Multi-touch: Many questions to one desire....
joerg.twinklephone at gmx.de
Wed Mar 5 05:04:10 CET 2008
Am Mi 5. März 2008 schrieb Marco Trevisan (Treviño):
> joerg ha scritto:
> > I'm investigating to squeeze some multi-touch properties out of the GTA02
> > 4wire-resistor touchscreen, by hacking the basics and probing the ts in a
> > couple non-recommended completely different manners. There still is a
> > little hope...
> What's your hope exactly?
> Do you have some "secrets" to share? :P
Well, it's basically a 4wire (2 metal coated transparent plastic foils)
design: you apply + and - at X+ X- which creates a linear voltage gradient
over the one foil, and you messure the voltage at any of both Y of the point
where the second foil touches the first. (and vice versa x<>y, for other
coordinate). So much for the classics.
However there are a couple of other permutations to apply + and minus and/or
leave open and where to probe for voltage, given you have 4 wires and 3
states (+,-,probe+open) for each one of the 4 to choose from. Hard to explain
without some graphics showing a simplified circuit diagram.
And then, if all this fails, there still remains the dynamic approach
(pulse-response), for there are two planes creating a R-C-R-C-R... network,
resembling somewhat like e.g. a coax cable. You know you can analyze coax
(10Base2 ethernet, even BaseT) for position of sharp bends with an analysis
of pulse response. Somewhat like hitting a drum and telling from resulting
sound where there are the 2 stones on the drum membrane.
All this *VERY* theoretical anyway for now, for i got scope etc, but even
don't have real hw to test with it :-( (who was the guy with a fried NEO and
no idea what to do with it? I had a real use for it, even better when it's
broken), and i was too lazy (or too poorly gifted) to do the math yet.
What i expect/hope: At least i now got diagrams, so i know what _can_ or
_must_not_ be done with given hw in GTA01/02. Just dreaming for now... ;-)
But i'm quite sure it's feasible to get at least kinda force feedback for the
single touch. Maybe more... Amazing what 12 formulae with 5 (10) unknown
variables(R) can reveal (ok, this are just numbers for instance, didn't the
When it comes to dynamic pulse response analysis - with all sorts of filters
and the whole scary stuff (any volunteers?), we probably even get the weight
of user-C touching the screen ;-) In an ideal world. A/D converters of CPU to
sample the response are not _so_ bad.
All this [patent pending, (C), smells like me etc]:jOERG ;-). At least mention
me, and send me a beer each day, and this is prior art. Ehrrm.
This week CeBIT, let's see what's my schedule for next week.
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