Subject: Re: [linux-audio-dev] off topic, optical protocols
From: D. Stimits (stimits_AT_idcomm.com)
Date: Tue Mar 05 2002 - 22:03:17 EET
"Richard C. Burnett" wrote:
>
> What you are making in itself is an analog to digital
> converter. Questions that pop off the top of my head:
>
> 1. different angles will have different lengths to the detectors, are you
> thinking of making the detectors curve to the geometric shape?
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.
>
> 2. Since the laser will not be a point but a spreading of light, how will
> that affect the spreading at different angles on a curves surface, will
> you hit too many bits at once?
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.
>
> 3. The mass of the object and its center point holding method will
> probably provide some distortion due to momentum change and spring like
> affects.
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.
>
> 4. As I can see it, each optical detector is a level indicator. To get
> the same resolution as a 16-bit A/D you would need 65000 detectors, is
> that feasible?
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.
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.
D. Stimits, stimits_AT_idcomm.com
>
> Rick
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