Idea for Theremin Staccato Pedal

Yup, we have done that, haven't we. Damn clients! Who'd have 'em? 

It seems to me that when you're designing a heck of a fancy switch, which is exactly what this is - in fact, it is literally a switch with knobs on! - the top priority must be to get the actual switch part of it right.

I agree with you, it could be the best choice. Given the number of possible mechanisms we may never know if it is the best choice. It is certainly the choice which best fits all of the requirements (thin, sensitive, robust, etc)  from the various suggestions made on this thread, and a heck of a lot better than anything I have found elsewhere.

But I consider myself forewarned - if it turns out to be problematical the switch mechanism could need changing.

I've been thinking about where this started.

The original idea was that the pedal would be like a second volume loop, but foot operated and with a very small sensing field, capturing the rapid movement of the foot during descent and ascent between silence and full volume. This was ruled out as infeasible for technical reasons.

So we looked at a variety of sensing mechanisms, and concluded by giving up on the idea of tracking the motion of the foot up and down and got to a sensitive switch triggering a predetermined ADSR envelope. The switch would use a variable capacitor with a fixed lower plate, then a springy dielectric and a movable upper plate (with a special shield on top of it) that remains in contact with the player's foot during ascent and decent between the upper and lower limits of its movement. And then you refined it into a sort of club sandwich with three plates separated by two layers of springy dielectric.

Did I get that right? Because given the right maximum separation between the plates and the right springiness (i.e. light enough to not impede movement noticeably and responsive enough to stay in contact with the foot during rapid ascent) I don't see that it would be different in effect to measuring the movements of the foot as I originally imagined it would happen.

Hi Gordon,

The difference is that the foots "ground capacitance" does not affect this new scheme - but maybe I misunderstood more - I never dropped the 'analogue' idea until the end of last month when you challenged the need for 'tap force detection' - I was not thinking in terms then of 'controlling' the volume, but was stil thinking it terms of a harder 'tap' having a greater effect than a soft 'tap'.

... And to be honest, I still like this idea - With PSoC it could possibly be added anyway without any hardware - by changing the Capsense from a switch to proportional sensing... Everything the same, low pressure "on/off" (trigger) sensitivity, but an additional "note-on velocity" parameter available............. But thats for the future - if ever.

The ground on top, sensor at centre, ground at bottom, with equal thickness squidge separating the layers is I think the best way to realize the switch (and could be the best way to implement an analogue sensor) - it gives complete electrical immunity (I came to it when realizing that the shield drive signal could be subject to ESD and would need protection), is lowest current (shield drive needs to work against any capacitance-to-ground, so could have problems if the shield pad had a large to-ground capacitance), and the double grounding with squidge separating sensor and ground layers means that sensitivity should be good (both upper and lower grounds reducing in distance to the sensor as the pedal is squidged)..

In fact, I think there could be a "squidgymin" in this ;-) Two spongy handles one grips, each covered in conductive elastic fabric (sadly, this stuff costs a fortune - I have looked into similar ideas before - got loads of samples and only the most expensive was suitable)  .. Like a bicycle handlebar.. metal bar (sensor) in the centre.. Squeeze the handle and the capacitance lowers, the more fingers, the lower the capacitance.. wire both ends of this squidge capacitor to an oscillator and one (with a reference oscillator + heterodyning) has a pitch only squidgymin.. Use the other as a volume control.... These bars need not be physically connected, one could be held in each hand.

Now if one had the ability, you could play a theremin at the same time as playing the squidgymin - duophony with independent control of both pitch and volume on each!

LOL ;-) .... But if you combine squidgymin (low latency) with theremini, you could use the squidymin to play the theremin parts, and the theremini to play..... something else (?)

Fred.

Someone already invented the squidymin. Don't listen to this if you value your ears...

http://youtu.be/PxLB70G-tRY

The difference between an on/off trigger and a proportional sensing device is the size of the sensitive zone. For a trigger we want the smallest movement possible - if we can detect a compression of one micron that would be fine, but for something that detects foot movement and uses it to ramp the volume up or down I guess about a centimetre would be more appropriate. This is, I imagine, outside the practical range for a squishy pad and more in the range of a rigid plate that moves up and down under the foot.So I looked at some options and ended up at bass drum pedals. And this reminded me of a toy I made a few years ago - a magnet powered ballista that would fling a small projectile an impressive distance. Combining these two ideas led to this...

It is mechanical, but trivially so - one moving part with a hinge at one and neodymium magnets in opposition at the other end to push it back to the up position.

It fits the criteria well - the hinge end is very narrow and the magnets' inverse square law behaviour means that a downward motion can be initiated with a light touch, while the force from the magnets to move it up from the lowest position will be strong. The highest position would correspond to full volume, while the silent position would be adjustable via the potentiometer - I imagine about half way up would be optimal to allow some room for the plate to accelerate on its journey upwards. The velocity of the plate would vary with the player's foot pressure and the position of the foot - near the hinge or further towards the magnet end.

