CurtPalme.com Home Theater Forum

[mikeshole]

Sony 1292. I just got my 5bnc to VGA cable in the mail, hooked it up and I noticed that the blue tube isn't as bright and sharp as the other two. The green is by far the sharpest, nicest one. Then the red is a close second, but the blue is way far behind. Blue just has a really weak contrast. The tubes have 4000 hours on them, so it should still be good, right? How should I go about trouble shooting this?

My camera isn't great, but I can try and take pictures of the tubes directly through the lens.

[CasetheCorvetteman]

Blue isnt meant to be as sharp as the rest, and it will hardly make a single bit of diffence to the overall image detail or sharpness even if the blue is reasonably out of focus.

It gets defocused in order to improve white balance. Look at the grey scale pattern on your screen and forget about how bright a single solid colour is, either on the tube face or the screen. You dont watch images on the tube faces, and you generally dont watch them in monochrome by shutting off 2 tubes either.

[mikeshole]

Wow, that's interesting, and a relief. I remember being surprised and confused as to why the white was perfectly balanced.

[garyfritz]

It's a combination of several things.

* Your eye is not as sensitive to blue, and does not focus well on blue. Try looking at some Christmas lights from a distance -- the reds are sharp and the blues are fuzzy.

* Blue phosphor doesn't emit as much light as green.

* Because blue can't pump out as much light, the blue phosphor "saturates" and doesn't increase its light output "linearly" like green does. (It's not really linear but pretend it is for this explanation.) If you drive the green at 75%, it has about 50% more light output than it does if you drive it at 50%. It's "linear." But the blue maxes out so maybe you only get 20% more light output at 75%. That would mess up your white balance -- at high IREs you would have too much red/green in your white because the blue can't keep up. To get around this, CRTs take advantage of the fact that your eye can't focus well on blue. They DEfocus the blue intentionally. This spreads the electron beam onto a larger area of phosphor. That means more phosphor can contribute to the light output, so you can get higher light output without "maxing out" the blue.

* This is the cause of the infamous "blue hump." If you set your white balance at e.g. 90% IRE without defocusing, the blue level is correct at 90% IRE. But because you didn't defocus, the blue is in the maxed-out "saturated" state. It's not linear. So instead of light output increasing linearly as you increase IRE, it increases quickly at lower IREs, then saturates and slows down at higher IREs. Since it's not linear, and since you set it to the right level at 90% IRE, that means you've got TOO MUCH blue in the midranges. It's a blue hump. Your midrange IREs will be too blue and "cold." You need to defocus the blue to get a more linear response; then when you set the white balance at 90% IRE, it should be pretty close at 50%.

[CasetheCorvetteman]

Doesnt a 1292 have an automatic blue defocus? Or is this only usually on NEC?

Here is an experiment for you mikeshole, put up a test pattern, like the fine cross hatch will do, then turn off red and green, get the blue focus control up on the screen, and go in and out of focus, youll see the blue get abit brighter as it goes out of focus.

You may see variation in brightness between the corners and center, and that is why astig and focus uniformity are still important on blue, unless you want your image to have an uneven blue!

Optical focus should still be set tight as you can get it on all 3 colours, but beam focus really should be defocused abit on blue.

[kal]

[kal]

[Spanky Ham]

I have always been curious if blue needs to be defocused if you are running the tubes with low contrast on a high gain screen.

[kal]

[kal]

[CasetheCorvetteman]

Yes if you play some good HD content and turn off the green and red, the blue is quite dim and not sharp at all.

[kabuby77]

My 1292 defocused too