One might at first suppose that an organist need only be concerned with studying technique, registration, repertoire, hymn playing, harmony and counterpoint, composition, improvisation, and song arranging, but such is not the case.
The study of orchestration, for example, the groupings and compass of individual orchestral instruments, and what others have done to transcribe important orchestral works for organ and vice versa, adds a new element of importance to an organist's education.
The organ in general, and the theatre/cinema organ or "Unit Orchestra" in particular, is likened to the grand orchestra with the organist comparable to an orchestral conductor in command of a machine which produces it own enormous range of pitches and timbres which must sound rhythmically on the beat and in harmony with itself.
Although the orchestral conductor never plays a single note and cannot conduct every single musician in a united orchestral ensemble, each of the four sections of the grand orchestra (strings, woodwinds, brasses, percussions) has its first lead, and the conductor has the first violin, first clarinet, etc., just like a leader cannot lead the whole organization but instead leads the leadership team to create synchronization.
This requires an "ear" to hear the beat of the drum in his team as well as the tender notes of the violin which represent the different personalities of his/her team members; the conductor adapts his/her style and leads accordingly.
This compares roughly with organ playing in the sense that organists also adapt their way of playing to the instrument at hand, its action, and its resources which correspond to individual orchestral players, its breathing in its own acoustical environment, and the types of repertoire which bring out the best the instrument has to offer, all the while using the ear to get things clear, achieving synchronization between its various divisions, working the machine, and being a musician.
The organist seated on the bench is very much an orchestral conductor in the sense that (s)he is in command of the instrument's united tonal forces, adapts to those forces, and determines their synchronization; beyond that, however, the similarity ends.
Groupings of organ pipe ranks by tone color do not correspond exactly to those of the orchestra; where the orchestra has strings, woodwinds, brasses, and percussions the organ has foundations, mutations, and reeds, with orchestral-equivalent numbers of tuned percussions and traps provided to theatre/cinema organs only.
Organists should never lose sight of the fact however that the instrument they're playing is not the orchestra but the organ, and that whatever the expressive tendencies of the music might be, the player "conductor" can do nothing but 1) open and close pallets which admit air always under the same pressure and quantity to each pipe, 2) open and close swell shutters, or 3) add or subtract individual ranks of pipes, each of which represents either a fundamental or an upper partial tone of the harmonic series.
Unless it's a compound harmonic-corroborating stop, every stop in the organ works this way: it sounds either a fundamental prime of the 8 foot, 16 foot, or 32 foot harmonic series, or reinforces one of its higher harmonics.
For example, when the fundamental prime or 1st harmonic is represented by a stop at 8 foot pitch the octave or 1st upper partial tone (2nd harmonic) is reinforced by drawing a stop at 4 foot pitch -- the twelfth or 2nd upper partial tone (3rd harmonic) is reinforced by drawing a stop of 2-2/3 foot pitch -- the fifteenth or 3rd upper partial tone (4th harmonic) is reinforced by drawing a stop of 2 foot pitch -- the seventeenth or 4th upper partial tone (5th harmonic) is reinforced by drawing a stop of 1-3/5 foot pitch -- the nineteenth or 5th upper partial tone (6th harmonic) is reinforced by drawing a stop of 1-1/3 foot pitch, and so on.
NOTE: By measuring the intervallic distance between the fundamental prime and its first 5 harmonic upper partial tones like this we discover the octave, perfect 5th, perfect 4th, major 3rd, and minor 3rd -- which sums up the entire Western system of major/minor tonal harmony based upon the triad and superimposition of 3rds as we know it today.
The ear recognizes the strength and number of harmonic upper partial tones of the fundamental prime generated by an organ pipe as its timbre or tone color, whereas the fundamental prime it generates, being of greatest intensity as the 1st harmonic in the vibration recipe, is what the ear recognizes as the pipe's pitch.
Obviously, to obey the natural laws of musical sounds which indicate that harmonic upper partial tones of a fundamental prime decrease in intensity as they rise in pitch, it's incumbent upon organ builders to voice the stops forming the upperwork (i.e. octave stops or higher) of the 8-foot, 16-foot, and 32-foot harmonic series less assertively as they rise in pitch.
Ideally then, those stops which sound the unison and reinforce the octave, twelfth, super-octave, and higher should NOT be all of the same quality and strength of tone, as when a single rank of pipes is unified and made playable at many different pitches, a trade method to which builders have often resorted to save cost and space.
Of those stops which reinforce harmonic upper partial tones, those which are off-unison in pitch, i.e. those having fractional numbers such as twelfth, seventeenth, nineteenth, etc. and sound one pipe with each key depressed are described as mutations.
When drawn in combination with single unison stops or in larger combinations, mutations add a valuable splash of color but must be used judiciously in solo lines and above middle C in order not to disturb the sense of pitch; also, in the manuals which are of 8 foot pitch, drawing the nineteenth (Tierce 1-3/5') without capping it from above with an assertive twenty-second (1 foot stop) -- a situation called an "unprotected Tierce" -- should be avoided in the minor key or when playing minor chords to prevent violent false relations.
With a little knowledge of orchestration, the organist can also use mutations to form voices not provided by the builder; for example, in smaller instruments which have no reed stop at all but have an 8-foot Gedeckt unified at 2-2/3' and 1-3/5', these 3 stops (perhaps with an additional 8-foot flute added to strengthen the unison pitch) can be combined to form a "synthetic Clarinet" for reed solos, the Clarinet's timbre deriving from an emphasis on odd-numbered harmonics.
Mixtures on the other hand are compound harmonic-corroborating stops composed of multiple ranks of small pipes which sound two or more pipes producing harmonic upper partial tones when each key is depressed -- sounds never produced in the organ.
Mixtures are often drawn to complete principal choruses or in combination with certain unison stops to create very bright solo voices.
The term "mixtures" therefore does not refer to mutation stops, even though the terminology used in certain publications of the past, due possibly to oversights in editing, may indicate otherwise.
It stands to reason then, that when one is playing orchestral transcriptions or improvising in an orchestral style at the organ the bringing together of these groupings of stops needs to be done judiciously, bearing in mind any orchestral instruments one might wish to evoke so the notes on the keyboard stay within the orchestral instrument's compass, the latter's range is never exceeded, and mutations and mixtures, if used at all, are voiced softly like those in the Saint Louis Scottish Rite Cathedral Kimball [See Photos 3].
At one time the organ compositions of J.S. Bach, which after 300 years still represents the kernel of the organ repertoire, was new music, and the same can be said for orchestral transcriptions of this same music, organ transcriptions of movements from his cantatas, and so on; the people who share such new material with other musicians are of no small importance.
The growth and development of the musician who plays the organ has always depended upon it.