraordinary ear. But these are two strengths that, clear and bewitching as they are to the popular ear, have been underrated by the pedagogues for not matching the sacred symphonic sanctions of a Beethoven. There has not been doubt about Tchaikovsky's brilliance, but it was as if his genius were more that of a decorator than that of an architect.

Now it is becoming clear that Tchaikovsky was not so much compromising the structure of the symphony, for instance, as bending it to his expressive needs--and creating a new model for Mahler and others to follow. More than most symphonies that came before it, the "Pathetique" is a subjective program work roughly in the conventional form of a symphony. The traditional need for a rousing ending is dispensed with, and Tchaikovsky brings the work to its resolution in a slow movement of the deepest despair. The two inner movements are cast, superficially at least, in dance forms, a waltz in number two and a march in number three; but as in the Mahler symphonies to come, the force of Tchaikovsky's passionate ideas would overflow the normal expressive limits of the forms.

It is this same passionate force of his imagination that led Tchaikovsky to expand the full-length story ballet from the restrained, often bland, almost dainty idiom--as in Adam's "Giselle" music--to music drama of almost operatic magnitude.

It is true that at this point we do not know his operas well enough to start making authoritative comparisons with the ballet, but one can safely say that Odette/Odile in "Swan Lake" is no less gripping a character than Tatiana in "Eugene Onegin."

Perhaps Tchaikovsky would not be so surprised at the extent of his present-day popularity, given the fame that he enjoyed during his career. Trips abroad were an annual thing and, in fact, he briefly visited Washington in 1891, as part of a trip to the United States for the dedication of Carnegie Hall, where Rostropovich will take the Tchaikovsky festival in mid-April.

The last such trip was to Cambridge several months before his death, where he received a doctorate, along with Saint-Sae ns, Boito, Bruch and Grieg. It was noted at the time that Tchaikovsky, a tall man, towered above the others. And now, 89 years later, it is clear that that was true in more than one sense.