Subject: Re: [linux-audio-dev] off topic, optical protocols
From: Richard C. Burnett (burnett_AT_tality.com)
Date: Tue Mar 05 2002 - 22:54:15 EET
> That is one variation. But if the "sphere" is really the "disco ball"
> type surface...a sphere that has a large number of small flat surfaces
> that altogether almost are a sphere...then that isn't a problem. Like I
> said, there are variations on the geometry to do different things. The
> base shape plus the surface details (such as facets) for the reflective
> device are half of it, the other half is the base shape of the detection
> system...e.g., flat or curved. Someone else mentioned interferometers,
> this is another variation, one can use interference waves for some
> pretty nice sensitivity. More can be done if the photo detector system
> is interlocked the way the rods or cones of the eye do: neighboring rods
> or cones alter the sensitivity of their neighbors, increasing the
> sensitivity of any otherwise almost imperceptible gradient. One can use
> a single source of light, and go for a single position for information;
> or use multiple sources and look for interference fringes.
Yes, I have worked with interferometer's before, and I can say they can be
extremely over sensitive for some applications. A friend of mine did his
thesis with one and made the mistake of mounting the whole assembly on
aluminium. The changes in air pressure and temperature in the room
GREATLY affected the phase differences and required him to redesign on a
platform of lesser thermal expansion (I think he used like 2 inch thick
steel). So your housing for this system might be VERY dependent on
temperature and pressue.
Also, if you are going to be detecting 'levels' of light hitting the
detectors, then you are going to need to understand the output power of
the light source, its spectral makeup, and the sensitivity of the
detectors. I have seen power plots and they are not simple gaussian
distributions, some have lobes that could give you some really interesting
issues.
With faceted surfaces you might not get the resolution you are looking
for, because changes in location are not going to be that great on sound
waves, and discrete flat reflectors over that range I would think would be
hard to do for quite a bit of ability. The sphere idea still sounds
better (or curved surface really). I can forsee an incredible tolerance
issue with all the parts.
> In the case of the variation that is similar to the rods or cones of the
> eye, with one detector able to alter the gain of another, it will not be
> a problem. They would be interlocked such that one and only one wins the
> war on gain, a kind of hysteresis/extreme case of rod/cone structure. In
> the case of facets, there is no spreading perceptible in such a short
> distance.
So this part of it is analog, because you are not dealing with complete
values. While you could set it for a threshold of gain, what happens if
two get the same value because the light is at the edge. At some point
you are going to need to compare bits, and the circuitry to do that is
going to probably have lots of variance due to part tolerances. IE, like
a large group of comparitors. There would probably be a time delay with
the comparison.
> True, but less than analog means. There is no reverse EMF to dampen it
> like a dynamic coil detector, no capacitive charge to add repulsion like
> in a condensor, no piezo electric crystal to resist movement. Regular
> microphones suffer from the holding method just like you mention above,
> plus they suffer from the movement detection also altering it and
> distorting it. This eliminates all but the effect of the photons hitting
> the surface...and I doubt the force in question is detectable even in
> the most serious of settings. Now if you really wanted to have some fun,
> and it isn't just a microphone you are interested in, you can create a
> vacuum packed container, and a superconductive magnetic suspension
> system, and literally float the reflecting device by levitating it (it
> would be overly sensitive, and dampening vibrations would be a problem;
> however, having a direct digital output, it is possible to implement
> fourier based filtering). By playing with geometry, the scheme can be
> made to isolate movement in particular directions...vector components of
> the central reflector. This latter part is why I am most interested in
> it, it could become a single point detector capable of knowing the
> direction of the force being applied to it; with fourier and the right
> geometries, it could detect the direction of the force being applied by
> different tones.
Outside of the superconductor example :) You would need something to hold
your moving part at a center location so it had freedom to move. It has
to have some force to bring it to a center position. This is no different
than a piezo-electic holding its position. (Unless I don't understand
what you mean). Because some force needs to hold it. If you don't have a
force, then how will it be able to retain location. Also, this 'thing'
that holds it will be frequency dependent, and react (vibrate) with
different resonant frequencies modulating the affect on the moving part
itself.
> The chip I was speaking of before that did not exist way back when is an
> optical detector array. But someone else mentioned interferometers, and
> the use of interference patterns can greatly reduce the amount of
> detectors required; one would no longer detect a single point location,
> but instead rate the interference pattern on a series of detectors. If
> you really want to know how sensitive such a thing can be, look up RLG,
> ring laser gyroscopes. The main limitation of how sensitive they can
> become is a two-part question: 1, as two coherent light sources are
> mixed and approach each other in phase, they tend to lock on to the
> phase of the other and you lose the phase difference, and 2, at any
> given tendency for closely phased coherent light streams to merge, a
> shorter wavelength still provides a finer measure. Using the newer blue
> laser diodes versus infrared you can gain a lot of ability to detect
> smaller variations. There is even a security type system based on
> interference patterns produced by the difference in path length of a
> single laser source being put there two separate fiber optic tubes
> planted under the ground. What happens is the pair or more of fiber
> tubes are just different strands of a multi-strand fiber optic cable,
> placed under the ground maybe 5 feet or so, and run in a circle around
> an installation; someone walking anywhere near, or cars driving in the
> area, distort that cable's shape through ground vibrations, causing the
> relative length of the two strands to change...the interference pattern
> then changes, which is detected, and alarms go off. So interference
> patterns can make the sensitivity part a whole new game, detecting
> distances roughly in the neighborhood of the wavelength of the light
> being used.
How many detectors does the array have? I guess you could use also like a
CCD type array. I know they have those.
On the fiber intrusion detection, a kid I went to school with had built
something similar with fiber inside a block of plastic. He would use it
to measure water pressure in the ocean, because as he lowered it, he had
pieced of plastic going perpendicular to the fiber, it would create
microbends and change the signal at the end (the level). Very similar.
> In any case, I had two things in mind, and a microphone is only one of
> them. The optical equivalent for joystick control probably requires only
> the equivalent of 8 to 11 bit to be useful; 12 bits would be extremely
> fine. And in no case am I doing this for anything other than fun, so I
> don't take the limitations very seriously.
Do you mean 8 to 11 sensors or 256 to .... sensors when you mention bits?
I think your project is very cool :) I hope my questions are helpful.
Rick
> D. Stimits, stimits_AT_idcomm.com
>
> >
> > Rick
> > +------------------------+-----------------------+
> > | T a l i t y | +------+ |
> > +------------------------+ +----+-+ | |
> > | Richard Burnett | +-+ | |
> > | Senior Design Engineer +---+ +----+ |
> > | burnett_AT_tality.com | | |
> > | | | |
> > | Phone: 919.380.3014 | |
> > | Fax: 919.380.3903 | | |
> > +------------------------------------------------+
>
+------------------------+-----------------------+
| T a l i t y | +------+ |
+------------------------+ +----+-+ | |
| Richard Burnett | +-+ | |
| Senior Design Engineer +---+ +----+ |
| burnett_AT_tality.com | | |
| | | |
| Phone: 919.380.3014 | |
| Fax: 919.380.3903 | | |
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