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Thread: balancing and modulation loss - help needed

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    Default balancing and modulation loss - help needed

    Hi,

    So this is my first RGB project and it is coming along allright but I need some input from more experienced people in the field at this point.
    The projector (view) has 100 mW 650nm, 50 mW 473nm and 100 mW 532nm so I would say that green is very overpowered, right? Well only when there is a lot of green in the scanned image. I find when blue and red are dominant in the image the green really drops in power. For instance when a square is drawn with 3 red/blue sides and one white side, the max green needs to be 198 out of 255 to achieve white (using Mamba). For a white square the same size it need to be 154 out of 255, otherwise the white really appears bright green.

    In practice I find myself changing the balance all the time for different still images, for animations it takes some time to find the best average value for the show. When there is only a small white element in a blue/red frame the white appears pink with full green - this is when it really became an issue to me. It only applies to green - it seems to be the nature of the laser, the blue dpss and red diode don't visualy show the same issue.

    I expect others have found a practical approach to this problem. Any thought on how to deal with this?

  2. #2

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    That's really awkward because it's time domain error. If it were just nonlinearity, you could make a compensating curve (hopefully laser software does this, the same way MIDI software uses keyboard velocity scaling curves), but in this case you might either need a change to the mod circuit, or to predict what happens to change the signal to compensate for the effect.

    If the change, already proportional to the on time per frame or per line, is also a change during the display of each line, you could tap the analog output and feed a capacitor to generate a similar effect, then subtract that from the main signal before feeding the scan amps. If it is NOT changing along each line, you might still get by with the same trick, but using a fast AGC amp instead of the capacitor.
    Last edited by The_Doctor; 05-21-2007 at 10:10.

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    It is a time domain problem. There is also non-linearity but that can easily be compensated with software. In my own coding I wrote a gamma curve compensation for it - which works quite well. The issue of this thread however I'm not so sure about. The green needs a lot of time to 'recover' to the right power after being blanked - the longer the blanking interfal the less power after.

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    I saw something like that with a LambdaPro laser. Can you get a sample over time, a log of the output's recovery after various blanking times? So long as it's a smooth curve there might be a way with the AGC idea I mentioned (AGC on one branch with fast attack, slow release, add output to other branch and readjust gain), but if as I suspect, it's got kinks in the curve due to mode hops or other discontinuous change, all bets are off; there might then be nothing you can do except compromise as you're already managing.
    Last edited by The_Doctor; 05-21-2007 at 14:59.

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    I don't see any modehops, the power increase is fairly smooth unlike the modehops I had with my old lambdapro. I'm guessing it might be temperature control for one of the crystals. Maybe the KTP cools down a bit too much when blanked and heats up again when the laser is on, heating the KTP to get closer to the optimum working temp. ... just guessing. Anyone with experience on this?

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    Try Bill Benners compensation circuit below

    http://www.pangolin.com/resguide03d.htm

    I have one by a unknown maker that has the same characteristics, keep it pulsing and your fine, cool it off for a while and its dim. I ended up adding a
    offset to the turn on voltage so I was on the edge of threshold at 0V in. In my case its the diode temp, KTP doesnt cool off that fast unless the heater is very poorly designed. Mine has lights on the KTP temp and its time constant is very slow. The diode emission spectrum has five or six or more "spikes" and the yag or vandate adsorption spectrum has three or four major ones, so as the spikes slide deeper across the holes, you get brighter in a nonlinear way.

    try adding a offset or using the crude log/antilog diode functions of
    Bill's circuit.

    Steve Roberts

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    let me change that word from crude, to simple, didnt mean to insult Bill's design, the diode mainly serves as a breakpoint, then gets a little nonlinear in the feedback loop. I had somebody bothering me when I typed that.

    maxima mea culpa Bill

    Steve

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    Thanks for that link. Correct me if I'm wrong but these circuits are put inbetween the DAC and the laser modulaion input - so they don't actually change the laser's characteristics but the modulation signal, right?
    If so I've already done the equivalent with software. For instance if the laser comes on at 0.8 V and reaches its max power at 4.7V, I can set values in my code to map the color code (0.0 to 1.0) onto the range from 0.8 to 4.7V. Besides that I can set a blanking value set slightly below the lasing threshold so there is still current through the diode without seeing green when blanked. As a final touch I added a gamma curve correction to compensate for non-linearity, thus green.out = green.in ^ gamma. I expect this is the software equivalent of the circuits you suggested. It all helps a bit but green brightness is still very dependent on the duty cycle of the laser.

    To my surprise the blue laser does not show any sign of this behavior, it is very consistent allthough lower power. I was under the impression that the 473 dpss is a harder laser to keep stable.

    I'm actually in the process of talking to the manufacturer about this.
    Last edited by Zoof; 05-22-2007 at 22:30. Reason: spelling :)

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    I looked at Bill's circuit, and it seems to adjust rise/fall slew rates as the only time domain changes it makes. This would correct lines that had a gradient in colour change or brightness due to scanner accelerations or slow diode driver responses, but I can't see an AGC in there, and I think you'll need one, especially as it seems you've got the offset correctly set for the lasing threshold in blanked state, and the problem still occurs. The AGC would make the signal higher after longer blanked intervals, so you'll probably want to make sure that it clips at 5V no matter what happens.

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    But if you can use the circuit and maybe add a agc, you get the full dynamic range of your modulation, in other words, you get a full 8 bits of color control, the agc will negate some of the resolution. So with the circuit set for offset, you'd have your fixed turnon bias, then every bit adds to the bias, giving you more color control. The .8 volts you describe sounds like your turning on a optocoupler in the laser, .69-.72 is the usual drop for a IR silicon junction in the led in the opto. The opto will be very nonlinear, explaining your problem. The time domain spiking from the circuit will cause a little faster heat up of the pump diode. BTW, optos turn on slow, so the bias will get you brighter faster.

    Steve

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