"I probably could have saved you some writing time if I had phrased that final question "Your technical thoughts on this?" I was mainly asking if you had any thoughts about RFI/EMI or signal integrity issues. I have my own, but I wanted to hear yours." - Roger
Do you mean: are there any issues with the LED tuner interfering with the playing fields? The answer to that is yes, and that's one minor reason I was working on the PLL dither (and some on the LED PWM waveform). In the very far field, where the mathematical subtraction from the null constant produces a number that is quite small and has poor SNR, you can obviously see some interaction going on in the way of LF oscillations on the tuner. You can see the interaction too if you pipe the highpassed pitch number directly to SPDIF. But you shouldn't see or hear this when playing normally. The tuner can be completely turned off via the UI.
Ideally the tuner would be located smack dab in the middle of the pitch plate, but that would probably be a much worse SNR scenario.
[EDIT] As far as signal integrity for the tuner interconnect, I believe the prescale counter divides the 180MHz core clock by 16, so the clock going to the tuner board is 11.25MHz and the data is at most 1/2 this. There are programmable FPGA resources to control output edge rate and drive strength, I've got them all set to the mildest settings. The tuner, like the LCD, is a write-only interface for the FPGA (no reading of data going on).
"But what I won't be doing is using plastic containers and Dupont jumper wiring, because I don't have to and I don't want to build it that way. I think time is best spent putting forth some effort making a few incremental changes that put the design a step or two closer to something that I like and would possibly appeal to more traditional thereminists as well. The antenna plates won't be in boxes because they don't need to be, and some things will be arranged differently physically."
Oh dear, I should have made myself clearer, I wasn't expecting you to duplicate the prototype to the level of plastic boxes and jumpers. They're fine for an alpha lab pass (and are incredibly freeing for development) but beyond that PWBs and sturdier cabinetry are called for. I was just hoping you wouldn't steer too far from the basic ergonomics, and it seems that you aren't planning on that.
"I am cautious enough to know that if I end up going that route it is better to do it from the position of having a working prototype, with plates, as a starting point. I have every intention of giving plates a fair test and I will spend some time with them to do that, but as you know I have legitimate concerns. Those concerns come from observations of the way my hand intercepts the constant pitch "surfaces" that surround the pitch rod. I have learned to work off those prolate-spheroid shapes both horizontally and vertically, and Electromagnetic Fields 101 tells me that the constant capacitance constant pitch surface contours for plates are going to be significantly different."
I don't have your deeper feel for analog rod Theremins, but I imagine that playing a rod or plate in the mid to far field probably feels similar. In the near field the rod is a smaller target so "aiming" the pitch hand horizontally is somewhat more critical. Every so often, usually when I'm playing tired, I notice that the near field sensitivity is low, and it's because I'm way off of the horizontal re. the antenna. Plates seem to be fairly forgiving targets. Maybe you'll end up hating them, but I'm glad you're going to give them a fair shot.
"Having configurable pitch linearity in software is a great thing. But linearity is very much in the arm (and hand, and body, and stance) of the beholder, and I think that plates may have a different feel at close ranges. If your pitch field is large enough that normal playing distances avoid flat plate distortions, that is good. But then I start thinking about the effects of z-axis arm movements in the more spherical far-field (I mean distant field, not the antenna terminology)"
The far field linearity is of course contingent on adjusting the null for my body distance, though when adjusted (it only takes a few seconds) I can open and close my hand right at my body and get the same response as with my hand closer to the plate. But I find that I play best (on pitch) in the mid and near fields. I sometimes touch the plate box going for a high note and there is no "feel change" (that I can detect anyway) indicating that I'm that close.
"Were you talking about doing the pitch preview in the FPGA?"
Yes, I would very much like work on this with you.
"What is your total current, approximately?"
I think around 330mA with the LEDs on the lowest setting. If you USB power via UART you get a ground too if the PC is grounded.
