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#2508986 - 02/10/16 07:06 AM Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha]
vince961 Offline
Junior Member

Registered: 02/09/16
Posts: 17
I heard an opinion that DP manufacturers use velocity sensors to measure an average velocity between sensor #1 and sensor #2. This average velocity later gets translated into MIDI velocity.

Question #1
What's the point to measure velocity and not the energy/impulse that is passed to hammers? Wouldn't that break the whole idea of action authenticity? I.e. one cannot play fast but quiet on DP (well, you know what I mean) So vendors, like Kawai, introduce GF actions to imitate the feelings - but not the physics. Why manufacturers would not add a special sensor that would be reading the strength of the impact in the lowest key position? What are the reasons to deliberately refuse building a truly authentic keyboard - the one with impact sensors at the end of the keyboard mechanism?

Question #2
They say that the most of DPs have their sensor #2 25%-30% above the lowest key position. So not only they use an average velocity instead of the energy/impulse but they also measure this velocity in an odd way. I heard that Korg SV-1 is one of the rarest kind that have the second sensor at the bottom. Kawai-65 has it 25% above the lowest key point. Yamaha is approximately the same or even worse. And so on. Why do they place the second sensor above the lowest point?

Does it make sense? Could anyone answer the questions or dispel my delusion? James from Kawai may be?

Thank you!

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#2508987 - 02/10/16 07:20 AM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
Pete14 Offline
500 Post Club Member

Registered: 06/12/13
Posts: 543
I believe that the Alpha piano uses 'impact sensors'.

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#2508997 - 02/10/16 08:14 AM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
alerique Offline
Full Member

Registered: 12/02/15
Posts: 127
I guess, it's physics. Kinetic energy of the impact E = (mv)^2/2, where m is the mass of the hammer and v is its speed. As m is a constant you can just measure velocity for the same result.

Further on, as the hammer goes flying after a short interaction with a key it basically moves with a constant deceleration defined mainly by the friction. In this case, you can measure the timing between any two points along the hammer's trajectory and calculate speed accurately. Having a longer track might give more accuracy, but doesn't change the model in principle.

Thus in terms of physics what they measure is completely all right. Accuracy is another question, but it depends not only on what is measured, but on sensors quality, mathematical model etc.

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#2508999 - 02/10/16 08:18 AM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
PM7 Offline
Full Member

Registered: 01/29/16
Posts: 20
In true pianos, the hammer is released after being accelerated by the key, about 20-30% before the key reaches bottom, so it's perfectly ok.
Still, many digital keyboards are horrible.

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#2509009 - 02/10/16 08:55 AM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
JoBert Offline
Full Member

Registered: 10/27/15
Posts: 181
Loc: Germany
Originally Posted By vince961
Why manufacturers would not add a special sensor that would be reading the strength of the impact in the lowest key position?
Because that's not how playing an acoustic piano works. Even on an acoustic, it's not the pressure that is applied to the keys that determines the volume of the sound, but the speed with which they are depressed.

Again: Not the pressure (of key into keybed) matters, but the speed, even on an acoustic!

That's because of the way how the acoustic action is constructed: The (very simplified) chain of events is, that when the player presses the key down, the other end of the key goes up, which moves up the wippen, which in turn causes the hammer to move up. At a certain point (which is not the lowest point of the key travel!) the so called "let off" occurs, i.e. at this moment, the hammer is thrown up and is no longer in contact with the wippen (and through it the key). So what you do with the key after this let off point no longer has any impact on the speed of the hammer (and thus the volume of the note).
And the only factor that determines the speed at which the hammer is thrown up is the speed that was used to depress the key from its rest position to the let off point.

Actually, a good teacher will tell you that you should not hammer the keys into the keybed with all the force you can muster to achieve forte/fortissimo, because that will, over time, cause injuries. Instead, a good pianist, will slow the downward movement after let off, explicitly so that the key is not "slammed" into the keybed.

So if a DP would measure the pressure/force at the end of the key travel, not only would it work differently than an AP, but exactly those pianists with good AP technique would be unable to properly play on such a DP. Making the DP unusable as a replacement/substitute for an AP.


Edited by JoBert (02/10/16 09:15 AM)
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#2509029 - 02/10/16 10:21 AM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
NormB Offline
Full Member

Registered: 04/02/12
Posts: 88
Loc: Vancouver, BC
Quote:
I guess, it's physics. Kinetic energy of the impact E = (mv)^2/2, where m is the mass of the hammer and v is its speed. As m is a constant you can just measure velocity for the same result.


