Photo by Kristoffer Juel Poulsen

I spend my first night in Copenhagen in an old slaughterhouse. The ILK collective (and record label) are presenting as many as six sets for each day of the festival, in unit 5e, which might be ultra-basic, but is strong on atmosphere, courtesy of flickering candelabras, a wicker-cased lamp and a grand total of two electric stage lights.

Habitable Exomusics is Jacob Anderskov on piano, Nils Davidsen on bass and Gerald Cleaver on drums, the latter clicking out an organic robo-techno repetition that fluctuats gradually, before ending up deep in the abstract realms. The Per Oddvar Johansen Trio play compositions with a ritual emanation, establishing a suspended state of spatial awareness. Cleaver returns for the late set, this time inhabiting a more aggressive field of conventional free jazz, with Danish saxophonist Lotte Anker and Norwegian bassist Ingebrigt Haker Flaten.

The following evening offers the polar opposite venue experience, with a sold out show at the new-ish, out-of-town DR Koncerthuset, a slick, acoustically-primed joint. Two of Brazilian music's starriest veterans give an intimate duo recital, Caetano Veloso and Gilberto Gil highlighting more of their similarities than differences, using just voices and acoustic guitars. Veloso tends towards a higher clarity, whereas Gil can veer right down to a sonorous bass depth. There were a surprising number of songs performed separately, and the secondary roles would shift, ranging from a hesitant backing vocal to a full duet. With no interval, and a two hour set, the sonic character began to feel limited towards the end, despite the communicative qualities of these veteran kingpins.

The next night's duo in the same sold out concert hall succeed in a more dynamic, far-ranging experience, as Herbie Hancock and Chick Corea range from grand pianos to electric keyboards. Hinting at an alternative career as stand-up comedians, the pair eventually sit down for the adventurous journey, taking in expected greats such as 'Cantaloupe Island' and 'Spain', but also diverting into wayward improvisation, only spoiling the good taste with a few outbreaks of Hancock's sometimes outdated electronics palette (more lumpy cheese than cool retro).

The highlight alternative arrive in the late afternoon or around midnight. The Nancarrow Trio are devoted to the compositions of Mister Conlon, the Mexico City resident who primarily wrote for mechanical player pianos. These pieces are given a fresh jazz life, with human pianist Christian Balvig daunted by the speeding complexity, and inducting a bassist and drummer to assist him with replicating the sound of a piano roll. Most of the pieces aren’t particularly recognisable as Nancarrow toons, but the feel is captured, and a new form of hyper-kinetic jazz is born. The small Mellemrumet café venue is sunk just below street level, and make a cosy haunt that suits the acoustic proximity.

The Stingray! threesome play in the front bar of Jazzhouse, Denmark's famed club, gathering an unnervingly large crowd for an improvising combo. Bassist Flaten returns, joined by the Swiss sticksman Raymond Strid, who sounds like he descended from Roger Turner and Paul Lovens, tightly tumbling over a range of muted drumheads, scrabbling with tiny bricolage. Reedsman Sture Ericson is the discovery of the set, a local player who leavens his serious throttling with flecks of absurd humour, the escapade veering from tightly contained cake tin mute-rattling to open stretches of minimalist invention.

Another late night highlight is Orlando Julius & The Heliocentrics, a sprawling London combo led by a Nigerian multi-instrumentalist, with Jason Yarde on reeds. The dominant style is afrobeat, but there are also streaks of pure funk and soul, and a playfulness that comes from Sun Ra. Their set could use an editing scalpel, particularly when they get into an extended band-intro vamp at what should have been the long set's climactic stretch. Otherwise, the Heliocentric groove is dynamically booting.

The Copenhagen Jazz Festival is one of the more extreme events of this type, on several fronts. In terms of sheer quantity, it takes over a humongous spread of the city's venues for 10 days, with gigs running from 11am until late, in multiple locations. Many of these are free admission sets. It's also stylistically very broad, spanning traditional New Orleans jazz right through to the hardcore improvisation found at the ILK fest-within-a-fest. Set as jewels in its crown are the guaranteed big-name concert hall fillers that happened to feature some of the music's greatest exponents.