Scanning speed and velocity mapping of Cybrid

xooorx So far I've coded up a physical model of a key/hammer/string system

Fantastic! Now we are getting scientific 🙂

If you care, you can introduce in your model the sensor response too. In fact the ADC number is not linear with distance for three reasons:

• current from the sensor (ADC measures voltage but V=RI is quite linear)
• ADC conversion (both in value and in time)
• time response for the sensor

For the first reason: see Pag.4, top figure for the 'RCA6 sensor (not sure what "negative distance" means in this plot). With this sensor (the one I am utilizing) at the distance of most interest, i.e. the last few mm, it's linear enough you can probably ignore this issue (assuming the datasheet is correct, which might or might not be). For the CNY70, the difference is big, see fig. 9 (notice that unlike the other one, the vertical scale is logarithm!) and also notice how it flattens in the most important last half mm. The same problem is shown in fig.2 for the QRE1113.

The second problem is described in general at https://microchipdeveloper.com/adc:adc-inl and the specifics of the ADC I am used are on fig. 116 and fig. 117 (in pag 588-9) and in the Errata RP2040-E11 on pag 647 of the datasheet (you would not think a datasheet would have 600+ pages eh?) -- I said repeatedly, I am interested in characterizing this platform, but I expect that eventually it would show its limits and not be adequate. So you might want to consider the other MCU which @JayKominek is already utilizing and which I plan to switch to when he has settled on a hw design. Here is the datasheet and the relevant data is in fig. 39 at pag 171 and fig 91 at pag 284. @JayKominek may chime in with more details, since I did not dig too deeply into these 356 pages (less than the other datasheet, but denser…)

Then, there is the time factor. The sensors do not respond immediately to a changing light, but they have a bit of delay. The sensors' datasheets report information about this either in a figure or in a table. Links above, let me know if the way they report it is too cryptic for you

The data is posted in https://github.com/davidedelvento/Mybrid/tree/main/data/RPiPico%2BEAITRCA6 in binary format.

There was a file uploaded here as plain text file (UNIX end line) rather than MIDI, but it got inadvertently deleted as described at https://pianoclack.com/forum/d/1222-i-have-inadvertently-deleted-some-files-on-the-forum/17 and I am not reuploading unless asked

RIP In fact the ADC number is not linear with distance for three reasons:
ADC conversion (both in value and in time)
time response for the sensor

Despite it not appearing in the CNY70 data sheet, I'd expect all of the sensors will have some time delay. But, I've largely discounted it:

1. They're interested in transitions from light-to-dark and vice versa, for barcode reading, and such. (Maybe even PWM'ing the LED.) So in those cases, the phototransistor has to charge, or discharge, "completely". We're interested in smooth and continuous motion with an always-on LED, and it takes place on "longish" timescales.
2. Note in the 'RCA6 data sheet that the times are a function of load resistance in their example circuit. That example circuit is what CyberGene used, and you're using, but not what I'm using. By hooking a transimpedance amplifier up to the output of the phototransistor, you show it a frequency-dependent load resistance. At the <=20kHz frequencies I'm dealing with, that load resistance is ~0. (It is technically a function of the gain-bandwidth product of the op amp you use.)

I believe any time delay can be safely written off as sub-microsecond when using a TIA.

RIP current from the sensor (ADC measures voltage but V=RI is quite linear)

Alas, the phototransistor is not an ideal current source, and V=RI is not the end of the story. If you look at figure 6 of the CNY70 data sheet, you see that the output current not only varies with the forward current of the IR LED, but also with the collector-emitter voltage. By just putting a resistor above/below the phototransistor and applying a constant voltage, the Vce voltage will vary. With no light shining in, and no current flowing, the resistor will drop 0V, and Vce will be the entire voltage of the supply. But as light shines in, and current flows through the phototransistor, the resistor will begin to drop voltage, and Vce will drop.

The TIA, again, shows the phototransistor a constant Vce. I set that voltage to 1.3V, which was a convenient value to hold stable.