Well, there is also the matter of factoring in acceleration. On an AP you can get to the final velocity of the key/action/hammer through different acceleration paths. An AP grand in particular gives one quite a long stroke to get the hammer up to speed through a slight but more or less constant acceleration. There also is considerable more inertia to contend with, which I think helps one's control. DPs don't seem to care anything except final velocity. I think this is one reason why it is easier to control soft playing on an AP grand than any conventional DP.

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#2509049 - 02/10/16 11:14 AM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
alerique Offline
Full Member

Registered: 12/02/15
Posts: 127
NormB - you must be quite right in terms of how action works, however as for the resulting sound only the final speed matters, not the acceleration path. So it's quite ok to measure it. How to improve the action is the whole different matter I guess.

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#2509103 - 02/10/16 01:29 PM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: alerique]
NormB Offline
Full Member

Registered: 04/02/12
Posts: 88
Loc: Vancouver, BC
Originally Posted By alerique
NormB - you must be quite right in terms of how action works, however as for the resulting sound only the final speed matters, not the acceleration path. So it's quite ok to measure it. How to improve the action is the whole different matter I guess.


I concur that nothing else but the 'hammer's' final velocity will determine loudness on an AP (recognizing of course that hammer mass, hammer hardness and string composition will play a varying role up and down the piano).

But I think acceleration rates should be something for DP designers to consider. If I hit an AP key sharply with high acceleration the hammer will detach from the key with a much shorter key stroke than if I try for a pp sound. In the latter case I get a long, controllable key stroke. Unless I am wrong, DPs just don't care about such things.

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#2509104 - 02/10/16 01:31 PM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
spanishbuddha Online   content
2000 Post Club Member

Registered: 11/08/09
Posts: 2781
Loc: UK
I think alerique has it. By measuring the velocity v, and with a fixed mass, you essentially have the equivalent of the kinetic energy. Besides, is there an off the shelf, inexpensive, reliable sensor that provides a continuous but variable electrical signal for kinetic energy on impact?

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#2509109 - 02/10/16 01:42 PM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
vince961 Offline
Junior Member

Registered: 02/09/16
Posts: 17
Thank you guys for all your replies, much appreciated.

@alerique
It would have been the velocity if it had measured the hammer's velocity. My understanding is that the velocity of the key is measured. And this is where the mass gets lost: Both a tiny kid and a massive adult can press a key with the same speed but the sound may be very different because such key movement passes a very different energy to the hammer. So the mass does matter.

@PM7
Thank you for this info - I guess this justifies the 20%-30% of the second sensor position and answers the question #2.

@JoBert
First of all, great explanation, thank you. Sorry, the lack of proper terminology didn't allow me to properly express myself. I meant a mechanical impulse (~ energy), which is mass x velocity. We move a key with our fingers, it's a single object: key + finger + some mass of a hand/body. This single object moves with different velocity. It's like leveraging, a weak man vs strong man: speed can be the same but the leveraging effect will be different. Essentially, we are hitting the hammer with our hand. So yes, velocity affects the sound but so does the mass. Therefore, an AP player should be able to keep the volume low when playing very fast: velocity is high but the touches are light (the mass is reduced). I suspect this is not achievable on DPs.

There must be a mistake in my logic because otherwise the whole DP industry doesn't make sense.

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#2509114 - 02/10/16 02:00 PM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: spanishbuddha]
Frédéric L Online   content
500 Post Club Member

Registered: 10/29/13
Posts: 551
Loc: France
@spanishbuddha: searching "force sensor", we can see some model. (e.g. https://www.adafruit.com/products/166 Here at 7$)... You need 88 of them, and a processor (or a coprocessor) with 88 analog entry. I don't know if this kind of sensor is adequate, which sample rate is needed (the force will be high a very very short time !)

Not as simple as an array of 2x88 switches. (put in an array of 15x12 you just need 15 digital outputs and 12 digital inputs)
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#2509123 - 02/10/16 02:17 PM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
krikorik Offline
Full Member

Registered: 08/25/09
Posts: 138
Loc: AZ
NormB may be correct on his acceleration statement. There is not only the kinetic energy stored on the hammer but there is elastic deformation energy stored on the hammer shank that can change the time of contact between the string and the hammer. Subtleties that still the digital world lacks behind

https://youtu.be/_BRvtUGDHJ8?t=38s

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#2509148 - 02/10/16 03:30 PM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
WarrenY Offline
Full Member

Registered: 10/08/15
Posts: 80
Loc: Denver, USA
Originally Posted By vince961
And this is where the mass gets lost: Both a tiny kid and a massive adult can press a key with the same speed but the sound may be very different because such key movement passes a very different energy to the hammer. So the mass does matter.


