I had a go playing Part 7 of Music for 18 Musicians. Using the mini score and a click track I played each part live into a digital recorder. You could call it one plays 18 (although there are only 9 parts on my version). My first try ended up sounding like complete mayhem as I was trying to play the piece at full speed and realised that I needed to put the conductor bits in first so I could work out where I was in the piece, so I slowed the pace down a tad and tried again, which worked out much better.
Instrumentation wise: I was using some guitars and a keyboard. :
For the pulse (usually two marimbas): choppy organ chords on the keyboard
For the vibraphone conductor process: a harpsichord (well, why not?)
For the continuous repetitive figures: classical guitar and electric piano
For the strings/bass figure: bass guitar and keyboard strings
Maracas: hi-hat on the keyboard (god this was a pain to keep going)
For the main theme figure (voices etc) : two slightly distorted electric guitars
For the breathing pulses (clarinet, voices) : classical guitar
This is my favourite piece of music and it was great fun to have a go at it - I have to take my hat off to the guys who play this live, I have no idea how they manage it, I could barely keep it going for just one part, never mind about a whole hour.
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