Having drawn it, it occurred to me that while it could use capacitive sensing, a resistive flex sensor would do just as well and be a lot simpler.

"The difference between an on/off trigger and a proportional sensing device is the size of the sensitive zone. "

Hi Gordon,

Nice diagram!

I think the confusion is perhaps getting deeper rather than shallower ;-)

For a "proportional control" where one wants to actually use force / positional data data as a continuous real-time parameter, what you say is correct.

But IMO, that could be comparable to a keyboard with aftertouch, versus a keyboard with velocity sensing only.. The aftertouch keyboard requiring a "control zone" the velocity keyboard only requiring two closesly spaced trigger points.

Another analogy (not a method I believe could be of use in your pedal, I must say in advance) is a Piezo sensor (as used for electronic drum pads) - with these one can obtain strike force (the "trigger" has variable amplitude depending on the force with which the piezo is whacked, and the amplitude is captured to determine the value of the attack peak - but further control is not possible.

The basic keyboard without velocity is the equivalent of your proposed requirement - sensing only an on/off condition, the velocity keyboard is my 'preference' and certainly something I would leave 'hooks' for at least if I was looking to manufacture the pedal..

In terms of sensitivity, there would be no difference - as soon as the minimum force 'trigger' was detected, the attack cycle would start - the 'velocity' could be determined by several different or combined means and perhaps  could be increased at any time during the attack phase only.

But it would require more elaborate firmware, and could be added later at any future time without any change to mechanics or electronics - so for now there's no point in thinking about it or discussing it further ;-)

Fred.

"Someone already invented the squidymin. "

The atomatone !?

Me thinks you visualize my concept entirely erroneously ! ...

Imagine one had a Tribble in each hand, a wire from each tribble running up each sleeve of your federation suit, and doen your trousers, where they emerge and are connected to the control unit.. The wires are screened so they improve your e-field gronding for other e-field instruments..

The tribbles EEG signals are used - one to control pitch, the other to control volume - These EEG signals are modified by the manner one squeezes, strokes or mangles the pests.

(Histerical note: due to concerns over potential interruption to the performance by tribble death, or protest from the alien life anti-cruelty lobby, the more primitive capacitance sensing squidgymin became the instrument of choice)

I think we just found your inner Klingon, Fred. They hate Tribbles! :-)

Thanks for the kinds words re: the diagram. I tried drawing it by hand, but it looked awful, so I installed a free 3d design app into my iPad and spent a good while drawing a little bit then searching the manual for the next step then drawing a little bit more. It's the sort of thing that encourages you to ignore low battery warnings! And then my battery died and I lost it all. So in the morning I downloaded the equivalent app for my iMac and drew it again.

I see the source of the confusion.

There are two ways of seeing the pedal.

1. A giant synth key. In this model one would measure the velocity of the key on the way down by timing its travel from top to bottom and using this to define the slope of the attack (or release in "wrong way round" mode) and then time it's travel from bottom to top and use this to define the slope of the release (or attack in WWR mode.) (No need for aftertouch, the volume loop is effectively aftertouch. Just velocity sensing in both directions.)

2. An expression pedal with a very light, fast action and an auto-return mechanism. In this model the height of the pedal is measured continuously, and mapped directly onto the audio signal (i.e. Pedal up = silence (WWR - max vol) pedal down = max vol (WWR - silence) and when the pedal is only half way up it is neither up nor down, but at half volume.)

I don't see any need to choose before building a prototype - both can be done in software - with a few more pots and switches, we could have both.

Gordon,

Further to your opposing neodymiums.. This is a really interesting idea for a switch / pad - the action would, I think, have an inverse-square feel - easy to push at first,  requiring more force the more pressure is applied. - great!

And it could work well with a shielded capacitance sensing plate - double sided PCB with shield layer facing the bottom, and sense layer facing up.. Perhaps no need for squidge!

(Personally, I much prefer the idea of mechanical to unknowns like the compression of rubber or whatever - Oh, I know that this is due to ignorance of the field, and that the chances of problems with mechanics are probably greater, and squidge would be cheaper and easier. .. But its just the way I feel..)

Fred.

ps - might be a good idea to avoid ferrous metal for the construction of the pedal - I have seen weird things happen to magnets in opposition when their circuit is closed - there have even been tales of them flipping their poles - but I have never seen this (one of the many magnet based free energy claims - push them close enough, do some hocus-pokus, then capture the free energy with a coil when they flip ;-) .. I dont doubt that there may be conditions under which they could flip - but strongly doubt one could ever get more energy out that what one put in to get them to do this.... But who knows - and if we can eliminate the pedals power supply, why not! ;-)

I'm good with mechanical side too. The worst that can go wrong with it over time would be a squeaky hinge, and that's easily fixed. 

It occurs to me that while it is not exactly the floor standing volume only theremin that I originally envisaged, it is a capacitive sensing levitating lever. It's a levermin! Do you think Lev would approve?

