Classical music: A heart-felt tribute to whom?

"I don't know who's conducting," said a neighbour to her companion. You get a different class of audience at a Jessye Norman event. What would it make of the toughest piece in the entire Pierre Boulez 70th birthday series? The LSO, carefully ratio ning the maitre's own music to one piece a night, slipped in Livre pour cordes at the start of the sure-fire public draw and, right on cue, another neighbour had begun muttering by bar six. Applause was polite, though, and tinged by the warmth with which both the conductor (as conductor) and the orchestra are currently regarded in London.

No wonder, because they had delivered playing of a refinement and spirit that were inconceivable when Boulez first made this version, nearly 30 years ago, of music that string quartets had found even harder. Here it was, revealing atmospheric textures and delicate shadings straight out of the fin de siecle - the previous siecle, that is. The logic and pace of thought, not to mention the harmonic idiom, remain as formidable as ever, but the old sound of a struggle to survive has faded. With it , too, hasgone some of the danger and violence, the passionate glare that characterised the young Boulez.

It's still there when needed in the performance of the music he champions: Berg and Bartok, on Tuesday. For Berg's Seven Early Songs, it wasn't forthcoming; but then, with Jessye Norman on stage, the air is charged with too many strong qualities for the rage of an angry young man to get a look in. In any case, the songs themselves, written in Berg's early twenties but revised later, always sound steeped in decadent sensuality, the more so when the voice projects them with such ample power, relishing words as much as melody. Against that, the orchestra continued to produce breathtaking subtleties at a steady, unexciting pulse. All the elements were exact, but they didn't gel; the Wagnerian sumptuousness needed less matter-of-fact sup port.

Berg's Altenberg Lieder brought them together. Less lavish and more extreme, the sharply focused instrumental sound builds with greater sureness and breadth and drew intensity as well as precision. Norman remained superb, dramatic, intimate; the orchestra entered a new dimension. Boulez's extraordinary ear was in its element, interpreting what might easily be a half-heard whisper as a musical utterance of perfect balance, right on the edge of sound.

In Bartok's Miraculous Mandarin, it realised an equally perfect balance at the opposite edge of sound. The neighbours were looking sceptically at each other again during the noisy prelude, but if they ever heard so much detail again amid the hectic activity of this score, they would be lucky indeed. Boulez kept up the electric charge of the previous songs and drew a positively rhetorical swagger from the bellowing brass at the moment when the ballet's central figure makes his entrance. It di dn't quite last the duration; the final few minutes, after the frantic silent-film-style chase, lost some of the tension and vividness. But, as sound for sound's sake goes, an exceptional encounter all the same.