The video shows the step sequencer app Xynthesizr playing the PO14 from an iPhone.
The unit is really easy to build made up from:
- the excellent Teensy
- the Hobbytronics USB Host module
- a rotary encoder
- a few buttons
- some signal diodes
USB (Device or Host) Midi Controller for Teenage Engineering Pocket Operator (with a arpeggiator thrown in for fun)