Re: bald mountain camera and new camo dude truck

Message posted by gary on May 04, 2002 at 13:29:20 PST:

Do you have a manufacturer's name and model number for a laser microphone. My understanding of such devices is that you need a reflective surface to detect sound, such as a glass window. The laser can detect the motion of the glass as the sound vibrates it.

I did some consulting work in the 80's for a company making a laser based measuring system that used a similar scheme, but a bit more difficult since they were trying to measure the position of a reflective surface. The laser was AM'd at 10MHz, and the reflected light was compared to the 10MMz reference signal. As the object was moved, the detected sine wave from the reflection would slip phase with the reference. If it moved sufficiently, the phase would change enough to slip a cycle. Thus you had to cound the cycles slips and the phase difference to get a position estimate. For sound, I'd say you pick the modulating frequency so there is no phase slip, then demod the phase difference with a PLL. Sounds simple on paper, but probably hard to perfect.

Getting back to Joerg's photo, that side view is pretty good. I know the spot where it was photographed, but every time I was there the camera was pointing at me!

It may be possible that in certain locations close to Bald Mountain, the view of the camera is blocked by terrain. This may be why they always send camo dudes to the central Bald Mountain observation point, but if you hang out at the points to the north (where Joerg's photo was taken) or the south (which has the dummy camera), the Bald Mountain camera can see you and thus no camo dude presence is required.

In Reply to: Re: bald mountain camera and new camo dude truck posted by John on May 04, 2002 at 12:33:38 PST:


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