London Sinfonietta/Valade, Queen Elizabeth Hall, London <!-- none onestar twostar threestar fourstar fivestar -->

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Take an ensemble of 31 players, seat them in sections separated by glass panels, mic up each player to a mixing desk, combine their sound with electronically generated sine tones, feed the resulting sound complexes through ring modulators and project them from loudspeakers, and you get the extraordinary sonorities of Mixtur by Karlheinz Stockhausen.

The electronic component of this pioneering effort in "real-time" sound transformation may now be neatly computerised, compared with the primitive means of 1964, when Stockhausen conceived the score. But the piece remains expensive to mount, retaining its reputation for technological gee-whizzery. No wonder the box office for this performance by the London Sinfonietta was besieged by readers of Time Out and The Wire.

In fact, the half-hour sequence comprising Mixtur was heard twice, as Stockhausen now likes it to be. The work also epitomises his preoccupation in the 1960s with what he called "moment-form" - the conceiving of scores as collections of self-contained units that could be shuffled into various successions. Here, Pierre-André Valade directed clear-cut readings of the score first in reverse order of Stockhausen's 1967 published version, then forwards. It seemed that the electronic modulation was more fiercely applied by Ian Dearden and David Sheppard of Sound Intermedia second time round.

Did one version seem more meaningful? Not really. The sounds are rarely less than arresting in their edginess or fuzz - as of a not-quite-tuned radio or chalk squeaking on a blackboard. And Stockhausen's gifts for textural layering and sudden dramatic gestures are everywhere in evidence. Yet there remains a curiously equivocal feeling in hearing instrumentalists capable of precisest nuances used to an essentially cruder end: the sense of a potentially focused composition reduced to a mere montage sonore.

As so often with Stockhausen, one felt the tragedy of a marvellously speculative and inventive mind, not quite musical enough to make the best of the sonorous means it has dreamt up.