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Thread: Modding from TTL to Analogue

  1. #31
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    Quote Originally Posted by marconi
    Quote Originally Posted by allthatwhichis
    Meaning you'd get dotted lines at a lower brightnes instead of full lines...
    Exactly.. not of much use unless you want them
    True. I forgot that the application for most people is projection displays.

    But still, for static type beam show applications it would work. Also, I haven't done the math yet, but it seems like if you plot scan speed vs. modulation rate, there are areas of the graph that would be in the "solid line" region. You might need very high speed modulation, but it still should work in theory.

    I guess I'm just trying to find a generalized method of conversion, without having to hack the PSU. (I guess it's called an AOM )

    Sorry for the tangent thread.

  2. #32
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    Pin6 isn't used, it only uses the other half of the amp (pins 1,2 and3), 5 6 and 7 all go to missing components.

    Jim

  3. #33
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    Quote Originally Posted by JimBo
    Pin6 isn't used, it only uses the other half of the amp (pins 1,2 and3), 5 6 and 7 all go to missing components.

    Jim

    Hmmmm... :roll:

  4. #34
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    Looking at the board layout the pins on the head connector are all in logical pairs.
    Therefore pin 3 will be paired with pin 11, both of which there is nothing connected in the plug to the head.
    Pin 11 can only be connected or biased to either Gnd or A3 Gnd.
    Pin 5 of N5 goes to pin 3 of the head, if that is an output from N5 it must follow that pins 1 and 2 are inputs.
    If they are it is being conditioned by the output of the AND gate.
    All thet said what do you expect would be in the head connected to pins 3 and 11?
    It obviously wont be anything that sinks a great ammount of current being connected pretty directly to N5.

    Jim

  5. #35
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    I'm pretty sure I figured out the missing parts. I believe these are for CNI's high stability option (low noise version). Basically the difference is that the low noise units are closed loop, they rely on photodiode feedback as opposed to straight constant current drive. The mystery pins on the laser head 3&11 are for a photodiode, and they get feed through some amplification (the mystery IC is probably some high quality low noise opamp)and that gets converted to an analog voltage which then gets fed back into the current control loop. So now it is basically under closed loop control.
    -illuzion

  6. #36
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    Quote Originally Posted by allthatwhichis View Post
    Meaning you'd get dotted lines at a lower brightnes instead of full lines...
    This topic has been a bit of a thorn in by brain for a while.

    Originally, I wasn't thinking of graphic scanning applications. If you have a static or slow moving beam, TTL can easily simulate the appearance of analog modulation - that is, give the appearance of varying brightness by switching on/off at maximum TTL frequency (using a "dithering pattern") for a given brightness.

    Well, after thinking about it a bit more today (and some quick calculations), I have come to the conclusion that most of the time you actually would not see dotted lines even during graphic scanning. Assuming 10khz blanking and 15kpps scanning, you would only see dithering artifacts on more complex graphics. If you go up to 20khz and 30kpps the dots pretty much disappear (unless you're doing raster scanning or very complex graphics).

    You guys kinda poo-poo'd my idea, but I really think that an external box that can convert any analog signal to TTL approximation would be totally useful.

  7. #37
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    Quote Originally Posted by marconi View Post
    Well, I thought jeejeedr answered that too!

    You see the TTL input only accepts either ON or OFF state...
    hence the reason this logic is used to start with
    You cant make a cow fly very well.. by doing something different.

    There is not much in-between.. to get to the ON state the voltage only needs to get to the threshold area
    of the chip., this is about 2v on most chips... see chart in link

    http://www.interfacebus.com/voltage_threshold.html

    that pulsed signal you want to provide will be just that a pulsed signal

    The blue area is the threshold region..the pink is the area of ON state

    Now this area between threshold and ON range very very small..
    this area could be used for vary-ing the brightness some but not very much
    you would still have pulses in your lines that are supposed to be fully turned ON or anything in-between
    I hope that makes sense?
    Just to be clear, I'm not talking about using the analnog slope of the transistor... I'm talking about full on/off at high frequency to simulate brightness. Remember that film (movies) is only 24 frames per sec, but it looks like smooth motion. It's the same principal as switching power supplies, except that your brain fills in the gaps instead of the capacitors.

  8. #38
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    For this to work, your pulse repetition rate (IE: your max TTL blanking speed) would need to be significantly faster than the time it takes your galvos to draw a short line segment between two closely spaced points.