@vince961
... and this is where your logic was flawed. The AP key-hammer latching mechanism is designed to decouple the player's finger/hand mass. Only the mass of the hammer and the hammer striking velocity matter.

So yes, as long as the striking velocity speed is the same, the loudness from the piano strings will be the same - by design - whether it's a tiny kid or a massive adult.

Only the noise/energy of bottoming to keybed will be different - more for a massive adult as you predicted.
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#2509225 - 02/10/16 06:31 PM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
Charles Cohen Online   content
2000 Post Club Member

Registered: 12/26/12
Posts: 2700
Loc: Richmond, BC, Canada
This discussion is verging on the "religious argument":

. . . "Is the velocity of the hammer, when it strikes the string,
. . . the only thing that determines the tone of the note?"

I have an opinion on this question, but I won't try to convince anyone.

If I were looking for an action that was closest to working like an acoustic action -- actually releasing the hammer from the key, and measuring the velocity of the hammer (or its kinetic energy) -- I'd check the "hybrid pianos":

. . . Yamaha NU1, and N1/ N2/ N3 "Avant Grand", and

. . . the newly-released Casio "grand hybrid" DP's (GP-300 and GP-500).



Edited by Charles Cohen (02/10/16 06:53 PM)
_________________________
. Charles
---------------------------
PX-350 / microKorg XL+ / Pianoteq / Lounge Lizard / Korg Wavedrum / EV ZXA1 speaker

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#2509228 - 02/10/16 06:42 PM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
Mental Nomad Offline
Full Member

Registered: 10/17/11
Posts: 67
Originally Posted By vince961

It would have been the velocity if it had measured the hammer's velocity...

[clip]

There must be a mistake in my logic because otherwise the whole DP industry doesn't make sense.


I think your logical error is in thinking that the DP takes the key velocity and uses that directly as the volume of the note.

In fact, there is a very finely controlled velocity curve used to relate the many possible speeds of the key to various resulting speeds of the hammer, and therefore to various resulting volumes.

While you're correct that in an acoustic piano it's the flying hammer's velocity that dictates the loudness of the sound, it's equally true that the velocity of the key always directly determines the velocity of the flying hammer - as long as you're measuring key velocity at the point where the hammer is released, which, as others have noted, is about 75% of the way down, or 25% above the key bed.

If you measure the key speed at the right point, and use an appropriate formula/curve to determine the flying hammer speed, then you've got the correct result... and the better digital pianos also let you modify the curve to match your taste or preferences for feel.


Edited by Mental Nomad (02/10/16 06:43 PM)

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#2509229 - 02/10/16 06:47 PM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
Mental Nomad Offline
Full Member

Registered: 10/17/11
Posts: 67
Oh, and since most DP's still use a physical sensing mechanism, measuring the hammer velocity directly would like have a large impact on the hammer speed, becoming less reliable and requiring fine regulation. By measuring on the heavier and stronger part of the action - the key - you can minimize the influence of the switch on the mechanism, making it more reliable.

Some of the latest systems do optical velocity measurement, but they are many times more expensive - and some of those are still measuring the key speed, not the hammer speed. It just works well.

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#2509239 - 02/10/16 07:18 PM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
Kawai James Offline
Yikes! 10000 Post Club Member

Registered: 09/06/07
Posts: 11570
Loc: Hamamatsu, Japan
vince961, welcome to the forum.

I'm afraid I don't have an answer to all of your questions. However, I'm inclined to believe that keyboard action engineers working at Yamaha, Kawai, Roland, Casio, Korg, Fatar, etc. are all committed to making a digital piano feel and respond as closely as possible to an acoustic piano.

By the way:

Originally Posted By vince961
I heard an opinion...

Originally Posted By vince961
They say that the most of DPs have their sensor #2 25%-30% above the lowest key position.

Originally Posted By vince961
I heard that Korg SV-1 is one of the rarest kind that have the second sensor at the bottom. Kawai-65 has it 25% above the lowest key point. Yamaha is approximately the same or even worse.


May I ask where you heard/read these opinions/generalisations?

