Cultural Onmibus. Berlin's 750th birthday bash features a chance to learn about the city's architecture.

THERE isn't much doubt about it. What Berlin is celebrating this year is Berlin. The key date will be Oct. 28. That's when the new Chamber Music Hall will open and a ``big music festival'' will commemorate the day, 750 years ago, when Berlin - or rather the twin city of C"olln-Berlin - was first mentioned in an official document.

The city is marking its 750th anniversary with a train of exhibitions, performances, concerts, outdoor sculpture, a gymnastics festival, a theater meeting, a historic fair, a regatta, a historical pageant, jugglers and jazz, magic and Japanese fireworks, to name but a few events. And this is just in West Berlin, where things will really start to roll toward the end of April. In East Berlin they've been going strong since the beginning of the year.

Berlin has been variously described during its history. Karl Scheffler said, ``It is Berlin's fate to be forever in a state of development, never resting.'' Goethe said, ``My whole heart yearned to quit this city of splendid misery and miserable splendor.'' Mark Twain called it ``the German Chicago ... the newest [city] I have ever seen.... All of Berlin is stately and substantial, and it is not merely in parts, but uniformly beautiful.''

But to the quick visitor today it does seem to be ``in parts'' and not always ``stately and substantial.'' Buildings of monumental character, both old and more recent, still accost you - though the cool, ordered neo-classical/romantic work of Karl Friedrich Schinkel, which transformed Berlin early in the 19th century, is now piecemeal. (Most of his major buildings are in the Eastern sector.) Historians observe that 18th- and 19th-century Berlin survived almost intact until the end of World War II. At that point, it suffered two devastations - by bombing and by planning.

First came the war rubble; then came the destruction wrought by insensitive policy of demolition and rebuilding, a policy dominated by the priority given to constructing an efficient transport system.

Modernism's disregard for the past is today blamed for much of the inhumanity of this ``second destruction.'' But today West Berlin is making definite efforts to turn the city into a pleasanter, more humane place to live, either by cautious rehabilitation or by building and planning that is more sensitive in scale and placement to people's daily lives and to the historical arrangement of the city.

Something of this is brought out by an exhibition now at the National Gallery, a building of modernistic and leisurely space by Mies van der Rohe (opened 1968) - an antihistorical building, if you like. The show, ``750 years of Architecture and Urban Planning in Berlin,'' is presented by ``IBA,'' the 1987 version of the ``International Building Exhibition.''

IBA's current guiding light, architect Josef Paul Kleihues, conceived the show's layout. Visitors are almost made to feel that this layout's symbolism is the show's basic message. This is further encouraged by a minimal use of verbal explanation, and that only in German. Still, the symbolism is clear.

Eight parallel galleries trace the history of the city's development through its 750 years by means of maps, panoramas, architect's drawings, models, and photographs. In fact, there isn't a lot of evidence of anything until the 17th century. From then on it is the story of European architectural styles as adapted to Berlin, with the grotesque hiatus of Hitler's plans for it as an empire capital filled with vast public architecture intervening toward the end.

Across the ends of these eight historical galleries run seven small rooms that take a look at IBA's current activities. As Professor Kleihues' right-hand man, Claus Baldus, explained the symbolism: ``This is so visitors are free at any point to leave history and come into the present.... Modernist architecture ... pushed historicism away. But one aspect of IBA is the rediscovery of history - history of the city, of style, of form, of classicism and so on.... On the other hand IBA is very modern.''

IBA, in fact, seems remarkably tolerant of various architectural philosophies. Under its banner fall a variety of projects. At one end of the scale there is the Phosphate Elimination Plant in the northern district of Tegel by Gustav Peichl, who opposes reactions against modernism (such as ``historicism'' and ``post-modernism''). His plant is self-consciously modernistic and at first sight looks like the prow of a ship designed in the 1930s. At the other end of the scale is architect Rob Krier, whose urban villas in the Southern Tiergarten area bear witness to what Dr. Baldus describes as ``a completely regressive stage of consciousness.... [Mr. Krier] said, in 1984, that `remembrance' is all he wants and he is completely against the future and industry and technical experience.'' In between these extremes the architecture IBA has fostered in Berlin in the last seven or eight years is by an international cross-section of architects, including such names as Rossi, Eisenman, and Stirling.

The exhibition (open daily 9-5, except Mondays) is expected to continue through June. But from May 16 to Sept. 30, ``IBA on-site'' takes place. This program will consist of conducted tours to IBA projects now under way in Berlin. (For further IBA information, Tel. (030)-3124936.)