[sdiy] A different kind of polyphonic aftertouch

rsdio at audiobanshee.com rsdio at audiobanshee.com
Fri Jan 5 11:50:23 CET 2018

I’m certainly not a physics expert, but I’ve long considered the question of whether velocity as a time measurement (fixed distance divided by varying time) is a reasonable measure of impact. The answer that I came up with is that it would seem impossible to move ones fingers faster and faster while generating less impact pressure, and likewise that it would seem impossible to increase the impact pressure of your fingers without also moving faster. Whether speed and impact match up perfectly or not is perhaps a more detailed question, but they certainly are related and not possibly in opposition. What little I know of physics is that Force Equals Mass Times Acceleration, and thus it seems like a reasonable approximation to measure Velocity in order to determine Force (or impact as I was calling it).

As for the Velocity Curves that are available on synths, I would not say that they are evidence that the velocity measurement itself is flawed, but more that it needs a little tweaking to be useful.

I estimate that the largest factor in these Velocity Curves is that human perception is relative, and often logarithmic. Pitch and Decibels are relative, such that a few Hertz in the lowest octaves might span a Half Step whereas a few Hertz in the highest octaves might go completely unnoticed (in isolation - without beating against another oscillator). Every octave has double the Hertz of the lower octave, and thus Pitch is logarithmic and the perceived interval is relative to the starting Pitch; conversely, the perceived interval is not linearly related to Hertz, because it depends upon the octave. Similarly, every 6 dB increase in perceived loudness corresponds to a doubling of power, so Voltage or Wattage needs ever increasing values to produce the same perceived loudness increase. This is another logarithmic relationship, where doubling the measured value is require to produce a linear change in perception. Seems like everything we hear is on a curve.

To put it more simply, we probably need some kind of Curve in order to perceive the parameter changes in a meaningful way, and that is why the simple Velocity time measurement benefits from mapping onto various curves.

I also imagine that a smaller factor might be that the circuits measuring the Velocity timing have a fixed time resolution, and this gives more resolution at softer force levels and less resolution at harder force levels. Since Velocity is Distance divided by Time, then the fact that time is discrete steps means that the fractional results have varying resolution. Long times mean slow Velocity; short times mean fast Velocity. At some point, though, the Time steps from 0, to 1, to 2 ticks of some discrete clock, and that’s like changing from Infinity, to 1, to 1/2. Continuing on, you get 1/3, 1/4, 1/5, and you can see how the Velocity steps get smaller and smaller, meaning the resolution increases. I tried to calculate the resolution of the eighties and nineties synths based on their crystal frequencies and Timer peripheral resolutions - I can’t remember what I came up with, but the Velocity measurements are far from exact or linear. Since MIDI only allows for 127 Velocity values anyway (and the DX7 only went to 100, in the earliest models), and humans probably don’t have super fine control over the range and variation of force, I don’t think there’s a lot of loss to worry about here.

In other words, I think that the Velocity Curve choices might help make up for the fact that the measurement resolution varies with force. I’m not 100% certain about this one without running through the numbers, though.

By the way, in addition to the Roli Seaboard and Continuum, there is also the Eigenlabs Eigenharp and the Madrona Labs Soundplane. The Eigenharp primarily has keys with four optical sensors on each edge, such that pressure as well as slight movements in other dimensions can be sensed. The Soundplane has a custom capacitive sensor with a neoprene dielectric that can sense a very wide pressure range at five hundred points and interpolate between them to track multiple fingers in three dimensions. The Soundplane supports both MIDI and OSC - while we’re familiar with the limitations of the MIDI protocol for things like non-piano-keyboard musical expression, you might be interested to know that OSC had to be expanded to handle three-dimension touch data (prior to t3d, OSC really only supported non-velocity, two-dimensional controllers like the monome).

I think the biggest challenge is the unfamiliarity, but that also holds the greatest promise. If a musician were to put in the time to learn any of these new controllers, the ability for expression would probably far exceed the typical MIDI piano, MIDI saxophone, MIDI guitar, MIDI drum, or whatever other traditional instrument might be mapped onto a modern sensor.

