Say Tristan and Isolde, and most people think of Wagner, but his opera tells only one version of the legend. When the Swiss composer Frank Martin began work on Le vin herbé ( The Magic Potion) in the late 1930s, he took his text from Tristan et Iseut, a novel written 40 years earlier by Joseph Bedier. There he found two Isoldes, not only the one we know from Wagner (Isolde the Fair), but also a second, Isolde of the Fair Hands, whom Tristan marries when he thinks he's lost lsolde the Fair. Not unreasonably, the second Isolde wants Tristan to herself, but all she manages to do is to kill him off. Fair hands, dark heart: his mother should have warned him.

Say Tristan and Isolde, and most people think of Wagner, but his opera tells only one version of the legend. When the Swiss composer Frank Martin began work on Le vin herbé ( The Magic Potion) in the late 1930s, he took his text from Tristan et Iseut, a novel written 40 years earlier by Joseph Bedier. There he found two Isoldes, not only the one we know from Wagner (Isolde the Fair), but also a second, Isolde of the Fair Hands, whom Tristan marries when he thinks he's lost lsolde the Fair. Not unreasonably, the second Isolde wants Tristan to herself, but all she manages to do is to kill him off. Fair hands, dark heart: his mother should have warned him.

Wisely, Martin decided not to write an opera but a "dramatic oratorio"; after all, who'd want to compete with Wagner? Even so, Le vin herbé is sometimes staged, although last Friday's performance by Opera Decentralise Neuchatel stuck to the concert format. It would be good to report that it revealed the work to be a resounding success, but that would be misleading.

In Le vin herbé Martin found a way of marrying serial technique to late-Romantic tonality, which opened a rich vein of lyricism for him, yet this performance, conducted by Valentin Raymond, proved austere rather than sumptuous. Martin scored the piece for a drastically small orchestra of seven string players and, as harmonic motor, a piano. Over the work's 100-minute span, they provided thin accompaniment for a vocal ensemble of a dozen singers functioning variously as characters, narrators and chorus. At times when all 12 were singing, the orchestra was barely audible, and all the way through the lack of textural variety made one long for a clarinet, a bassoon, anything to break the pattern.

Still, there were moments when the sombre massing of strings carried real emotional weight, as when King Mark discovered his wife in Tristan's arms and a spasm of light passed through the orchestra. As for the voices, most of Martin's word-setting is syllabic, with an evenness of tempo that renders long passages into a kind of etiolated, sub-Debussyan recitative.

It didn't help that few of the singers enunciated their French with ideal clarity, although the performances weren't without character. Jeannette Fischer (Isolde I) got genuine passion into her long farewell to Tristan (Peter Wedd, showing a natural intensity). Not for the last time, though, it was the villain who stole the show, Sibyl Zanganelli breathing fire as Isolde II when too much around her had an alpine chill.