Kind regards,
James
x
_________________________
Employed by Kawai Japan, however the opinions I express are my own.
Nord Electro 3 fan & occasional rare groove player.

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#2509333 - 02/11/16 02:07 AM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
mezzopiano Offline
Junior Member

Registered: 11/15/15
Posts: 13
Very interesting post!

As I understand, only the final (@25% from keybed) *instantaneous* speed matters in order to calculate the sound intensity of a key (loudness is not a physical measure, but a perceptual one).
Of course the calculation is not trivial, but through a pretty complex mathematical model governing the mechanical process of the key action.

That means, given external parameters such as air temperature / humidity / string material / tension / material tiredness / hammer composition etc., the only user-related parameter which affects the final intensity is the final instantaneous speed.

However, I kindly disagree that it is being measure in today's DPs (except for optical continuous sensor based DPs). What is being measured by most models is the *average* speed between the sensors, which may or may not be associated with the final instantaneous speed (and the sound intensity), depending on whether the user's speed curve throughout the key depress is uniform or not.

So I think it's in the details, but still DPs have a long way to go in order to fully represent all the precision aspects of an AP.

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#2509340 - 02/11/16 02:30 AM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
mezzopiano Offline
Junior Member

Registered: 11/15/15
Posts: 13
Thinking about it some more, I actually wonder whether in a triple sensor system, they use the third (middle) sensor to get a better representation of the final instantaneous speed in addition to the "original" use of it for fast trills or re-pressing a key.

If they put the third sensor closer to the bottom one, such DP could have a really good representation of the intensity you'd get on an acoustic (but I guess the re-pressing performance will go down...)

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#2509350 - 02/11/16 03:13 AM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: mezzopiano]
Frédéric L Online   content
500 Post Club Member

Registered: 10/29/13
Posts: 551
Loc: France
Originally Posted By mezzopiano
Thinking about it some more, I actually wonder whether in a triple sensor system, they use the third (middle) sensor to get a better representation of the final instantaneous speed in addition to the "original" use of it for fast trills or re-pressing a key.


When you repeat a note, only the middle and the bottom sensors are involved. I don't think they use two different way of measuring velocities, then I think the middle sensor is always involved and if it is near the bottom one, the measured velocity is close to an instantaneous velocity.
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#2509376 - 02/11/16 05:41 AM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
vince961 Offline
Junior Member

Registered: 02/09/16
Posts: 17
@WarrenY
The AP key-hammer latching mechanism is designed to decouple the player's finger/hand mass.
I am not sure if it's possible to implement a mechanical device that would neglect the mass component of an impact.

@Mental Nomad
In fact, there is a very finely controlled velocity curve used to relate the many possible speeds of the key to various resulting speeds of the hammer, and therefore to various resulting volumes.

That's correct but note that an equal key velocities may result in different hammer velocities. Identical key velocities may incur different hammer velocities. Key and hammer velocities depend on each other like this: Mass_of_finger/hand/key x Key_initial_velocity = Mass_of_hammer x hammer_velocity. An impulse from one object is passed to another (note that the hammer's initial impulse and the key resulting impulse are zero due to their velocities).

@Charles Cohen
Thanks for mentioning that.

@Kawai James
Thank you for getting in! Actually, there is only one question: why most DP manufacturers (OK, - Kawai) prefer to use key velocity instead of key impulse (or hammer's velocity)? Such approach effectively destroys the authenticity - at least in theory, right? What's the reason behind ignoring the mass component?

I can only guess that:
- Imitations are cheap and well-known in production. E.g., no way we want to take a different path as long as the general public buys the imitators with a great commercial success.
- Non-trivial technical solutions must be implemented. E.g., costly, non-trivial, technology that is not ready yet.
- Little-to-none difference against the current solution. E.g., we did some tests but pro piano players could not tell the difference.

But none of these items convince me.

Please do not get me wrong - I am not holding you accountable neither for Kawai nor for the whole DP industry. I am also far from questioning the skills of Kawai, Yamaha, Roland, Korg, etc engineers - that'd be silly. I am just trying to understand WHY? I believe there must be some explanations from engineers about that. So from one side I see brilliant teams, from another side I see (I think I see) non-authentic imitators. That gives me a cognitive dissonance.

To you question: let me ask you first - does an answer to your question matter? OK, you know where I got these opinions from - would that change anything? If these opinions are incorrect in their essence then please let me know and we will close the topic. If this opinion is correct then it doesn't matter who had it originated, right?

