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Using galvanometers for a rastor scan?

mullockt

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Hello :)

I am currently devising a way of displaying the output from a 'spinning disc camera'. It's an approximate recreation of the first ever live TV camera made by John Logie Baird in 1936.

It's outputting signals for position x and y (in a rastor scan pattern) and brightness over time. I thought that one way of displaying this output might be to create a laser projector with a pair of galvos positioning the laser spot. I appreciate that usually when creating a rastor scan with a laser other systems are suited better, such as using a spinning polygon mirror or a resonant mirror, but since the camera is outputting a live signal with slightly varying frame rate, it would be far simpler to take the ouputs already coming from the camera and just plug them into a laser projector.

I have been looking for as much information as I can find about how the specs for galvos work, and it seems as if the answer is fairly complex! The fast axis of the projector requires the x-axis galvo to be scanning in a sawtooth pattern at around 900Hz (the camera outputs 60 lines x 15fps = 900 lines/sec). If a 20kpps galvo were able to move from any point to any other point on the screen in 1/20,000 s, then that would be fast enough to produce a decent rastor scan, but I beleive things may not be this simple?

Can anyone offer any ideas as to how galvos might cope with a rastor scan at this rate? Is the idea feasible?

Cheers for any help!
 

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Hello :)

I am currently devising a way of displaying the output from a 'spinning disc camera'. It's an approximate recreation of the first ever live TV camera made by John Logie Baird in 1936.

It's outputting signals for position x and y (in a rastor scan pattern) and brightness over time. I thought that one way of displaying this output might be to create a laser projector with a pair of galvos positioning the laser spot. I appreciate that usually when creating a rastor scan with a laser other systems are suited better, such as using a spinning polygon mirror or a resonant mirror, but since the camera is outputting a live signal with slightly varying frame rate, it would be far simpler to take the ouputs already coming from the camera and just plug them into a laser projector.

I have been looking for as much information as I can find about how the specs for galvos work, and it seems as if the answer is fairly complex! The fast axis of the projector requires the x-axis galvo to be scanning in a sawtooth pattern at around 900Hz (the camera outputs 60 lines x 15fps = 900 lines/sec). If a 20kpps galvo were able to move from any point to any other point on the screen in 1/20,000 s, then that would be fast enough to produce a decent rastor scan, but I beleive things may not be this simple?

Can anyone offer any ideas as to how galvos might cope with a rastor scan at this rate? Is the idea feasible?

Cheers for any help!
There is a little bit of galvo basics and how to drive them in "The Laser Cookbook" by Gordon McComb. It is not even close to the complexity you are talking about, but some of it may be useful to you. That is all I know about the subject. Hope this helps. Good Luck.
 

Cyparagon

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If a 20kpps galvo were able to move from any point to any other point on the screen in 1/20,000 s, then that would be fast enough to produce a decent rastor scan, but I beleive things may not be this simple?
You'd believe correctly. The speed rating in thousands of points per second (kpps) is based on display of the ILDA test pattern. With a different vector set, your maximum speed before distortion/overshoot will deviate from this value by a lot. For instance, if you plot a circle with a million points on the circumference, you could easily display perhaps billions of "points per second". If you have a program that plots random points in the display area, you may only get a few dozen points per second.

The speed rating is only to have a standardized comparison, not to calculate frame-rates. Raster can be done with standard laser projectors, but the frame rate will be low, even with low resolution. A spinning hexagonal mirror for the Y axis would yield better results.
 

Benm

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If you were to scan it like a CRT tv did, that'd be extremely hard on the X galvo - it'd have to move from left to right to scan the visible line, and then back to the left in a blanking interval between lines. For a system with no moving parts like a CRT this is doable, but for a galvo getting from one extreme deflection point to another is difficult.

Usually laser shows are not raster scanned, but the galvo's draw the desired graphic moving in an optimized path that requires the least amount of travel between all the points to complete the drawing. If you draw something like the outline of a person you just trace around that outline, you do not raster scan it.

If you must raster scan something, at least do it zig-zag (drawing even lines left-to-right and odd ones right-to-left). This prevents the galvo to have to move over with the laser off between lines. This will still be very hard on the galvo though as it has to make hard turns for every line and you still have the mirror inertia to deal with.

Don't expect TV quality images from this at all though: To scan a 480p image at 30 fps would require 14.400 lines a second. I doubt a galvo that can do the ilda pattern as fast as 60.000 pps could even get close to this, certainly not without additional cooling (these are mechanical systems with friction, intertia, electrical resistance etc).

If you -only- want to raster scan the spinning hexagonal mirror would be a solution - that's pretty much how laser printers do it ;)
 

mullockt

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Thanks for the advice!

I'll look further into using a polygon mirror at this point. I was optimistically hoping that I could just get some galvos and plug them straight in, but even at 60 lines 15fps, it's probably too fast.
 

Benm

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60 lines at 15 fps may be feasible with galvo's, though it would not be easy on them and the whole system would also be pretty noisy. In a club or outdoor environment that usually isn't that big of an issue, but i you wanted to project a movie on w wall at home or something like that it'd be big factor.

In laser projectors systems for video scanning like that is never done - a projector can use a laser as the light source, but the actual image is normally produced by running that laser through an lcd screen or cluster of tiny mirrors.
 

mullockt

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60 lines at 15 fps may be feasible with galvo's, though it would not be easy on them and the whole system would also be pretty noisy. In a club or outdoor environment that usually isn't that big of an issue, but i you wanted to project a movie on w wall at home or something like that it'd be big factor.

In laser projectors systems for video scanning like that is never done - a projector can use a laser as the light source, but the actual image is normally produced by running that laser through an lcd screen or cluster of tiny mirrors.
Mmm, they'd probably have to be fairly high spec galvos in that case (at least the x axis one)? I haven't got a huge budget!

Noise isn't really an issue unless it gets obscene, this will just be a demo project to display short periods of live video without any sound.

For the moment, I've sourced myself a broken laser printer and am playing with the spinning mirror in that, which could be an option.

Cheers again for everyone's advice!
 

Benm

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Yeah, you'd probably be looking at 60.000 pps compatible galvo's, and the problems that come with them (power use, heat generation etc).

If you want to raster scan with a laser and are just experimenting a bit i'd try for a solution with a spinning mirror to do the X axis and a galvo to do the Y. It will probably not become a perfect projector, but on a limited budget it'd be interesting how good a result you could get on a limited budget.

With the spinning mirror design it'd be a challenge to get good horizontal sync, but i'm sure there would be some way of doing that (perhaps a led/photodiode pair near the mirror so you know when to start drawing the next line).
 




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