A sneak peek!

[DZ]

Did you end up using Hobbs little bundle of joy transimpedance circuit?

I tried that circuit but didn't have much luck with it. Currently the photodiodes are feeding jfet amps with the bootstrap and cascode removed. That is one part of the circuit I want to revisit and improve a bit.

Dave, the 60Xs all used AR coated glass as the pickoff. With either a mylar or alumina disk as the attenuator.

The pickoff I have now I got from dsli_jon, and is very nice, the splitter actually is kicking about 10% into the photodiode. The glass I plan on using will be closer to .5%, so current configuration would represent a system with much higher power lasers. Still plenty of playing around to do before I have anything remotely available.

As for beta testing - you should know by now that you don't even have to ask. Just let me know when you're coming and I'll rip the covers off the projector, get some steaks out of the freezer, and fire up the grill!

Thanks Buffo, probably going to be quite a few more weeks.

[dave]

Nice work!

[Frixxion]

HE Laserscan has a built in Colour correction function, will this do the same? I have not yet played with this feature since my lasers haven't arrived yet.

[tocket]

No. If I understand things correctly this is an optical feedback circuit. No software can do this.

Very nice work DZ! Hopefully this can improve the poor modulation of my laserwave greenie.

[Frixxion]

but it would take away part of the problem right?

[Doc]

but it would take away part of the problem right?

No DZ's circuit is actively monitoring the output from the lasers, keeping it's output linear with the control input. Lasers unfortunately are quite unstable, for example; as you try to increase it's output in a linear fashion, it may behave to a degree but it will generally flicker high, low, or increase and decrease in steps, peaks and troughs, further; if the required output is say 50%, it may fluctuate a fair bit above and below 50% before it settles. All of this means poor colour blending/brightness gradients in the display. DZ's circuit will actively keep control of the lasers at high speed, by actually looking at the output with a photo diode, counteracting these characteristics.

[fuzcub]

Very nice - well done! The comparison videos really show the improvement.
B.

[mixedgas]

but it would take away part of the problem right?

That menu takes care of color balence and color linearity. It does not help with the lag in blue turn on time compared to the green lasers, or the lead in turn on time in the red lasers, compared to the green lasers.

Light feedback, plus some creative work with a derivative circuit, will greatly improve the blue situation.


Steve

[Zoof]

Light feedback, plus some creative work with a derivative circuit, will greatly improve the blue situation.

Sorry to be skeptic, but I highly doubt it. The reason is that the blue laser has slow dynamics. An optical feedback circuit closed around the slow internal dynamics will not speed it up. In fact it could make the output unstable as you will probably try to increase the outerloop gain.

Think of it as trying to control a sluggish Boeing 747 with an F16 autopilot. The autopilot will not make the 747 any more agile.

What DZ is doing is slowing down the faster lasers, flying an f16 like a sluggish 747, which will work. Since the the environment of the lasers are well controlled (temp mainly) there are no significant outside disturbances so an extra feedback loop is not needed. Feedback is used in control to take care of outside disturbances. If those are not significant and you are controlling a predictable system (like our lasers) feedforward is the way to go.

To be complete: the laser's internal feedback control (current, temperature) make it a predictable system so is can be regarded as a 'black box' with a known mapping from modulation input to light output.

[Stuka]

OK, so a couple of questions from someone who still has the big "?" over their head ():

1. In "less-electronically-oriented" layman's terms, what is the end-result functional difference between this particular circuit, and the color correction board?

2. Would this be applicable in a "less common" projector setup, such as RGBV?