The fame of Glasgow- born composer Craig Armstrong rests on his reputation for creating purpose-specific instrumental music.

A graduate of the Royal Academy of Music, his provision of incidental music at the Tron Theatre in director Michael Boyd's era set him on a path that has seen him join the select pantheon of globally quoted creators of movie soundtracks, particularly through his collaborations with Australian filmmaker Baz Luhrmann on Romeo + Juliet, Moulin Rouge and, most recently, The Great Gatsby. Next year, Armstrong's music will features on Thomas Vinterberg's Far From The Madding Crowd, with Carey Mulligan and Michael Sheen, and Paul McGuigan's Frankenstein, with Daniel Radcliffe and James McAvoy.

But there is another, parallel, side to Armstrong's work that focuses on the voice, and that has a won a large share of his time in what has been the composer's busiest and most prolific period. The new album under his own name, It's Nearly Tomorrow, released last month on BMG/Chrysalis, is a 70-minute sequence of songs and interludes with lyrics, some of his own writing, delivered by home nation associates Jerry Burns, Katie O'Halloran, James Grant (of Love And Money) and Paul Buchanan (The Blue Nile) as well as Brett Anderson (Suede).

The composer also hopes that his Herald Angel-winning adaptation of Ibsen for Scottish Opera, The Lady From The Sea - premiered at the 2012 Edinburgh International Festival - may be his next venture into the recording studio, and that such a release become the catalyst for further productions.

Less well known is Armstrong's collaboration with the Lewis Psalm Singers, the preservers of a unique Scottish vocal tradition, and his contribution to the cultural programme that accompanied the nation's great 2014 summer of sport.

The work was heard in the Queen Anne Hall of Edinburgh Castle at a gala dinner hosted by whisky company Diageo to celebrate its association with the Ryder Cup as the owner of Gleneagles, where the tournament was played. Its beginnings, however, lay at Celtic Connections in Glasgow, when the event's artistic director Donald Shaw introduced the composer to Calum Martin, who had brought the choir - and their remarkable singing technique rarely heard outside of Gaelic church services in the Outer Hebrides - to the festival.

Armstrong dates his own fascination with the Gaelic psalm singing of Lewis to having heard a BBC recording in his student days. "I think it is one of the most interesting cultural forms in Scotland, and I wanted to know how it worked," he says.

The combination of the texts from the kirk's Scottish Psalter and traditional sean nos Gaelic singing in a choral style led by a precentor, embraces improvised variations in pitch and tempo that are passed from voice to voice.

The singing style has been handed down through the generations and is quite alien from conventional Western music, yet audibly akin to oral traditions elsewhere in the world.

"My way to start was to notate what the singers were doing," says Armstrong, "and that was easier said than done."

Because there is no way for each singer to perform their own line in isolation - this is very much collective music-making - the composer had the choir perform as normal, but with each singer very close-miked to record the way individuals reacted to one another. "I wanted to understand the nuance and complexity of it, the use of micro-tones and slight delays in the differences of melody and tempo."

Committing this to paper was a considerable challenge, but one which led Armstrong to note parallels in the work of 20th-century composers he reveres, including Gyorgy Ligeti, Steve Reich and Terry Riley.

"There's lots happening, but it can create a feeling of stasis," he explains.

"The singers learn to do it instinctively, but when I isolated the voice of Calum's daughter Isobel Ann, the grace notes she was adding were so complicated. At times it almost sounds Arabic or African - and it is known that there was trade between the Scottish Highlands and Islands and Western Africa."

For Armstrong there was also a personal dimension to this musical exploration, as his mother's family hail from Balintore in Easter Ross, and a relative had been a presenter in the Kirk. His first instrument was the violin and he had played traditional fiddle himself as a young man.

It was important, therefore, that he was able to travel to the church on the edge of the ocean on Lewis and record the music in its native habitat.

"It is very intuitive, like waves of sound, and I have not heard anything exactly like it anywhere," he says. "I don't think anybody else had ever recorded the singers individually in that way and I believe it is very important to preserve it, as it is in danger of dying out. It is completely religious and yet so contemporary."

Armstrong's resulting composition, still untitled, combined the 12 singers with players from the Scottish Ensemble - another reason why it had to find concrete form in conventional musical notation. He sees his role as being as much documentation as it is composition, but also hopes it is just the start of his relationship with the Lewis Psalm Singers, now that they are taking their music into different musical settings.

"Calum Martin is a fantastic front person for the choir, and I'd love to work with them again," Armstong concludes. "And we have now opened that conversation."

Craig Armstrong's latest album, It's Nearly Tomorrow, is out now.