Question - do we need to use digital tech if we just go for the "expression pedal with a very light, fast action and an auto-return mechanism" model rather than the "measure the velocity and shape an envelope from it" model?

Oh, and there would still be some squidge involved. Well, some thin impact absorbing layers to stop the pedal from clicking at the top and bottom of its travel. 

"Question - do we need to use digital tech if we just go for the "expression pedal with a very light, fast action and an auto-return mechanism" model rather than the "measure the velocity and shape an envelope from it" model?" - Gordon

Sorry Gordon, I am not following this.. I think ive been infected by some AU in one of the crazy videos recently ..

Reasons for digital:

simpler and lower cost control of ADSR. If CapSense is not used, any MCU with an ADC and usual timer/PWM etc could probably be used.

Reasons for PSoC:

Reconfigurability and Capsense primarily.

The above can also work with

(1) simple contact.

(2) could be adapted to 'keyboard' velocity type changeover contact.

(3) can also work with analogue sensor (FSR,HALL,CAPACITANCE etc) in either (a) Single contact mode (b) changeover velocity contact mode (c) continuous analogue mode

The big advantage to the above is that the interface to the sensor can be implemented entirely in the PSoC and changed with configuration / firmware.

The big disadvantage to a analogue front-end is that one must get the sensor right and stay with this - as in, the front end can only be developed with a sensor / pedal assembly close to its final form.

The ""measure the velocity and shape an envelope from it"" was, I thought, just a possible future option .. I saw the primary requirement as "light touch for on/off - drive envelope from this" as defined by your specification.

The simplest:

If we dont need to worry about amplifying a 'sensor' and determining a threshold (as in, a single make / break contact is available) - OR if we are using a FSR as a switch with a simple comparator - Then, for a simple AD envelope analogue is possiblyy easier.

BUT:

When we start to add anything beyond simple AD, even adding invert and certainly if adding ADSR and reverse ADSR, the digital option has clear benefits - the ONLY big benefit analogue has is that it could be put together by any hobbyist and eventually be made to work.

CapSense:

My "And it could work well with a shielded capacitance sensing plate - double sided PCB with shield layer facing the bottom, and sense layer facing up.. Perhaps no need for squidge!"

Is only possible due to the shielding function - Active shielding effectively eliminates any ground influence from the shielded side.. Its how I have made conductive objects "invisible" to the theremin and implemented directional antennas. The problem with shielding is that the shield must be driven at exactly the same voltage and perfectly track the signal on the sense antenna - it is a bugger to implement on high voltage antennas.

Capsense now provides shielding - without shielding the bottom side capacitance of the sensor would reduce sensitivity and hugely increase drift - It may be that there would still be enough sensitivity - but there will be at least a 100% and probably more like a 600% improvement by using shielding..

Without (active) shielding, all the prior arguments against using capacitive sensing return..

The "sandwich" sensor arrangement with ground on both sides of the sensor overcomes the need for shielding because both grounds are 'squidged' towards the sensor - this should increase the capacitance change, and the 'bottom side' capacitance is not merely a "capacitive load" on the sensor...

Also, the way CapSense works (and here I am talking about an area I am not an expert in, so I really cannot say how well it works) is by monitoring base-line drift so that slow changes in the base-line due to thermal factors or whatever are compensated.

To me, changing the pedal mechanics is a side issue if one is looking for an 'on/off' detection, and a side issue if one enhances to dual threshold "velocity keyboard" detection - it only becomes an issue if one is looking for proportional analogue control.

Perhaps my DAC/ DCA frightened you a bit.. Sorry ;-) ..

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Hi Fred, no your DCA/ADC didn't frighten me - I didn't entirely understand it, but I'm used to that. It was more your post at the bottom of the page before last pointing out that we could be carrying some assumptions over from the initial proposal, which prompted me to challenge every part of the "yellow pearl" specification.

So I revisited the "flat plate sensor on the floor and no moving parts" assumption which had led to the discovery that the best we could achieve with a thin, flat sensor was a simple on/off trigger, and that led to the rest of the "yellow pearl" specification. The extra understanding that I have accrued during the design process led to the "drum pedal with magnets" idea, which put capacitive distance sensing - very much like the initial proposal - back on the table. 

Now I have challenged the "we need to go digital" assumption. Thank you for providing a very clear answer to that. Yes, we do need to go digital if we want shielded capacitive sensing because it is complicated and the PSoC 4 does all the hard work for that, and because going digital provides a lot of envelope shaping options that would also be deeply complicated without a digital processor.

So there's going to be a new spec to supersede "yellow pearl" - the Levermin specification, based around the drum pedal with magnets. I'll be thinking about the user interface - the knobs and switches - I don't want a display with menus - and what the software will do, but I'm feeling that the hardware aspect is settled now - a levitating lever feeding a PSoC 4 via shielded capsense (with the caveat you pointed out that this may make it hard to find a builder, so retaining the option to switch to a flex sensing resistor) and the processor driving an analogue VCA.