Around 1980 I came up with a keyboard layout that could accommodate microtonality, including just intonation. Much more recently I have become interested in alternate key structures on top of microtonal pitch sets. Now I see how to remap my keyboard layout to support the alternate key structure I have been exploring. I would like to share that remapping here.
The keyboard layout covers a flat surface, like a table top. It consists of a hexagonal array of buttons, perhaps something like the chord buttons on an accordion. Of course this keyboard could extend any amount in any direction. A musician needs to be able to reach all the buttons and of course the cost of the instrument will increase with more buttons, but generally a practical instrument will be considerably larger than what I have illustrated here.
I have picked some selected buttons to have a different color, just as an aid for a musician to more easily keep track of location, a bit like the marks often placed on a guitar fretboard. The arrow shows the direction in which pitch increases. The pitches are roughly constant along the line perpendicular to the arrow. I.e. by moving along that line one can explore very small pitch variations.
The notes are laid out on the keyboard by mapping geometric relationships to harmonic relationships. The fundamental harmonic relationship is the octave. Every pair of notes in a the same column with two buttons in between will be an octave apart. An octave is a frequency ratio of 2. To illustrate these frequency ratios, I have picked a button in the middle of the keyboard to serve as a reference frequency and then will show the frequencies of the other buttons as a ration with respect to that reference button. So far all I show is just the two buttons that have octave relationships with the reference button. Of course on a real practical instrument one would generally like to have a much larger series of octaves, so the keyboard would be considerably extended in that octave direction.
The 1980 keyboard layout and the 2013 layout have the same geometrical representation for octaves. They start to split apart from there, though. The next harmonic relationship to consider is the perfect fifth. The 1980 layout uses the perfect fifth as the next building block. Every pair of buttons with the sort of diagonal relationship illustrated will differ in pitch by a perfect fifth. This lets us fill in many more frequency relationships, given that a just perfect fifth is a 3:2 frequency ratio.
The next harmonic relationship is the major third, which is a 5:4 frequency ratio in just tuning. Now things get a little complicated. Three major thirds make a just tuned ratio of 125:64 which is almost but not quite an octave. So the geometrical pattern for the harmonic relationships will be a bit irregular. The columns will follow a pattern of two major thirds, which combine to make a 25:16 ratio, and then a 32:25 ratio to make the complete octave. This completes the mapping of buttons to frequencies: One could use this keyboard layout to play in just tuning, but it can also be used with various temperaments. For example, the conventional 12 pitch equal temperament would look like: Now, instead of frequency ratios, the buttons are labelled with the number of steps up or down from the reference note. In this case the step size is 2^(1/12).
In just tuning every button has a unique pitch and one can find pairs of buttons that differ with arbitrarily fine spacing, limited only by the extent of the keyboard. Temperament limits the number of pitches within any interval. Thus with a tempered pitch set there will be multiple buttons mapped to any given pitch, for example: This geometrical mapping of harmonic relationships can also be mapped to the microtonal 53 pitch equal temperament: Here of course the step size is 2^(1/53). With more finely spaced pitches to choose from, there is less repetition of pitches among the buttons: Up to now this has been a description of the 1980 keyboard layout. In 1980 I had started with just intonation and was exploring various microtonal tempered pitch sets but it was not until the 1990s that I became interested in the 53 pitch equal temperament. Just a couple years ago I became interested in the kleisma, a small interval which is tempered by the 53 pitch set. The kleisma can be considered as primarily constructed from minor thirds. This opens up the intriguing possibility of a keyboard layout based on minor thirds: The 1980 layout had minor thirds for some of these geometric relationships but not for all. There is no way to keep all the minor thirds and all the perfect fifths in a single layout. So the new layout prioritizes the minor thirds and keeps only some of the perfect fifths. Of course musically this is unconventional and probably well nigh perverse, but all the same it might well open up some new and fruitful musical territory. This new layout actually preserves some of the frequencies of the 1980 layout: The mapping to 12 pitch equal temperament is exactly the same as the 1980 layout. The distinction between the layouts is subtle enough that it only appears in a microtonal layout such as the 53 pitch set. The tempering of the kleisma means there is quite a bit of pitch repetition in this layout: Maybe it's time to get out the soldering iron!