    I've never found a good conversion for PPS to actual galvo slew rate in degrees per second, so I'm forced to do an aproximation. But if you assume that it is reasonable for a scanner to draw a very short line segment in 100 microseconds (1/10,000th of a second), then your laser will need to pulse at least 5X, if not 10X that speed in order to make the line visible. So we're talking about 50Khz or 100Khz TTL blanking speed.

    This is assuming that the short line segment would require at least three points and the scan speed is 30,000 points per second.

    The key is that the eye's persistance of vision really isn't the issue here. Granted, the eye will not detect flicker even at frame rates approaching 16 frames per second. But the eye certainly can detect breaks in an otherwise continuously scanned line. So if the galvos move any appreciable distance while the laser is off, you will end up with a visible line break in the scanned image.

    Now, it's true that the scanners will be continuously scanning the pattern, so it is possible that the next time through the scan the line break will appear somewhere else in the line, and in that case the eye's persistance of vision *will* average everything out just like it does in a movie theater.

    But you can't count on that always being the case. There will be some patterns that, given the correct coincidence of scan speed, blanking frequency, and number of points in the pattern, will always end up with the line break in the same spot in the pattern. And in those cases the line break will be clearly visible.

    So to eliminate this problem you're going to need to modulate the laser at a much higher frequency than your scan frequency. My guess is that 100 Khz would be the sweet spot, and anything over 200Khz would probably be overkill.

    Now, there certainly are laser drivers that support TTL blanking at these speeds (and even higher), but I think the cost is high enough for these drivers that you'd be better off just getting an analog driver.

    Still, you are right in that the circuit wouldn't be all that difficult to design. (I think I'd use an off-the-shelf PWM chip myself, rather than trying to cobble some 555-based circuit together...) If you've got a laser that supports really fast TTL blanking to begin with, maybe you might want to give it a try and post some pictures of your results?

    Adam

  9. #39
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    Quote Originally Posted by Buffo View Post
    Now, it's true that the scanners will be continuously scanning the pattern, so it is possible that the next time through the scan the line break will appear somewhere else in the line, and in that case the eye's persistance of vision *will* average everything out just like it does in a movie theater.

    But you can't count on that always being the case. There will be some patterns that, given the correct coincidence of scan speed, blanking frequency, and number of points in the pattern, will always end up with the line break in the same spot in the pattern. And in those cases the line break will be clearly visible.

    So to eliminate this problem you're going to need to modulate the laser at a much higher frequency than your scan frequency. My guess is that 100 Khz would be the sweet spot, and anything over 200Khz would probably be overkill.
    Or modulate the frequency around a lower value, as only the pulse width affects the apparent brightness. I don't know how well it would work (various rates and depths and mod waveforms would need testing), but it makes less demands on hardware, and also with a schmitt trigger on the output to make the pulse edges sharp, brings the entire signalling within the capability of professional (96KHz sample rate) sound cards and other DAC's of similar spec.

  10. #40
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    Quote Originally Posted by Buffo View Post
    Now, it's true that the scanners will be continuously scanning the pattern, so it is possible that the next time through the scan the line break will appear somewhere else in the line, and in that case the eye's persistance of vision *will* average everything out just like it does in a movie theater.
    This is the key assumption for the idea to work (I think, even at less than 1x the draw speed).

    Quote Originally Posted by Buffo View Post
    But you can't count on that always being the case. There will be some patterns that, given the correct coincidence of scan speed, blanking frequency, and number of points in the pattern, will always end up with the line break in the same spot in the pattern. And in those cases the line break will be clearly visible.

    Adam
    That sounds right. The difference in frequencies will create dark spots in the pattern. "Good" frequencies will blur out the breaks completely, and "bad" frequencies will reinforce them and make them more visible.

    But - The worst possible scenario, first harmonic matching - leading to a static dark spot - is fairly unlikely and fixable by adding or removing a point.

    More likely, you'll have some some higher harmonics creating blurry dark zones. (I'm picturing the way a fan appears as the blades change speed).

    This is where dithering may help. It would require some TTL speed, but the idea is to use a calculated pattern to avoid harmonic generation. Another strategy is to randomly dither the signal to approximate an average (I bet that would look weird, though).

    Without enough speed, artifacts are unavoidable. But, at other times I think you may be able to get a reasonable approximation.

    Good feedback. Thanks.

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