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Old 22nd April 2008, 12:28 AM   #11
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Unfortunately the CD and DVD read lasers have short, mm or less, coherence lenghts. The laser mice lasers OTOH are single mode and would be great in this application. You would need to do some homework, they work at 1-2mA and burn out at just 4mA or so.
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Old 22nd April 2008, 03:37 AM   #12
jcx is offline jcx  United States
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for phonograph readout you may want to consider that the better line conact styli have groove wall conact patch dimensions on the order of a single wavelength of red laser light in the scan direction - the realities of practical F# focusing mean a direct optical readout of a spot on the grove wall isn't likely to compete for extracting high frequency info

I would suggest letting highly evolved styli do their thing and use the laser to watch the stylus motion rather than trying to get down into the groove with the optics
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Old 24th April 2008, 05:42 PM   #13
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This seems a very good suggestion. At the moment i'm working in my short free time at using old cdrom transports to move a more coherent laser. There are some relatively cheap ones out there that have linewidths at arround 50 MHz, which i belive to be long enough to be focused with a cdrom transport.

My plan is to start with the microphone once i get all the necessary information and a semireflectant glass as it seems easier to get. Now i belive it will have a nice quality since nothing has to be placed close behind the membrane, so it can be used to try other things like placing an imput and output waveguides and it will be easier to control its resonance frequency.
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Old 27th April 2008, 06:45 PM   #14
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Quote:
Originally posted by jcx
for phonograph readout you may want to consider that the better line conact styli have groove wall conact patch dimensions on the order of a single wavelength of red laser light in the scan direction - the realities of practical F# focusing mean a direct optical readout of a spot on the grove wall isn't likely to compete for extracting high frequency info

I would suggest letting highly evolved styli do their thing and use the laser to watch the stylus motion rather than trying to get down into the groove with the optics
Actually SOTA laser measurment on surfaces goes many orders of magnitude below a wave length. The catch with the laser turntable is the fact that there is no stylus to push the dust out of the grooves so the noise, unless you have class 10 cleanroom record surface, is unbearable.
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Old 28th April 2008, 04:16 PM   #15
jcx is offline jcx  United States
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I was referring to diffraction limited spot size - the practical minimum dimension of the laser illumination on the groove wall, assuming hobbyist mechanical/optics fabrication limits na (~2/f#)

http://en.wikipedia.org/wiki/Gaussian_beam

Click the image to open in full size.

(from: Computers, Software Engineering, and Digital Devices By Richard C. Dorf)

I assume you can squeeze a little in one dimension with an elliptical spot but I doubt you can match a line contact stylus with classical optics and red diode lasers

fine resolution of phase is possible in interferometry and evanescent-wave scanning optics can increase resolution normal to the beam propagation direction

maybe these techniques could be used to scan/digitize a phonograph groove at high resolution but real-time phonograph playback with sub-wavelength scanning aperture seems beyond the current electro-mechanical-optical SOTA
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Old 30th April 2008, 04:03 PM   #16
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The laser turntable guys claim it works with a 10 micron spot, so I have no idea what their doing. If the highs were THAT bad you would think they had a non-starter.
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Old 1st May 2008, 04:00 AM   #17
jcx is offline jcx  United States
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a 10 μm spot would be 15-20 wavelengths of the blue and green lines of the gas laser used by the elp
you can have much higher f-number optics and correspondingly longer focal depth that cuts way back on your focus tracking servo bandwidth requirement - the actual audio modulation information in the elp is the angle of the reflected light - which turns out to be proportional to the audio information carrying cutting head velocity normal to the groove tangential velocity
not to mention the easier construction if your whole focus assembly doesn't have to actually fit in the groove as it would with more tightly focused optics

on closer examination 10 μm isn't so bad as its only ~ 1/10 a 20KHz groove wall modulation wavelength at the inner dia of the record so the high frequency amplitude loss is minor

for archival non-contact playback the elp scheme is great but I think the hobbyist would have a better chance just looking at the stylus motion - a far more "cooperative" target with your cited advantage of pushing the dust bunnies out of the way
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