Part fact, mostly fiction, Poulenc's Dialogues des Carmelites (1956) holds up an unbroken mirror to the essence of faith. The nuns of Compiegne, martyred in the French Revolution, face terror within and without, on the streets and in their souls. They sing of their joys and struggles in music that ranges from stark to radiant, mawkish to arresting, near plainchant to baroque extravagance with bells on, plus two harps, a piano and an excess of minor thirds.
The music smells of incense even in a production as scrubbed and minimalist and generally antibacterial as Robert Carsen's, which has arrived at the Royal Opera House nearly two decades after its first outing, in Amsterdam, in 1997. Designed by Michael Levine and with 18th-century costumes by Falk Bauer, it can be dispatched rapidly here. Nicely choreographed movement, an upturned table or two and a few candles, all set in a blank black space, doesn't offer much in terms of dialogue, or even dialogues.
The production has won praise elsewhere - Milan, Madrid - for being intense, though intensity is what it lacks. Removing the physical and concentrating on the metaphysical, as Carsen does, is almost acceptable if you know the opera intimately, or have remembered the voice types (mezzo, soubrette and so forth) of the past operatic heroines Poulenc had in mind when characterising his nuns: among them Verdi's Desdemona, Mozart's Zerlina, Massenet's Thais. That is quite an ask. Otherwise, distinguishing several women in habits and veils - I will stick my neck out and say they came in all shapes and sizes - is hard when almost all Poulenc's finely graded relationships are flattened and belittled by the empty vastness of the stage.
The agonised death of the old Prioress (fiercely sung by Deborah Polaski), full of dread after a life of devotion, felt less tormented than usual. Even the central character of Blanche, first timid then full of assurance, is occluded. Her friendship with the open-hearted Constance shows little of its usual touching sparkle, despite outstanding performances from Sally Matthews and Anna Prohaska, the one ardent, the other bubbling with life.
Luckily Simon Rattle, who was warmly applauded as if to encourage him in his reported plans to return to London post-Berlin, was in the pit to add musical roughage and depth, grit, dirt and anguish. From the opening bars, vital and brisk, he made it clear that this would be a taut performance, without indulgence. All Poulenc's religious choral works have a strong whiff of the theatrical - he was a devoted, if idiosyncratic Catholic. Yet this opera offers the reverse: with its settings of the Ave Maria and the Salve Regina, this is as close as many will get to liturgy in an age of reason.
In the women-dominated cast, Luis Gomes, a Jette Parker young artist deftly replacing an indisposed Yann Beuron mid-performance, Thomas Allen and Alan Oke contributed sharply etched cameos. Sophie Koch as Mother Marie and Emma Bell as Madame Lidoine led the beautifully sung ensemble of nuns. Apparently 26 actors and a "community ensemble" of 67 people, recruited from the Department for Work and Pensions and elsewhere, together with principals and chorus, made a grand and record-breaking total of 167 people on stage - a statistic the virtues of which I am still pondering. Rattle himself has suggested that this huge crowd is the scenery. Eloquent on all things, he almost convinces me.
The final scene, in which the guillotine falls on the nuns one by one - aurally only, in keeping with Carsen's conceptual style - still held its power to shock, not least because Poulenc writes the rhythm of the falling blade, created by a sliding crescendo which cuts off abruptly, into the music itself. No mere sound effect, it is intrinsic to the score, and all the more chilling for it. The Royal Opera orchestra made perfect sense of this oddest of operas, with Rattle a powerful evangelist.
An opera about counting sheep strikes one as decidedly normal in comparison. Harrison Birtwistle's rarely seen 1984 work Yan Tan Tethera (alas the same night as Carmelites) was one of the array of events at the Barbican this past fortnight to celebrate the British composer's 80th birthday. This has been an all-join-in gala, with the Britten Sinfonia exploring English pastoral and Birmingham Contemporary Music Group, conducted by Oliver Knussen, revisiting the works which first established Birtwistle's career: Monody for Corpus Christi and Tragoedia, from 1959 and 1965 respectively.
A stunning account of Gawain (1991) won a standing ovation for the composer, as well as for the BBC Symphony Orchestra, the conductor Martyn Brabbins and an exemplary cast led by John Tomlinson, Leigh Melrose, Laura Alkin and Jennifer Johnston. The London Symphony Orchestra played Birtwistle's epic-sized Earth Dances (1986). Conducted with mesmerising authority by Daniel Harding, the LSO was on fire. I could not take my ears off the orchestra.
In this busiest of seasons, I caught only the opening event of the increasingly essential London contemporary music festival, which took place at Second Home, a former carpet warehouse in Spitalfields, attracting a large crowd. Some of Birtwistle's near contemporaries, more associated with the art world and gathered here under the title British Underground, played works by Gavin Bryars and Christopher Hobbs among others.
That untiring guru of all that is experimental, the pianist John Tilbury, was there to contribute insight and wisdom, as well as to perform one of John White's 172 sonatas. In Bryars's 1, 2, 1-2-3-4, each musician plays along to different recorded sounds in his or her headphones but no one, audience included, hears the whole. The effect is a kind of enigmatic, sleepy blues in which phrases bump into one another like a slow-mo game of bagatelle.
As part of LCMF's eclectic refraction of sound and light, the 80-year-old Italian artist Michelangelo Pistoletto has made an installation of 10 large mirrors, which created ghostly reflections during the performances. Tonight, in the UK premiere of Ten Less One, the artist will take a hammer to these mirrors to create a shattering finale.
Dialogues des Carmelites at Covent Garden: 'The music smells of incense even in a production as scrubbed and minimalist as Robert Carsen's.' Photograph by Donald Cooper