Pipe organ
Action

An organ contains two actions, or systems of moving parts: the keys, and the stops. The key action causes wind to be admitted into an organ pipe while a key is depressed. The stop action causes a rank of pipes to be engaged (i.e. playable by the keys) while a stop is in its "on" position. An action may be mechanical, pneumatic, or electrical (or some combination of these, such as electro-pneumatic). The key action is independent of the stop action, allowing an organ to combine a mechanical key action with an electric stop action.

A key action in which the keys are connected to the windchests by only rods and levers is a mechanical or tracker action. When the organist depresses a key, the corresponding rod (called a tracker) pulls open its pallet, allowing wind to enter the pipe.

In a mechanical stop action, each stop control operates a valve for a whole rank of pipes. When the organist selects a stop, the valve allows wind to reach the selected rank. The first kind of control used for this purpose was a draw stop knob, which the organist selects by pulling (or drawing) toward himself/herself. Pulling all of the knobs thus activates all available pipes, and is the origin of the idiom " to pull out all the stops ". More modern stop selectors, utilized in electric actions, are ordinary electrical switches and/or magnetic valves operated by a rocker tab.

Tracker action has been used from antiquity to modern times. Before the pallet opens, wind pressure augments tension of the pallet spring, but once the pallet opens, only the spring tension is felt at the key. This sudden decrease of key pressure against the finger provides a "breakaway" feel.

A later development was the tubular-pneumatic action, which uses changes of pressure within lead tubing to operate pneumatic valves throughout the instrument. This allowed a lighter touch, and more flexibility in the location of the console, within a roughly 50-foot (15-m) limit. This type of construction was used in the late 19th century and early 20th century, and has had only rare application since the 1920s.

A more recent development is the electric action, which uses low voltage DC to control the key and/or stop mechanisms. Electricity may control the action indirectly by activating air pressure valves (pneumatics), in which case the action is electro-pneumatic. In such actions, an electromagnet attracts a small pilot valve which lets wind go to a bellows (the "pneumatic" component) which opens the pallet. When electricity operates the action directly without the assistance of pneumatics, it is commonly referred to as direct electric action. In this type, the electromagnet's armature carries a disc pallet.

When electrical wiring alone is used to connect the console to the windchest, electric actions allow the console to be separated at any practical distance from the rest of the organ, and to be movable. Electric stop actions can be controlled at the console by stop knobs, by pivoted tilting tablets, or rocker tabs. These are simple switches, like wall switches for room lights. Some may include electromagnets for automatic setting or resetting when combinations are selected.

Computers have made it possible to connect the console and windchests using narrow data cables instead of the much larger bundles of simple electric cables. Embedded computers in the console and near the windchests communicate with each other via various complex multiplexing syntaxes, comparable to MIDI.
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