I may have written this better/differently on my CNY70 notes page. https://github.com/jkominek/piano-conversion/wiki/CNY70

RIP Here is the datasheet and the relevant data is in fig. 39 at pag 171 and fig 91 at pag 284. @JayKominek may chime in with more details, since I did not dig too deeply into these 356 pages (less than the other datasheet, but denser…)

That's just the data sheet for the STM32H743; there's also a 3319 page reference manual. I'd consider the RP2040 data sheet to be combination data sheet, and reference manual.

While I'm writing, I'll note that I successfully got my hardware sampling at 20kHz, by making use of the non-uniform memory architecture of the part, and finally working out how to get the 3 ADCs to sample out of phase from each other. Unfortunately going much past 20kHz begins to show some severe ADC issues. It's possible they'd be resolvable, and the ADCs could be driven up to 30kHz, but at that point I'm also constrained enough on clock cycles that the current code can't run without dropping/ignoring samples. That might also be solvable, with significantly tighter code, and removing the convenient abstraction layer library. I don't want to bother with that unless/until the entire system is proven to work.

JayKominek Despite it not appearing in the CNY70 data sheet, I'd expect all of the sensors will have some time delay. But, I've largely discounted it:

Yes, I expect that too and I discounted it too. One other reason to discount it is that if the delay is (somewhat) constant, it's just a simple latency on the all system (small enough to be neglible). On the other hand, if the delay depends on the value of the signal, than distortion happens and that affects the computation of the velocity. Hence I want @xooorx to be aware of it, since he's building this nice model.

JayKominek Alas, the phototransistor is not an ideal current source,

I see. I knew something like that would happen, but I thought it would happen at loads with much lower impedance compared to the ones we are using.

JayKominek The TIA, again, shows the phototransistor a constant Vce. I set that voltage to 1.3V, which was a convenient value to hold stable.

Very nice!

JayKominek That's just the data sheet for the STM32H743; there's also a 3319 page reference manual.

Ahaha! Now we're talking! That's what I was missing. The data sheet felt too short and too terse 🤣

JayKominek While I'm writing, I'll note that I successfully got my hardware sampling at 20kHz,

That's a great result! Congratulations on your march forward!

JayKominek and the ADCs could be driven up to 30kHz, but at that point I'm also constrained enough

I think I found a way to collect all the data the ADC of the RP2040 can spit out (3 channels at 167kHz per channel or even at 500kHz in only one channel). If that works as I hope it will, in the next weekend or two I will produce some "definitive" data for all the three sensors (CNY70, QRE1113 and 'RCA6) collected at these frequencies (if I can all 12 bits, but since the ENOB is slightly less than 9 I may collect just the highest 8). I suspect these very high frequencies are overkill, but with the data in hand we can confirm if 20kHz suffices and/or if the stretch to 30kHz would be useful, and/or if even 30kHz is too slow and we really need to go higher frequency -- which I guess will be the final answer to the question that made this thread start. For other people reading this, if necessary we can use the RP2040 in Jay's design as he mentioned here -- so it's not a either-or his-way vs my-way.

RIP On the other hand, if the delay depends on the value of the signal, than distortion happens and that affects the computation of the velocity.

I believe it will depend on the rate of change of the signal, rather than the absolute value. (Which might be what you meant, but, helpful pedantry?)

RIP I knew something like that would happen, but I thought it would happen at loads with much lower impedance compared to the ones we are using.

Hmm, not really an impedance thing, I don't think. More of an IV-curve thing, if that makes sense.

Main items I dropped in for:

#1 I just put up some data where I measured the output of the CNY70 while moving it in a controlled fashion. So you can make a distance-signal plot. The raw data and notes on interpretation are at https://github.com/jkominek/piano-conversion/tree/master/analyses/jkominek/cny70

If anyone can fit some reasonable function to that data from ~5mm on out, I'd really like to see it.

I'm interested in trying to convert the voltage readings I get off the CNY70's phototransistor into something which is at least a linear function of distance. I could build a lookup table, but I'd like at least the smidge of physical insight which would come from seeing a function fit to the data.

#2 I've made some additional improvements to the firmware for my ADC boards, cutting out some unnecessary CPU usage. That completes the last of my mental todo-list for the non-signal processing portions of the firmware.

#3 On the subject of the signal processing, I got curious and tried a quantized version of the FIR filter I was using. Worked just fine. So at least for that, floating point math isn't even necessary.

