User:Cor anglais 16/Organ key actions

In a pipe organ, the key action is the system that transfers the motion of the key to the windchest. The system can be either mechanical or electric.

Mechanical action
If key action consists of mechanical linkages, the organ is described as having mechanical action or tracker action. In mechanical-action instruments, the keys are connected to the windchests via a linkage system incorporating rods called trackers (usually made of wood), rollers (horizontal cylinders that transfer the vertical motion of a tracker across a horizontal distance), and squares (hinges that allow a tracker to turn at a right angle). When well-built, mechanical actions are very responsive and allow the organist to control the speech of the pipes with great precision. Because the degree of responsiveness is dependent in part upon the length of the tracker run, with a short run being more responsive than a long one, organs with mechanical key action typically feature keydesks that are built into the main organ case, or consoles that are no more than a few feet away from it (perhaps ten or fifteen feet).

Suspended action
In a suspended action, the key pivots from a fulcrum at the back end, and the tracker is attached towards the center of the key. In this arrangement, the key is held in position ("suspended") by the force of the pallet spring and the pressure of the wind inside the windchest. A suspended action allows for a particularly responsive touch, as every motion at the key is transferred directly to the tracker and from there to the pallet. Suspended actions are found in organs of the French Classical period.

Balanced action
In a balanced action, the key pivots from a fulcrum in the middle, and the tracker is attached at the back end of the key. In this arrangement, the key is not held in place by the spring but simply by the weight of the key as it balances on the fulcrum. Therefore, a balanced action is not quite as responsive as a suspended action, because more of the motion of the key is lost in the system than in a suspended action. Balanced actions are common in seventeenth- and eighteenth-century organs from north Germany.

Electric action
If the system consists of electric circuits, the organ is described as having electric action. In this system, the keys operate electric circuits via metal brushes (on the key) that touch electrically-charged contacts (in the keybed), thereby completing an electric circuit. The circuit is connected to the windchest via either electric or pneumatic means. In the case of a pneumatic connection, the action is referred to as electro-pneumatic. Electric action can be combined with various kinds of windchest design, including the slider chest (electric slider). Because an electric circuit is an absolute system—it is either "on" or "off"—electric key action does not provide the organist precision control over the pipe speech. However, because there is no need for a physical connection between the console and the windchests (other than an electric cable, or a pneumatic relay in electro-pneumatic action), electric action allows for a considerable freedom in the placement of the console relative to the pipes—the console may be located a hundred feet or more away from the pipes, or around a corner, or even (conceivably) in a different room.