Let me put it this way: it's a consolidated opinion from a small group of pro AP players who also have a solid experience with various contemporary DPs / VST and technicians who assemble/fix/tune DPs - which I obviously do not belong to. Please note that there are AP players who are not technicians but there are NO technicians who are not a pro AP player.

However, these details are irrelevant. The biggest question "why the mass is being ignored?" is an independent question.

P.S. To re-iterate - I am not confronting/blaming anyone, just trying to understand the situation. If I am wrong - please let me know where.

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#2509381 - 02/11/16 05:53 AM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
Frédéric L Online   content
500 Post Club Member

Registered: 10/29/13
Posts: 551
Loc: France
I don't understand "why the mass is being ignored?". The mass for a given key/hammer is a constant. No need to measure it.
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#2509398 - 02/11/16 08:11 AM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
krikorik Offline
Full Member

Registered: 08/25/09
Posts: 138
Loc: AZ
Vince, you have some conceptual errors on the reasoning. Simplifying it the acceleration of an object depends on its speed history. If two keys have the same speed history they have the same acceleration, so their hammers, assuming the key and hammer are linked, will have the same acceleration. What is going to change for you if the force you'll make on the key, for a heavier hammer larger force.
If you apply the same force on a heavier hammer key than on a light one, their speeds are going to be different.

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#2509405 - 02/11/16 08:28 AM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
prout Offline
1000 Post Club Member

Registered: 11/14/13
Posts: 1919
Loc: Southwestern Ontario
As a classical acoustic pianist, who has a M&H BB grand, and two DPs, I find this whole discussion fascinating.

Re-reading the posts, I find no one is actually discussing how a basic midi keyboard works. It's all about time people. There are no 'velocity' sensors, or 'acceleration' sensors. The only thing that matters is the length of time that elapses between sensor one being activated and sensor two being activated. The distance is known, the time is known, therefore the average velocity is known. If you choose to slow down the key movement at the end of the key depression, the average velocity will be lower. Mass, inertia, acceleration all have an effect on the feel of the keyboard, but have no effect on the average velocity.

You can stick a resistance or capacitance based force sensor on the bottom of the keyboard, but why would you? Every pianist knows that any energy put into the keybed is just wasted energy and potentially damaging to your hands. The keybed is a great place to rest your fingers on an acoustic piano, but nobody plays the keybed, only the keys. Try a fast trill. The repeat is all in the first 75% of the key travel.

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#2509419 - 02/11/16 09:26 AM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
WarrenY Offline
Full Member

Registered: 10/08/15
Posts: 80
Loc: Denver, USA
@vince961
You should google or lookup the latch mechanism in a piano book yourself. It is designed to decouple, separate or "let-off" the key-to-hammer connection near the end of a key stroke. This connection is then re-established as a key travels back upwards. Once you understand this, things everyone else is telling you here will start to make sense.

Because of this temporary disconnection, enabled by the latch system, the final impact to keybed does not transfer to the hammers. This is an important characteristic of piano and why it is so easy for any person (no skill needed) to just press a key and get a crispy sound - which is nearly impossible on other instruments, like violin for example, without practice.

On a DP, without real hammers striking any real strings, this "let-off" is completely unnecessary but it's there in many DPs - only for the purpose of simulating the "feel" as on acoustic. And, without real hammers, we can't measure hammer velocity. Measuring key velocity instead and apply a math is what is done and rightfully so.
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#2509439 - 02/11/16 11:03 AM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
spanishbuddha Online   content
2000 Post Club Member

Registered: 11/08/09
Posts: 2781
Loc: UK
I think you've been given answers to your questions, but based on an incorrect understanding of the action and physics, just continue to to ask the same questions.

The mass is fixed, so not ignored but assumed, if velocity is used.

Also you ask, Why is the sensor on the hammer though? All DP's (except the hybrids mentioned) have the hammer directly coupled to the key (there is no escapement), so measuring the key velocity is effectively the same thing.

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#2509442 - 02/11/16 11:09 AM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
vince961 Offline
Junior Member

Registered: 02/09/16
Posts: 17
Originally Posted By Frédéric L
I don't understand "why the mass is being ignored?". The mass for a given key/hammer is a constant.


That's correct. However, when a key is played it's played by a finger/arm, so it's a composite object. You cannot determine the amount of impulse knowing just the velocity of the object. Again, I may be using wrong physics terms but essentially a player's effort gets transformed into the hammer movement. And the quantity of such movement (energy) depends on the FORCE which is a function of both M and V. While DPs only measure V.