JayKominek If anyone can fit some reasonable function to that data from ~5mm on out, I'd really like to see it.

For light reflected off a large card, the density of light reaching the card falls off with inverse square. The light intensity getting back to the sensor falls off with another inverse square, but the field of view also increases with the square of the distance, so overall distance to photocurrent is inverse square. Just eyeballing the data (plotting vs 1/dist²), it looks like zero current voltage is at around 1.97V, so to invert things, estimate dist~=1/sqrt(1.97-V)

I expect the response for a shank will differ from the large card.

luns Ah, I was even mentioning inverse squared drop off to my wife last night and then didn't even try it with this. Thanks! I got the same graph, and a nice fit linear fit between 5 and 15mm.

My target was only maybe 20mm wide by 30mm tall, so I'm not surprised the fit would start failing further out.

JayKominek If anyone can fit some reasonable function to that data from ~5mm on out, I'd really like to see it.

One drawback of these larg-ish sensor is the drop when the hammer become too close (which happens for geometric reasons). What do you plan to do for the close-by measurements? If you have a properly regulated action, with the sensors at the same distance as the strings would be, the let off should be 1.5 to (at most) 2.5mm so if you ignore that part you ignore the whole free fly of the hammer. Of course it is possible to regulate a larger let off, but if it's too large I suspect that would feel weird to the player. It is also possible to identify the peak and use velocity estimate to determine if the subsequent drop is the hammer becoming close (if speed was faster than x) or falling back (if speed was slower), however since that is happening when the hammer is still controlled by the key, I expect edge cases to happen, when the player moves the key in unexpected ways.

Seeing this problem in your graph (as opposed to the datasheet) makes it feel worse than I initially thought. What do others think? I think that we should more seriously look at the QRE1113 and EITRCA6 (which are already my first choice -- and I hope to post some better files with their data today).

RIP One drawback of these larg-ish sensor is the drop when the hammer become too close (which happens for geometric reasons).

It was already apparent in the data sheet that things became a bit strange as you got too close. The collected data makes that seem much more extreme, though. Both the amount of distortion, and the distance at which it starts. Their graph does note that Vce is 5V. I was using ~1.3V. Perhaps the phototransistors's low-current linearity improves as voltage increases. I can trivially go up to a Vce of 3V with my setup, so I'll try that this afternoon and see if the behavior changes.

RIP What do you plan to do for the close-by measurements?

…the same thing I'm already doing? The plastic protectors I've got around the CNY70s keep the hammers at least a few millimeters away from the sensor surface. They seemed to eliminate all of the weird dipping in my early experiments, but they're not holding the hammer off by the 5mm that this data suggests is necessary. And yet things seem okay-ish.

I've got my 16MB files of ADC capture. Here is a plot for the p strike

and one for the f strike

If everything goes well, I will share my data later today

RIP I've got my 16MB files of ADC capture.

With high temporal resolution, as explained at https://github.com/davidedelvento/Mybrid/tree/main/data/RPiPico%2BEAITRCA6

Including some multichannel (i.e. multi keys) strikes (this was deliberately not synched across keys)

Here is the data in txt format for those willing to take a look

EDIT: This got inadvertently deleted as described at https://pianoclack.com/forum/d/1222-i-have-inadvertently-deleted-some-files-on-the-forum/17 and I am not spending the time to re-upload it unless someone asks

It might not buy you much, but if you prefer I posted the original RAW data (binary, not MIDI, simple python suffice to extract it) at https://github.com/davidedelvento/Mybrid/tree/main/data/RPiPico%2BEAITRCA6

There is also the script to do extract the text (or plot these figures) if you so prefer.

Note: the speed of writing to the SD is not as good when also sampling the ADC, for doing both at full throttle I should use one of more of DMA, interrupts or multicore. Each one requires a bit more work than I am willing to put in for just a test, so I am calling it good for now. Open to revisit that decision should someone prove (or even just speculate) that a further increase in time resolution is useful for this analysis.

In case anybody is still interested in this discussion, I've resumed it with a slightly different focus in a new thread:

https://pianoclack.com/forum/d/916-performance-of-diy-controller-prototype/4