Brian Willoughby

On Jan 4, 2018, at 8:51 PM, Mike HEQX <mike at heqx.com> wrote:
> Good points Tom. Ok so velocity sensitivity is the attack velocity, in the piano paradigm, yet synthesizers are not pianos and their actions are not hammer oriented. Conversely pianos do not have aftertouch.
> I was referring to the velocity curve that you can select on some digital synths. It's as if your fingers can't tell the instrument what you want it to do directly because, it has to have an additional layer of processing to simulate some type of mechanical action that is not inherent in the instrument, thus it is psuedo-realistic sensing because you are playing one way and the instrument is artificially creating a response. Now that is not to say it is a bad thing to have artificial curves as many controllers do, but it is certainly not directly representing actual playing physics.
> If you read the descriptions from some manufacturers of their velocity curve settings you can see that they are compensating for deficiencies in the human player, from the physical interface as well as the conversion to midi data.
> "All that said, more sensing and different sensing is all good, so I agree with the thrust of your argument. I’d still like to see a serious return of polyphonic aftertouch - how is it possible that so many years after the CS80 that isn’t a *standard* feature on all synth keybeds?!?"
> I agree heartily and I'll add that something like the Roli Seaboard or the Continuum are breaking that piano paradigm even though they have a key basis that is familiar. Now we need more interfaces that are familiar ( beyond the saxaphone, and trumpet) and also some that may be even more efficient and intuitive than the piano key layout.
> Mike
> On 1/4/2018 6:10 PM, Tom Wiltshire wrote:
>> There’s nothing “pseudo” about measuring the time something takes to get from A to B and then calling that velocity. Ok, “average velocity” if you like, but it’s definitely velocity. Saying that the attack curve is “artificially generated” is a bit of stretch too. There’s a relationship between how hard you hit and the eventual velocity value. Once upon a time, that would have been organised mechanically, but now it makes sense to do it in software and make it more flexible. It’s no more artificial than using  bits of trees and elephants tusks and cast iron and wire to build a mechanical solution.
>> Similarly, for a hammer action, velocity sensitivity *is* attack velocity, since once the hammer leaves the key, you haven’t got any control over it, despite all those pianists massaging their keys. Synth keypads do better than hammer action in that measuring release velocity is actually possible and realistic, even if rarely done (and rarely that useful, I’d argue).
>> All that said, more sensing and different sensing is all good, so I agree with the thrust of your argument. I’d still like to see a serious return of polyphonic aftertouch - how is it possible that so many years after the CS80 that isn’t a *standard* feature on all synth keybeds?!?
>> Tom
>> On 4 Jan 2018, at 22:29, Mike HEQX <mike at heqx.com> wrote:
>>> As we see in the Ensoniq SQ-80 there is no physical switch. It was a change in inductance that was being measured and they did have a rubber pad under the key and they did give you polyphonic aftertouch ( not related I know ). So they used inductance, and someone else uses light, and yet another uses capacitance, and these are all linear sort-of systems that in my opinion are better than the two switch method of generating a psuedo-velocity sensitivity that measures time.
>>> So really the majority of synth keybeds that claim velocity sensitivity are really just sensing attack velocity. Some offer a choice of attack curve, which is artificially generated for you. Very few are actual hammer actions ( in the piano paradigm ), and very few are doing anything unique. Long live linear sensing!
>>> On 1/4/2018 2:36 PM, cheater00 cheater00 wrote:
>>>> You put a spring or piece of rubber under the key and suddenly the
>>>> linear sensor is a pressure sensor.
>>>> On Thu, Dec 14, 2017 at 6:46 PM, ASSI <Stromeko at nexgo.de> wrote:
>>>>> On Thursday, December 14, 2017 5:09:40 PM CET MTG wrote:
>>>>>> https://deskthority.net/wiki/Adomax_Flaretech
>>>>>> "There are no electronics in the switch modules themselves. Instead,
>>>>>> surface-mounted infra-red optoelectronic components on the circuit board
>>>>>> provide the sensing in conjunction with a prism in the module's
>>>>>> transparent slider."
>>>>> So a vertical mouse, sort-of.  Which means it's very likely measuring key
>>>>> travel rather than pressure, which is a lot less useful for a musical
>>>>> keyboard.
>>>>> Regards,
>>>>> Achim.

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