Originally Posted By krikorik
If two keys have the same speed history they have the same acceleration, so their hammers, assuming the key and hammer are linked, will have the same acceleration.

This contradicts to the law of conservation of momentum: the keys are not moving just by themselves. They are pressed by fingers - it's a complex system that has momentum. And this momentum is not just velocity.

@prout
You are very correct about MIDI-keyboard mechanics, I think we are on the same page. And this mechanics is exacly the problem - because AP "is not using" an average velocity, it "uses" a momentum / force at point.

Sensors at the bottom? Well, you are right, it appears that it would not make sense. What would make sense is to either measure velocity / acceleration of hammer OR measure the momentum that is passed to hammer and leave the keys alone.

I also think that you are right about the most of energy being dissipated.

@WarrenY
Again, you cannot ignore momentum in a mechanical system. The let-off effect has nothing to do with that. We must consider momentum (m and v) and not just velocity. Do you agree?
I am not talking about the final impact. I do agree that the impact at the keybed bottom is ignored for a reason. What I question is that measuring the average velocity of the key matches the hammer momentum. In AP there is a concrete point where the keys pass the momentum to hammers - so I am talking about measuring the momentum at that point. Average velocity defines a very different physical model of a piano, essentially it introduces a very different instrument (at least, in theory).

And I do not get why the manufacturers would not move from "average velocity" to "momentum" (or force at point, whatever you prefer). It doesn't look too complex in implementation but would allow to get a killer-feature on board.

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#2509444 - 02/11/16 11:21 AM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: vince961]
Garf Offline
Full Member

Registered: 08/27/14
Posts: 101
Originally Posted By vince961
Originally Posted By Frédéric L
I don't understand "why the mass is being ignored?". The mass for a given key/hammer is a constant.


That's correct. However, when a key is played it's played by a finger/arm, so it's a composite object. You cannot determine the amount of impulse knowing just the velocity of the object. Again, I may be using wrong physics terms but essentially a player's effort gets transformed into the hammer movement. And the quantity of such movement (energy) depends on the FORCE which is a function of both M and V. While DPs only measure V.


As already explained, M is constant and known. The fact that there was initially an arm and pianist attached doesn't matter because of the escapement. At the moment the hammer strikes, M is constant and does not involve arms, and V can be measured, thereby entirely defining the system.

I'll repeat again: once the hammer is flying free, its (constant) mass and its (measured) velocity entirely define its momentum.

There's no point in arguing further if you cannot accept this point.

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#2509447 - 02/11/16 11:26 AM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: spanishbuddha]
vince961 Offline
Junior Member

Registered: 02/09/16
Posts: 17
Originally Posted By spanishbuddha
I think you've been given answers to your questions, but based on an incorrect understanding of the action and physics, just continue to to ask the same questions.

Yes, some of my action understanding may be incorrect - and I have that admitted - and will admit.
But what exactly is wrong with my understanding of the physics?

Originally Posted By spanishbuddha

The mass is fixed, so not ignored but assumed, if velocity is used.


The mass of a key is fixed, that's right. But the quantity of energy that is being passed to string can be different for any given key velocity. Do you agree?

Originally Posted By spanishbuddha

Also you ask, Why is the sensor on the hammer though? All DP's (except the hybrids mentioned) have the hammer directly coupled to the key (there is no escapement), so measuring the key velocity is effectively the same thing.

Yes, sorry - I somehow mixed AP with DP. But I am not expected to propose technical solutions anyways, right? Yes, for DP it might have been a system that measures the momentum of a key press at the point of sensor #2 - but that's my guess. In fact, this is my question #1 - why similar system is not used.

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#2509452 - 02/11/16 11:34 AM Re: Avg velocity&sensor positioning vs impact[Kawai,Korg,Yamaha] [Re: Garf]
vince961 Offline
Junior Member

Registered: 02/09/16
Posts: 17
Originally Posted By Garf

I'll repeat again: once the hammer is flying free, its (constant) mass and its (measured) velocity entirely define its momentum.

There's no point in arguing further if you cannot accept this point.

I am not arguing with that, why? But before hammer is flying free it gets the momentum itself, right? It's from player's arm, which is transferred to hammer by (a) velocity of arm/key movement (b) mass of the arm/key. Where is a flaw in this logic?

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