Initial impressions of DZ's new color correction board

[buffo]

Hey folks!

DZ drove up to Charleston this weekend so we could geek out for a couple days, and he brought 2 of the new color correction boards with him. Yeah, these are the one's that we've all been talking about for a few months now. Well, they're here, and he's been assembling them for the past week or so. (Still lots of work left to do to get them all ready, but he had two of them with him so we could test them.)

I'm going to write up a formal review soon, and I'll also be producing a tutorial on how to adjust these things for maximum performance. But since there has been so much talk about these boards, I thought a little preview was in order.

First, for those of you that may not be aware of the project, these boards are designed to provide a variable delay on the color signals sent to your lasers' modulation inputs. The idea here is to compensate for the relatively slow response of DPSS lasers to modulation inputs. Essentially, you want to slow down all your *other* lasers so that their response is just as slow as the response of your slowest laser. The end result is that you will eliminate unsightly "tails" and other color irregularities in your scanned image.

The board accomplishes this by giving you the ability to delay the rising edge of the color signal (eg: how fast the laser turns on) as well as the ability to delay the falling edge of the color signal (eq: how fast the laser turns off) for all three color channels. (If you have more than three different color lasers in your projector, you'll need a second board for the other colors.)

The board also has adjustments for gain and offset for each channel. Offset is used to compensate for the fact that most lasers don't turn on until you have anywhere from .7 to 1.5 volts on the modulation line. By adjusting the offset, you can have the laser come on at any voltage you want, all the way down to zero. Likewise, the gain adjustment controls the top end of the modulation curve, so if the laser reaches full power at less than 5 volts, you can set the gain lower and still have a full range of power variability, rather than having the laser reach full power prematurely.

We spent about 12 hours testing the board on Friday night (and well into Saturday morning), before installing it in my projector late Saturday morning. Then we went through another 6 hours of tests on Saturday afternoon. In all, it was a busy weekend! (And my hat is off to DZ, who was willing to drive all the way up here just so we could work on this!)

What we learned is that adjusting these color boards is very tedious and time consuming. There are similarities between adjusting a color board and tuning a set of scanners, but trust me when I say that adjusting a color board is *much* more difficult. It is a job that is best performed by two people, with one person standing close to the scanned image looking for problems while another person handles the adjustments. Also, a long throw is preferred. (Makes it easier to see the problems in the image.)

You can get the basic adjustment completed in roughly 30 minutes. And if you already have a serious mismatch between your lasers like I did, then this basic adjustment will improve your scanned image quality a good bit. But this is only the beginning. After the basic adjustment, you'll be about 75% of the way to perfection. Getting that last 25% though is *HARD*. It can be done, but you need to be prepared to try lots of combinations. (Also note that even a small "tail" on a scanned image is still noticeable, so to correct the problem you really do need to go all the way to 100%.)

We frequently ran into situations where we needed just a little more delay, and ended up having to adjust the offset or the color shift in software to get it. But making such a change immediately changed everything else, so we'd have to go back to the beginning again. There are a total of 5 different settings / adjustments for each color channel (4 pots + a jumper that doubles one of the settings) on the board, plus you have the color shift setting in your software (which of course changes all 3 channels equally). So you can see how this can be very time consuming!

Also, we made an interesting discovery. The color board works extremely well on graphics and beams, but it actually makes raster-scanned images look worse. The Pangolin "Linea" show looked a lot worse with the color board enabled. Now, raster images are few and far between, but if you really like watching them, you may want to consider installing a bypass switch on your projector. (I installed such a switch on my projector, but for me the main reason was so I could show people the difference the board makes.)

Another point that is worth mentioning is that this color board will not correct any non-linearity in your laser's modulation curve. It will adjust the high and low points, but beyond that it assumes that the laser responds linearly. True, if you are running Pangolin's LD-2000 system, then you can use the color palette wizard to compensate for this, but I don't know if other software packages have such a feature. Also, this board does nothing to combat the noise (or jellybeaning) that you see on many DPSS blue lasers when they are scanning a solid line. The noise will still be there.

