Fons about the manual/division/group concept:
For the midi matrix it's simple: the 'keyboards' lines are for note on/off, and the 'division' lines are for swell and tremulant control. Otherwise it's probably of little importance except when you want to write your own definition file, or use one of the alternatives.
To understand the structure of Aeolus you need the following concepts:
- Ranks - Divisions - Keyboards
- (audio) Sections - (midi) Channels - (GUI) Groups
The first three map directly to real organs. Each rank belongs to a division. There can be up to 32 ranks in a division, and there can be up to 8 divisions. Tremulant and swell operate on a division. A keyboard is a virtual 'manual' or 'pedal'. The last three are required only beacuse Aeolus is a software application.
To undertand this we need to consider:
- the audio structure - note on/off control - effects control - the GUI structure
1. All ranks that belong to the same division are mixed together
to a division signal. Tremulant and swell are applied to this mix. Although the implementation is a bit different, you can imagine a division signal as a stereo signal that has a width of about 90 degrees in the horizontal surround circle.
2. Each division belongs to an audio section. The division signals
for each audio section are added, and then the the set of controls you find in the audio window - azimuth (surround panning), width, volume, early reflections, and reverb send - are applied to this mix. There can be up to 4 audio sections. In the default instrument, each division has its own audio section.
3. The outputs of all audio sections plus the reverb output are mixed
to produce the final output signal. The only controls on this level are the 'position' fader, and the master volume. Position is only used for the stereo output, and it determines 'where you are' in the surround scene, going from front over center to back.
In the midi matrix each midi channel is mapped to one of the virtual keyboards. This is a simple one-to-one relation.
The second mapping is from keyboards to divisions. In the default 'Aeolus' instrument, there is a fixed link from each keyboard to a division, and this enables the keyboards and the divisions to have the same names : P, I, II and III. Additional keyboard to division links are enabled by the couplers, in this case only 'upward'.
Now this second mapping can in fact be a bit more complicated. First, the default keyboard to division links exist only because they are specified in the definition file - they are not encoded into the software structure as they were in 0.3.1. In the 'Aeolus2' instrument definition for example, there is only one default coupler - from the 'Pedal' keyboard to the 'Pedal' division - and the three other divisions can be coupled to any combination of keyboards. French romantic organs (e.g. the one in the cathedral here in Antwerp) often permit this sort of flexible coupling. Second, it is also possible to couple a rank directly to a keyboard, bypassing the division grouping. Ranks used in this way should be in a separate division that itself has no couplers. This is the case in 'Aeolus1' where the three reed stops are in a fifth division named 'IV', and can be used from both the I and the III keyboard. These three stops are independent of division couplers: if you have for example the Oboe on for keyboard III, and you engage the I+III coupler, the Oboe will still not respond to keyboard I because it is not part of division III. In this instrument the I and P divisions are also combined into one audio section in order to give the extra 'reeds' division its own one (so you can put it at the side or back).
Since it is now possible to have no fixed relation at all between keyboards and divisions, it is necessary to route midi events for each of them separately - hence the extra lines in the midi matrix. You just can't see the need for this with the default instrument because it has that fixed keyboard to division relationships.
This flexibility w.r.t. coupling also requires the concept of a 'group' for the GUI. Compare 'Aeolus' to 'Aeolus2'. In the first we have the default keyboard to division mapping, and the groups of buttons in the GUI represent both a keyboard and a division. In this case the couplers are placed with their corresponding keyboard, and this seems logical - 'I+III' means 'the ranks of division III should also respond to keyboard I, as if these ranks were added to division I. In 'Aeolus2' this is no longer possible - the groups represent divisions only, and the couplers are now placed with their division.
So it is necessary to control the visual grouping of buttons independently from the division structure. A 'group' can in fact be any set of up to 32 stops, couplers, or effects. If you want to combine all 8' stops into a group that is perfectly possible but of course quite silly. MIDI control of stops etc. if ever implemented, will be in terms of these groups, not of divisions.
I hope this helps a bit...