We can separate pipe organs into two main classifications based upon their operating principles, and these are mechanical or tracker action, and then electrical action instruments. We can further subdivide the electrical action instrument into two subcategories, the direct-electric and the electropneumatic. The term action, in reference to a pipe organ refers to all of the mechanism between the keyboards and the pipes which is responsible for getting the musician's key and stop maneuvers to the appropriate pipes of the instrument.
In the mechanical action instrument, there are actual mechanical linkages between the keys, pedals and stops to appropriate valves in the instrument. We will in this series however, not devote too much attention to mechanical action, as compared to the two major forms of electrical action, because the mechanical system imposes a number of significant limitations upon the ultimate possibilities in the pipe organ for both development and playing. For that matter, we won't spend too much time with the direct-electric action either as its usage represents a very small percentage of instruments.
The large classical, concert and theater organs all utilize electropneumatic action, where both electricity and also the pressurized air that is abundantly available in pipe organs combine forces to make the instrument work, essentially using electricity for control and transmission of signals from the console to the actual instrument, and air pressure to accomplish all of the necessary mechanical work.
There is, however, still a market for mechanical action instruments, and there are many musicians who are often referred to as "purists," who are passionately devoted to the mechanical system, and indeed the mechanical system was the only system available during Bach's time, Bach being considered one of the greatest composers of classical musical literature for the organ. Although most of us believe that this sentiment is really not true, many of these "purists" feel that only an instrument which uses mechanical action can accurately play the music of Bach and his contemporaries. It is probably extremely likely, however, that if electro pneumatic action had been available to Bach, he would definitely have preferred it inasmuch as Bach was a very progressive innovator as far as his music is concerned.
The various electronic instruments that we at the NSHOS encounter and use, however, were developed from those pipe organs which primarily use electropneumatic action, and that is what we will devote the major portions of this article to; the modern pipe organ which uses e-p action.
The two systems of electrical action that we will find made possible the development of the modern pipe organ which made for a very fast and positive key action; a touch that would not become impossibly "heavy" in a large instrument; the placement of various sections of an instrument in many different parts of a large hall or building; the use of higher air pressures which greatly benefit certain classes of organ pipes; and also greater tonal flexibility by making some ranks of pipes independently playable on different keyboards and/or at different pitch levels.
Electrical action also makes it possible, in a large installation, to place the organ console at a central location where the musician can hear all of the various divisions of his instrument in the proper balance, and it also allows the console to be on a suitable rolling platform so that it can be moved to different parts of a large hall depending on the needs of a particular event.
For example, a console for a large church instrument could be placed somewhat out of sight for a Sunday service where watching the musician could be distracting, and then if the instrument is to be used for a public concert, it can be rolled out to a more prominent location where the audience can watch the organist. Mechanical action by definition requires a fixed and immovable console location which, once the instrument is installed, can never be changed again without a major redesign of the action along with the accompanying expense and downtime.
It stands to reason that if the console is very close to the instrument, or to a particular division of a larger instrument, the musician will perceive that which is closer as being generally louder than other, more remote divisions. This can also make it very difficult in a church with such a pipe organ layout to accompany a choir, for example, if the organist must be very close to all or at least a portion of the instrument. With either form of electrical action, the console can be located at a place where the musician is equidistant from the organ pipes and the choir. This lets him achieve a proper tonal balance and gives a much better over-all musical result.