Having said all that, though, the board *will* improve your graphic images, and your beams and atmospherics as well. I found that the annoying color tails were completely removed (once we got the thing adjusted correctly), and the color mixing and blending was visibly improved as well. Granted, I've always been bothered by those color tails, so perhaps I'm more sensitive to it than the average person. But I think most people will be able to notice a marked improvement in their images nonetheless. Just understand that there is a lot of work involved in getting the board dialed in just right. (Installation, by comparison, is a breeze.)

More info later, once I have the time to take some pictures and work out a tuning tutorial for this board. That should give DZ time to finish soldering up the rest of the boards. Then the stampede can begin!

Adam

[DZ]

Well I'd like to extend a huge thanks to Buffo and his family for having me over to work on this. I absolutely appreciated thier hospitality. These boards will be available in a few more days, I really wanted folks to see that these are not going to be easy to tune up. I also came to the realization that I needed to add another part to go along with the board, a trimpot adjustment tool! So I managed to order those up when I got home last night, right before I had to go right back out to work till about 4am, talk about being tired. Anyway, after doing some thinking on just how difficult it is to adjust these color boards, I've decided that I'll probably offer something like a 30 day right to return.

Regarding raster, if you enjoy sitting down and watching the linea show over and over, then this board is probably not for you. We didn't take the time to specifically tune for raster scanning so I'm not sure if there could be any improvements or not. However when properly tuned for typical graphics and beams, the Linea show was noticeably worse. Of course in my opinion, rasters look terrible on solid state RGB's anyway!

[Pangolin]

Sounds like an amazing product. Where on earth did you get the idea for such a thing?

Bill

PS: What this really does is shift colors. If you have one slow laser, it probably would ruin the possibility of doing good rasters anyway, so I wouldn't go as far as to say that a product like this, in and of itself, ruins the possibility of doing rasters or makes them "worst".

[buffo]

Sounds like an amazing product. Where on earth did you get the idea for such a thing?

HAHA! Good point, Bill. We should have credited our sources!

For those that may not know, the whole idea for this circuit comes from this page on the Pangolin website, which describes a few different color smoothing solutions. The specific circuit that David has implimented on this board is a hybrid of the simpler sallen key filter and the more complex "ready for anything" circuit.

Bill had a *lot* of input on the design of this board. David sent several drafts to Bill, and Bill would suggest changes here and there. Once David had a working prototype, Bill agreed to do some testing at Pangolin to ensure that everything worked as it should. Since then, David has tested the circuit on two separate projectors, and it does indeed perform as expected.

What this really does is shift colors. If you have one slow laser, it probably would ruin the possibility of doing good rasters anyway

This is true. Rasters never looked really good on my projector, because my blue and green lasers were so slow compared to my red Maxyz modules. But they still looked "OK".

However, adding the color delay has a marked effect on raster graphics. While they never looked great on my projector, they do look a lot worse now that I have the board installed and tuned. As you said - a slow laser is going to make rasters look bad anyway. Having 3 slow lasers is even worse.

But it's a fair trade-off in my eyes, because all the other images look *so* much better. And if I really want to run the Linea show, I can just flip a pair of switches inside my projector and the color board is completely bypassed.

Adam

[LesioQ]

Rasters require high modulation bandwidth. I think every 'typical' analog filter-style delay will limit it's high end.
In the past I've been trying a different approach:

- for TTL blanking delay it was a long CMOS static shift register + variable clock
- for analog modulation I'd use older SAD1024 or other modern audio effect analog delay line (2 phase capacitor charge shifter), also clock-tuned.

Piotr.K

[dave]

- for analog modulation I'd use older SAD1024 or other modern audio effect analog delay line (2 phase capacitor charge shifter), also clock-tuned.

Piotr.K

Bucket brigade!

[-bart-]

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