Linked by Thom Holwerda on Tue 10th Jul 2012 01:24 UTC
Microsoft "Microsoft and Perceptive Pixel Inc. (PPI) today announced that they have entered into a definitive agreement under which Microsoft will acquire PPI, a recognized leader in research, development and production of large-scale, multi-touch display solutions." Yes, Jeff Han is now a Microsoft employee. This demo still amazes me - from 2006. Before the iPhone. Before Android. Before the iPad. Remember that the next time you wind up in a discussion about who supposedly invented what.
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RE[5]: I built one too!
by Neolander on Wed 11th Jul 2012 20:27 UTC in reply to "RE[4]: I built one too!"
Member since:

Regarding night clubs, I don't know if that would be enough to perturbate an acoustic touchscreen, but I agree that doing the experiment with a prototype would be a good idea...

I think that tap amplitude and tune are not very important for this design, however, as long as the following conditions can be met :
1/It can be assumed that the acoustic properties of the screen are uniform and there are some absorbers on the sides to address the problem of reflections
2/It is still possible for the mics to detect a reproducible signal that is reasonably far above the noise level

That is because, for the design which I have seen at least, finger position tracking is based on sound propagation delays, not attenuation or wave shape. If we detect a pulse on mic 1 at a time t1, on mic 2 at time t2, and on mic 3 at time t3, we can measure t2-t1 and t3-t1, then deduce spatial positioning information using such calbration data as the speed of sound inside of the screen material and the well-known mic network geometry.

I would gladly draw you a sketch that explains this in more details, but my main computer is currently undergoing repairs and cellphones are not very good for that kind of things.

Since we are only interested in the direct signal flowing from the impact region to the mics without undergoing reflections, rubber casing should not be a problem. In fact, they could even help addressing the problem of reflections and the one of friction on the back and the sides of devices that I mentioned above.

To conclude, I am not sure that acoustic touchscreens could play the same role as capacitive touchscreens either. The guys who worked on them at that lab preferred to market them as a path towards cheap large touchscreens (such as touch-sensitive shopwindows). I was just playing with the idea in thoughts experiments to see what would be the advantages and drawbacks, and found that it might work surprisingly well in that scenario, with much simpler hardware than the capacitive electrode mesh insanity. But I'd probably need to build a prototype (which is way beyond my knowledge of embedded electronics) in order to test some of the potential issues that have been raised here.

Edited 2012-07-11 20:43 UTC

Reply Parent Score: 1

RE[6]: I built one too!
by Alfman on Wed 11th Jul 2012 21:14 in reply to "RE[5]: I built one too!"
Alfman Member since:


Do you think acoustic reflection is that big a deal for software to handle in your model?

Granted, I don't know how much attenuation to expect, but I'd be surprised if it's difficult to differentiate between the primary and reflected signals. My guess is that secondary waves would loose high frequency components far more quickly than low frequency components.

Instead of trying to absorb/eliminate reflections, we may be able to use the information from reflections to our advantage. Consider a sensor on the right hand side of the table that picks up a pulse, shortly followed by one on the left hand side. Ordinarily the algorithm would measure the time lapse difference and register a corresponding touch on the right side. However the pulses may have been triggered by environmental sources off the table.

If the reflection isn't too week, we might be able to prove that the pulse originated on the table because from anywhere else the timing of reflections would be off.

Instead of attempting to write an explicit algorithm for this, it might be easier to use a generic pattern matching algorithm and perform a hough transform across it. This would produce a 2d graphic with intensities corresponding to the areas that have the strongest matches. This could be projected onto the table so that it gets lit up in the spots where it is touched. Then try to fool it with external noise.

Edited 2012-07-11 21:19 UTC

Reply Parent Score: 2

RE[7]: I built one too!
by Neolander on Fri 13th Jul 2012 10:47 in reply to "RE[6]: I built one too!"
Neolander Member since:

Well, theoretically-speaking, a few reflections on the sides of the screen from time to time are not that big of a deal. Without even going into waveform analysis, they would be detected as taps coming from outside of the screen, which can be eliminated by means of simple bound checking.

The first potential issue with reflections, however, is when you have lots of reflections going on with little damping. In this case, standing waves of significant amplitude would form inside of the screen after a short while, and potentially bring a significant amount of noise into the mics for a few seconds (think of a guitar string), which is bad news.

Another potential issue is reflection on the back and the top of the screen. These should probably be quickly damped, but if they aren't, they would result in the detection of multiple taps on a direction of space which the mics aren't sensitive to. I don't know if there would be an easy way to ignore those, maybe an heuristic like "Ignore taps which originate from nearly the same position within a delay smaller than 1 ms" could work.

I don't have enough of a signal processing and acoustics background to know whether your proposal to discriminate taps from the outside from sound from the inside would work, but spontaneously I would think that once an exterior pulse has hit the screen at some point, hard enough for mics to detect a signal, the perturbation would propagate inside of the screen just like a regular tap.

Reply Parent Score: 1

RE[6]: I built one too!
by Laurence on Wed 11th Jul 2012 22:34 in reply to "RE[5]: I built one too!"
Laurence Member since:

I follow what you're saying so no diagram is required (thanks for the offer though).

Actually, you've raised some interesting points there - to the point of nearly convincing me (it's a real pity I can't up vote once I've posted).

I don't think there's necessarily a "right" or "wrong" answer though, at least not without building a prototype and testing it (sadly it's above my level too). I do think you're right about the huge potential of using such a solution for larger surfaces though (if it's not already in wide-spread use).

Reply Parent Score: 2