Rainbow emerged from an ambitious process of collaborative creativity that reached across continents and cultures, and across musical categories and conventions. It represents at once a continuity and a turning point in the Smithsonian Folkways-Aga Khan Music Initiative CD-DVD series Music of Central Asia, whose previous seven releases each focus on a particular region or musical idiom, or on the artistry of a particular musician. Like the artists featured on earlier releases, the lead performers on Rainbow are all consummate masters of their own musical domain. Indeed, Azerbaijani singers Alim and Fargana Qasimov and Afghan rubâb player Homayun Sakhi have already debuted on Music of Central Asia (Sakhi on volume 3, The Art of the Afghan Rubâb, and the Qasimovs on volume 6, Spiritual Music of Azerbaijan). But how could it be otherwise in our era of cultural globalization that in their sonic imagination, these inquisitive artists, no less than the famously intrepid Kronos Quartet, should hear their own music both enriching, and being enriched by sounds and sonorities from elsewhere. These days, the instant availability of so many musical "elsewheres" makes the prospect of artistic collaboration at once alluring and daunting, full of promise as well as potential pitfalls: where in the world to begin? How to move beyond superficial grooves toward deeper levels of music connection? What standard to use as a measure of artistic success?

To the first of these questions, David Harrington, first violinist and founder of the Kronos Quartet, offered a straightforward answer: "I follow my ears," he said. Harrington's relentless musical curiosity has been the engine behind Kronos's adventurous musical travels. "I started playing string quartet music when I was twelve," Harrington related, "and I remember one day, as a young teenager, looking at a map of the world and realizing that the only string quartet music I'd ever played was written by men who lived in the same city: Vienna, Austria. That just struck me. And so at that age it became a part of my thinking to try to learn more about other parts of the world through music." Harrington has devoted his career to doing just that—in the process, forging under the aegis of Kronos a living legacy of pioneering and wide-ranging collaborations with musical creators from many cultures.

David Harrington's global musical explorations led him to recordings of Alim Qasimov and later, Homayun Sakhi. Harrington recounted his first impressions of listening to Qasimov: "I realized immediately that there was a quality I had never heard before from a singer. The way he inhabits the notes he makes is profoundly beautiful. It's like he's molding and shaping these notes in a way that we can only try to do with our bows. I hoped that one day we might be able to meet and find a way of making music together."

That musical meeting happened in 2008, as the first project in an ongoing collaboration between Kronos Quartet and the Aga Khan Music Initiative. In its mission to revitalize and assure the onward transmission of musical traditions in regions where they are endangered, the Music Initiative came to understand that cultivating creative processes which lead to artistic innovation and evolution is as important as conserving links to the past. Kronos's long experience in creating new music—oftentimes with artists from other cultures who didn't share a common musical lexicon of terms and concepts—offered a successful model of how to do such work.

To launch the Kronos-Qasimov collaboration, Alim Qasimov and two members of his ensemble, kamancha player Rauf Islamov, and tar player Ali Asgar Mammadov, traveled to San Francisco to rehearse a set of Azerbaijani songs drawn from Qasimov's repertoire. The challenge of the weeklong rehearsal period was to create a seamless interface between the note-reading Kronos players and the Qasimov Ensemble, whose performances typically feature an ever-shifting blend of memorized and extemporized musical gestures.

The details of working out the Kronos-Qasimov interface fell to Jacob Garchik, a multi-talented performer-composer-arranger from New York who had collaborated with Kronos on previous projects. "It was difficult," said Garchik of the arranging work. "You have highly skilled, virtuosic musicians, but they have very different ways of learning and teaching music. Alim Qasimov's music is more improvised and unpredictable, but at the same time, it's based on arrangements. Someone in Azerbaijan had already composed and arranged these songs and then, in the process of recollecting them, Alim turned them into quasi-improvisations. Now his improvisations are being turned back into arrangements again."

Just as Kronos and Jacob Garchik were challenged by the Azerbaijani musicians' "unpredictable" improvisations, Alim Qasimov found it challenging to work with note-reading musicians. "Our traditional musicians don't learn folksongs or classical mugham from notes," said Qasimov. "When I perform with my ensemble, I'm usually free, but here, everything depends on notation. If something is written two times, I have to sing it two times. To absorb this music deeply and get close to its inner sentiment, you have to put the notes aside and learn how to improvise."

"I remember the first time we tried to do that," said David Harrington of Kronos's efforts to "move away from the page," as he characterized the process of improvisation. "It's something that we've become more and more comfortable with as time has gone on. From Kronos's perspective, there's a lot of improvisation going on in the Azerbaijani songs, but hopefully the listener won't even know when those moments are."

