Excerpts directly quoted from the Rodgers 550 Brochure (circa 1972):
Friedrich Bessel, the famous German mathematician and astronomer (1784-1846), provided the mathematics which ultimately led Rodgers' engineers to a breakthrough in organ design. His well-known theory, called Bessel Functions, provided a basis for exploration and insight into the nature of certain effects which make possible truly accurate generation of pipe organ tones.
Bessel Tone Generation produces the correct harmonic structure of tone, including the necessary strong, higher-pitched overtones, without an artificial, buzzy edge. The tone is enriched, and virtually comes alive, under the influence of an effect called "Pipe Edge Tone" which precisely simulates the sound of wind impinging on the lip of the organ flue pipe.
Positive Exponential Attack yields the gentle bloom and prompt decay of sound characteristic of the pipe and also provides proper initial transient quality.
Heterochromatic Chiff is an authentic chiff effect, percussive and complex in timbre, which grows naturally with the initial speech of each flue stop.
The scope of this achievement in tonal realism is perhaps most evident in the singing, elegant tone of the Principal (Diapason) voices.
In the lower registers, the tone is powerful, but without a trace of "boominess". In the upper registers, it is rich and colorful. The Great 8' Principal, for example plays a commanding role upon its entry into any combination, yet blends with the cohesiveness unheard in any other contemporary organ. Its initial speech blooms naturally, slowly at first, under the parcel of initial harmonics, then promptly to a full, living steady-state tone.
Pipe Edge Tone actually "carries" this sound to the listener with precise pitch definition. The effect of Pipe Edge Tone on the steady-state sound is analogous to the effect of Heterochromatic Chiff on the initial sound; the final degree of articulation which provides a real Principal...not only in name, but in substance as well.
To accommodate the acoustical contours of any particular installation, the American Classic provides on-the-spot voicing and adjusting capabilities.
The console is prepared for up to eight separate speaking channels: Six Main Channels, an Antiphonal Channel, and a channel for outdoor Carillon. Each channel has its own master level control.
There are also level controls for each stop, and finally each stop and finally each individual note is separately adjustable.