Diy laser microphones and turntables

[ionomolo]

I have a lot of old cdrom players at home and i was trying to do something interesting with them, but I've been looking arround and it seems that there is really few information about laser pickups out there, well, there is a lot of information about repair and operation but little on other experiments.

My idea is to try both a laser microphone and a laser turntable, starting with the laser microphone as it seems far easier since tracking the grooves will probably be a hellish job.

I would like to ask some advise with both where to get datasheets for laser pickups (I have some SF-PXXX sanyo's) and reports from people who have tried similar things.

At first i will keep the laser photodiodes (once i know how to read theyr output), but probably it will end up with a photodiode better suited to linear operation. Any advise about good photodiodes for audio operation will also be helpful.

I have both CDrom and DVD lasers and i don't know which ones to choose, since the ones from a dvdrom player seem easier because they are visible, they actually have a pair of laser diodes for cd and dvd operation and this makes them more complicated.

Somebody has information about the coherency length of standard cdrom and dvdrom diodes?

 

[sreten]

Hi,

You have lost me. CD lasers are designed to indicate whether a pit
or not exists in the disk at a particular point. They are not designed
to do laser inferometry which is what you need to measure distance,
either in a microphones diaphragm or the shape of a record groove.

A capacitor microphone is as near perfect as it gets, why complicate it ?

/sreten.

 

[SY]

Are you thinking of using the diaphragm as one leg of an interferometer?

 

[ionomolo]

Originally posted by sreten
You have lost me. CD lasers are designed to indicate whether a pit
or not exists in the disk at a particular point. They are not designed
to do laser inferometry which is what you need to measure distance,
either in a microphones diaphragm or the shape of a record groove.

I know how a laser pickup works and i also know that laser diodes aren't suited for interferometers since they have a very low coherency lenght (well, they can work as long as the two paths are kept exactly the same lenght if used in a Michelson arrangement, in this case interferometry can be done even with white light), but i was looking arround a simpler idea, something that could be called an "unfocusometer", since the mirror-focusing lens system will create a virtual light source that whose position will depend on the mirror position, if you use a linear photodiode you can detect distance since the amount of light that impacts the detector will follow the inverse square law and this should be enough for extracting the audio information from the membrane/groove.

Originally posted by sreten
A capacitor microphone is as near perfect as it gets, why complicate it ?

I'm not thinking in doing anything perfect, but rather in having fun and experimenting with cdrom lasers. These are very perfect things and it's low expense is only due to the fact that they are mass produced, some of these have been used in doing high-precision velocimeters for scientific applications for example (with extensive modifications, of course).

I feel this to be a very interesiting project but your post may help in preventing people wanting to get immediate perfect results from exposing theyr sight to a 200 mW invisible cd-burner laser (Good point to say that writing lasers should NEVER be used for experimenting).

Originally posted by SY
Are you thinking of using the diaphragm as one leg of an interferometer?

No since i don't have the equipment to exactly match the two optical paths, but this might be a good idea for people owning an optical bench.

 

[sreten]

but i was looking arround a simpler idea, something that could be called an "unfocusometer", since the mirror-focusing lens system will create a virtual light source that whose position will depend on the mirror position, if you use a linear photodiode you can detect distance since the amount of light that impacts the detector will follow the inverse square law and this should be enough for extracting the audio information from the membrane/groove.

Hi,

I you want to do it just to do it fair enough.
I cannot see that your idea will give anything like the
resolution required, or in other words it will be too noisy.

/sreten.

 

[ionomolo]

Yes, you are right about the noise, this is only a first attempt and if i suceed i may mess into something more complicated. It seems that placing the virtual light source nearer from the photodiode will give better noise performance but worse distortion.

Do you know if the photodiodes are exposed directly to the interconnect or if they have some active circuitry built in the same die?


Somebody has a datasheet for the PXR-550X? Everything on google is in asian languages and does not lead to anything downloadable.

 

[scott wurcer]

DANGER WILL ROBINSON!!!

The lasers from DVD and CD burners are extremely non-eyesafe.

That being said, the interference microphone idea fails because a single phase wrap gives very little dynamic range. The noise level actually can be incredibly low. I don't think anyone has solved the keeping track of the phase wraps problem.

I can probably get any answers on the lasers that you want.

EDIT - Sorry you already said that.

 

[ionomolo]

Originally posted by scott wurcer
I can probably get any answers on the lasers that you want.

I would really thank you if you can send me the pinout, optical power, coherency lenght and how the photodiodes are arranged in the PXR-550X.

The "unfocusmeter" approach will have little problems with coherency lenght but it will be very nonlinear, but it will be the only option if considering a turntable because distances will be far higher than 700/900 nm.

 

[scott wurcer]

I would really thank you if you can send me the pinout, optical power, coherency lenght and how the photodiodes are arranged in the PXR-550X.

The "unfocusmeter" approach will have little problems with coherency lenght but it will be very nonlinear, but it will be the only option if considering a turntable because distances will be far higher than 700/900 nm.

Sorry I can't easily get that specific datasheet. Typically there are focusing and tracking arrays. The diode outputs are matrixed on chip with transimpedance amplifiers as the first step in the process. I'll ask my optics guy about the coherency length. We had some chips in the first laser turntable (which I think is still being sold in small quantities). I know they are archiving historical 78's with them. Apparently you can average up and down the groove walls to give some serious noise reduction.

EDIT here they are http://www.elpj.com/about/index.html

 

[ionomolo]

I have seen some cheap lasers at online stores having linewidths arround 50 MHz, since i don't know the relation between linewidth and coherence lenght i'm waiting to ask at the university before buying one. Anyways, my idea is to use cd transports to move the laser and the reflecting mirror.

Somebody knows the accuracy of the pickup moving system?

 

[scott wurcer]

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.

 

[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

 

[ionomolo]

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.

 

[scott wurcer]

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.

 

[jcx]

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

(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

 

[scott wurcer]

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.

 

[jcx]

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