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The pure-tuning automat

In the fall of 2003, the pure tuning automat in the organ-house at Ekeberg was changed. The previous transistor-automat was replaced by a Macintosh computer with a MIDI keyboard, software for pure tuning, plus an interface that links the computer with the organ. The software performs the same tasks as the previous transistorbaased automat, and is programmed according to Eivind Groven's principles for pure tuning.

The organ consists of three parts: The organ pipes (180 pc.) with air flow, the automat for pure tuning, and the keyboard. When the keys are depressed, a signal goes to the digital automat, which calculates and makes decisions as to which organ pipes will be opened. Each organ pipe is connected to a chargeable relay that opens the air flow to the pipe when electrically activated. Each relay is wired to the pure tuning automat, which controls opening and closing of each pipe.

The wires for all 180 pipes were disconnected, and for each wire the corresponding pipe was identified. The wires were connected to the interface unit by six connectors. The interface-unit consists of five components that have been built into a common cabinet; three MIDI-parallel-cards, one USB/MIDI interface and one power supply. In addition to the connectors for the individual pipes, the cabinet is outfitted with connectors for the keyboard (MIDI) and the computer (USB).

Each of the three MIDI-cards translates from note-on messages to one of the 60 electrical circuits, each 16 V. The software determines which card and which output that will be used. Several outputs can be active at the same time. Electrical current is passed through the outputs to the appropriate relays, and the pipes sound.

The chargeable relays from the old system and their wiring have been kept, and the new interface-cabinet has been wired to this structure. In the new system, the analogue pure tuning automat has been replaced with computer software, and a custom interface. The keyboard has also been changed. The software performs the same calculations that the old automat did with transistors. The computer is an Apple ICube with a flat screen, and the keyboard is a MIDI Kurtzweil. The interface has been built especially for this purpose, and is well documented.

The new technical solution provides a flexibility that the old system didn't allow. The core is a computer program that can be changed easily, and the functionality of the automat can thus be adjusted according to need - for example in conjunction with adjustments and further development of the calculation methods for pure tuning, as well as new methods for controlling the organ that one might want to include.

The software is running under Max, a programming environment for real time processing of signal- and control data. Based on which keys are depressed on the keyboard, and which keys were depressed previously, the program calculates the appropriate pitches to sound based on Groven's rules for pure tuning.

The system works through MIDI-messages that are sent from the keyboard to the computer when the keys are depressed. After completing the calculations, the program sends MIDI-messages to the interface-unit. This in turn activates the chargeable relays, and the airflow to the pipes is opened.

The software has been developed by David Løberg Code at Western Michigan University. Henrik Sundt at NOTAM has constructed and built the interface, as well as re-connected the entire system. The project has been financed by and Arts Council Norway and NOTAM.

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In collaboration with Eivind Grovens Institutt for renstemming