The enthusiastically received world premiere performance of the Azerbaijani songs took place at London's Barbican Centre during Ramadan Nights, the Barbican's eclectic survey of music from the world of Islam. For the concert, Kronos and Alim Qasimov were joined by Alim's daughter and musical protégé, Fargana, and his full ensemble of four instrumentalists. The songs were recorded for the present CD the day after the concert. Alim Qasimov succinctly summed up the artistic results: "I think we have planted a tree, and now we have to work to make it grow and bear fruit. God willing, it will happen."

For Homayun Sakhi, the process of creative collaboration with Kronos unfolded differently. Whereas the starting point for Alim Qasimov's work with Kronos was a set of popular songs performed by Qasimov but composed by an older generation of Azerbaijani songwriters, Homayun Sakhi delivered to Kronos a musical work that he composed himself. The work was "Rangin Kaman"—"Rainbow" in Persian—for Afghan rubâb, string quartet, and percussion. The percussion instruments include Indian tabla and Central Asian frame drum (doyra) and clappers (qairaq). Like many contemporary composers who do not notate their compositions, but simply record them direct from instruments or computers to hard drives or digital recording devices, Homayun Sakhi composed and recorded the Afghan rubâb part on his own instrument and realized the string quartet sounds on a Casio synthesizer. The percussion was added during rehearsals, with the musicians improvising in traditional rhythmic cycles and patterns.

Sakhi's Casio and Afghan rubâb recordings were given to Stephen Prutsman, an award-winning concert pianist, composer, arranger, and music festival founder whose long collaboration with Kronos has resulted in over forty arrangements for the quartet. Prutsman transcribed the entire piece and wrote it out in Western music notation. In the sections of the piece where rubâb and quartet play together, Prutsman created the quartet parts from the implied harmony suggested by the modal melodic lines of the rubâb.

In sections where the quartet plays alone, Prutsman assigned each of the pitches he heard on the Casio to a particular instrument—what composers and arrangers call "voicing." "I voiced it according to what I thought would be most natural for the quartet," said Prutsman. "I also marked articulation, dynamics, slurs, and phrases. Most of the time, people don't do that, but I've found that it's always helpful as a guidepost to the quartet." During rehearsals, the quartet re-arranged of some of Prutsman's markings based on their experience of working with the rubâb and percussion. Traditional Afghan meters and rhythms in the piece presented another challenge for Prutsman and Kronos. Prutsman explained, "There are lots of groupings of seven, but where are the strong beats in those groupings? Oftentimes, strong accents don't correspond with down beats. We spent a lot of time getting comfortable with the rhythmic units."

Homayun Sakhi composed "Rangin Kaman" as a way to represent "all the peoples and regions of Afghanistan and connect them to other parts of the world." Sakhi elaborated, "There are influences from both classical and folk music. The folk music comes from Herat, Mazar-i Sharif, Kandahar, and other places; it's music that's performed by Hazaras, Pashtuns, Uzbeks—I tried to bring together a little bit from each region and people of Afghanistan."

"Homayun is a great band leader," said David Harrington. "He knew exactly what he wanted from us; he knew every rhythm that he wanted the tabla to play, and basically, he taught everybody. He has the whole score completely imprinted in his mind, so in one sense, there's nothing left to chance. But Homayun is very open to changes that arise from the experience of working together. For example, in one place I suggested using a practice mute to achieve a different timbre on the violin. He loved the sound when he heard it, and it got worked into the piece. As Homayun got to know us better, he refined the writing through a lot of little changes like that. In the end, ‘Rangin Kaman' was literally tailor-made for Kronos."

Homayun Sakhi added his own assessment of the artistic collaboration. "Working with Kronos, I realized that through music you can cross not only boundaries between different regions of Afghanistan, but even bigger boundaries, such as between East and West. When we play together, the music really connects well and we all get a lot of pleasure from it." In this impromptu critique, Homayun Sakhi may have inadvertently provided as good an answer as any to the question of how to measure the artistic success of collaborative music-making. If a piece "connects well," as Sakhi put it, and if performers with the keen musical sensitivity of Kronos Quartet and Homayun Sakhi enjoy performing it, there's a good chance that it has something to say.

David Harrington echoed Sakhi's critique. "If we're involved in a piece of music and I get a recording of a performance and can't stop listening to it and, when I wake up in the morning, the music is part of my consciousness, then I know it's something I'm really happy with." The riveting performance of "Rangin Kaman" on Rainbow makes a strong case for this piece, and for the prodigious talent of its young Afghan-American composer, Homayun Sakhi.