I think a perfect linear pitch interaction is essential, if a thereminist aims to develop his or her muscle memory beyond the point where it can exceed the necessity to rely so much on aural skills to accurately place notes. A linear playing interaction is not important if a theremin player is already happy with his or her accuracy, note transition time, margin of error and correction time or error masking time. I have spent the better part of the past six years focusing on this core element of my instrument's interaction, because I have learned the best way to achieve what I wanted musically was to have a firm foundation of what supports my musical voice... my interaction with the invisible interface.
I say perfect linear pitch "interaction", because, as Thierry has mentioned above, Pitch Linearity (what is perceived as a 1 to 1 ratio between changes in capacitance and changes in chromatic note output) is a product of a number of variables. These variables include, of course, the theremin circuit itself (which can be tweaked and modified), but there are many additional factors that are not easily modifiable, because they are difficult to see and or measure. also because of these additional variables, a perfect linear interaction can never be achieved with good circuit design alone.
Many theremin players may feel they have a linear playing interaction, but they may actually have only achieved a comfortable balance between their memorized movements and permissible margin of error according to their perception of intonation and playing style.
A pitch interaction can definitely be made "more" linear by the circuitry of the theremin itself, but it's also important for the player to become aware of all the variables that affect the interaction externally. it's even more important to understand that there is ultimately only one variable that changes. That is the averaged capacity displacement of the space around the external point on the theremin at which the pitch antenna begins.
The additional variables include the size, shape, mass, density of the instrument cabinet, the mass of a player's arm, position/posture of the body, proximity of nearby objects, and the player's technique that used to displace capacity.
Linear pitch interaction can properly be measured by using a momentary fixed change in capacity (made only by the player of the theremin) across various locations in space along the 3-5 octave range of the theremin. the most consistent method is by using the change in capacity of a closed fist to fully extended fingers. If the closed to open gesture creates the same interval, regardless of the position of the arm's location within the playing range, then the interaction can be considered linear. To what degree the interaction is perceived to be linear depends on the aural acuity of the person who perceives the sound.... unless a method of quantitative analysis is employed.
last year, I published a software application that can suit this very need. It graphically reflects pitch in a high resolution visual tuner that tracks incoming audio signal at an extremely fast rate. This allows for the intonation to be reflected back to a player in real-time with high precision. most tuners, digital or analog, have a non-real-time response, making it difficult for us to draw a synapse in the brain to effectively connect movement with sound.
I use an octave as the pitch interval (from closed fist to open hand) to measure the linearity, but any interval size can be used as long as the motion is consistently yielding the same change in capacity. This means, that if a player is creating changes in capacity that include the arm or body movement, it is much more difficult to create a consistent test movement.
The change in capacity of a theremin from an electronic point of view is very *easy* to measure because the theremin circuit only interprets a singular value, where as from the point of view of the player, in 3D space the variables that can influence the change in capacity across the full playing range include a wide variety of objects, shapes, masses, densities that all must be averaged together... this collection of external influences is much more difficult to measure.
The ultimate gain from having a linear pitch interaction is to create stability in theremin playing. We, most of us who play the theremin, all want one thing... and that is for the instrument to be as predicable as possible so we do not waste the energy that is invested in interpreting a piece of music or executing an improvised composition with exactly what was in mind (or the composer's mind). When the interaction is linear, it allows for memorized motions, whether they be complex or simple, (using the hand, wrist, fingers, or whole arm) to be easily repeatable with a low order of discrepancy at any point within the theremin's multi-octave range.
With all of the above said, I must admit that the pursuit of a perfectly linear interaction is an extremely difficult voyage that has mostly to do with a player's own awareness of his/herself... However the process of investing energy, attention, and awareness to this area will eventually reward anyone, who decides to jump down the rabbit hole. I say so, because with my Etherwave Pro, I have adjusted the instrument, and my playing posture, position, and technique to yield an extremely linear (measurable) 5 octaves.
The goal in the end is to be happy with your playing, so that you enjoy what you do, right? The nuts and bolts aren't always so important, unless you have a very specific need that must be met. for me, I think I've gone as far as I could possibly go to get what I wanted... In a few weeks I'll have a video camera again, and i can perhaps demonstrate what I've described.
sorry if that was a bit full-on for anyone. Info vom. Thanks Chobbs for starting this topic.