Glass’s revision of Music with Changing Parts, which premiered in New York in 2018, doesn’t fundamentally alter the musical patterns, those propulsive rhythms or the sense of predictability in a work which is always unpredictable - what has changed is the lack of astringency, the greater range and just different sounds in a revision which places it between two poles of Glass’s creative output.

If there is one thing which Music with Changing Parts does display it is a discipline to the intricacies of how every bar, every phrase and every line are carefully processed, even if you’re hearing the same ones, multiple times. This is the Boulanger effect on Glass’s early music. But what is equally striking about this particular piece - and this remains true whether you hear it in the original or the revision - is a debt to Stockhausen, if not consciously inspired then certainly as a musical parallel. The Glass of Music with Changing Parts is about the geometry of patterns, the polyvalent, unpredictable randomness which runs through a piece like Klavierstück XI, the clusters of electronic sound which reach a colossal apex and the psychoacoustic effects of voices which recall Stimmung, a work written just a few years before in 1968. What the revision does, and does so well, is turn Music with Changing Parts into an almost full-blown choral piece, and in doing so expand on those overlayered voice frequencies, the kaleidoscopic complexities of long and short vocal sounds into something significantly more minimalist and hypnotic than one usually experiences in Stimmung.

As the title of the work suggests there is nothing fixed about the orchestration of Music with Changing Parts. If I recall, the 2005 performance at Tate Modern used significantly more players than the Philip Glass Ensemble had used on their ground-breaking recording of the work in 1971. Whilst doubling of instruments is an option (as it was for the recording) what the revision does is to simply expand the number of players and instruments (beyond the normal keyboard and woodwind). For this particular performance, that was two horns, two trumpets, a flute, a tenor trombone and a bass trombone. What this does to the tone of the work is to markedly change its musical direction. It can, in one sense, just sound much less radical, much less primitive a score - much of the asceticism has been torn from its pages, its landscape less jagged and now more sensuous, almost opulent. The colours aren’t as shocking or vivid as they once felt; now they seem more tenebrous. In a work which sometimes seemed it was striding multiple musical genres, from minimalist classical to quasi-symphonic clusters of rock (at one time, I rather thought some of this music resembled the cumulative, kinetic power of Santana’s Soul Sacrifice - the 1969, psychedelic Woodstock version, that is), it now sounded comfortably, and eerily, closer to Akhnaten. Not a bad thing - just a different aural experience.

Originally, the use of voices in Music with Changing Parts was strongly tied to the improvisatory geometry of the work - and those voices were in every sense unwritten. The words had no structured meaning, they occurred only at breaks in the playing - and those breaks were themselves determined by the players, not by the score. The notes given out for this performance didn’t make anything clear (in fact, Mr Glass was extremely minimalist in telling us anything about what we were going to hear) so what follows is largely based on how I heard it.

If the fundamental randomness of the instrumentation remains largely untouched - that is, patterns of varying lengths, repeated as often as a musician wants - the choral writing is a whole new layer in itself. The momentum is clearly forward-looking, each section sounding different to the previous one. The chorus is divided - and part of the work’s spectral unpredictability is that often only the left or right chorus will sing - though sometimes one experiences the chorus in unison. Quite how far the impact of this is random, of whether the ear will ever be tuning in to one side or the other, to a stereophonic or a more monophonic distribution of sound, is difficult to tell. A choral conductor suggests this is entirely possible. Glass has also opted for a children’s chorus, which if it seems a touch Mahlerian is actually much more inventive. If this is now a work which is about contrast and depth, about the possibility of range and tone, the layers in it have become enmeshed in quite complex textures. The rhythm remains entirely uniform - what feels alien to the original Music with Changing Parts is an exploration of different pitch than we have in the later version.

The performance itself was beyond first rate. If I clearly heard parallels with some of Glass’s later works (especially the operas after Einstein on the Beach) this was an incredibly hypnotic experience. The biggest irony of Minimalism is that its simplicity masks huge complexity. What can seem structurally slight is actually vast in scale. It would be easy to take the virtuosity of the Philip Glass Ensemble for granted, but the concentration, especially from the keyboard players (Lisa Bielawa, Nelson Padgett and Mick Rossi), was of an exceptional quality. This was the kind of performance you were transfixed by - something you watched as well as listened to. This often went beyond music into a kind of visual art.

The London Contemporary Orchestra, who provided the brass players, were entirely immersed in this music. I warmed especially to the dark trombones (Iain Maxwell and Dan West) as well as the piercing and bright trumpet playing (Katie Hodges and Dave Geohagen). The Tiffin Chorus had, I think, something of a challenging evening - but this was a performance which lasted for ninety rather intense minutes. Glass makes few, if any, allowances for the fact that huge amounts of this score is to be sung by children. Much of the rapid repetition of same-note patterns, the varying length and intensity of the notes themselves, the shifts in register or the apparent discretion of the choral conductor were all taxing on them, but they acquitted themselves very well.

At times one sometimes felt that the audio and sound engineering (Ryan Kelly and Dan Bora) felt a touch over reverberant, especially when capturing the voices of the Tiffin Chorus. It was possibly meant to be deliberate. The theory is that Glass came to compose Music with Changing Parts precisely because of the effect of reverberation he heard in a performance of an earlier work. Distraction in this case may well have been purposeful effect. It did little in the end to weaken a performance which had been masterfully conducted from the keyboard by Michael Riesman and the choral conductor, Valérie Saint-Agathe.

The question one might ask after hearing this concert is whether Philip Glass intends this revision of Music with Changing Parts to completely supplant his earlier version. His notes were elusive about this, and I don’t know the answer either. Musically, it was a superlative evening - and I think compositionally it improves on the original. Performances of the latter were a rarity; perhaps this will change that.

Marc Bridle

Philip Glass Ensemble, Michael Riesman (music director), Valérie Sainte-Agathe (conductor), Players from the London Contemporary Orchestra, Tiffin Chorus (James Day - Tiffin Chorus director)

Barbican Centre, London; Wednesday 30^th October 2019.