NEC: Who believes they have excellent focus - AVS Forum
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post #1 of 36 Old 01-16-2008, 06:45 PM - Thread Starter
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I believe I have good focus on my red and green however my blue is not. I think it is not supposed be perfect, but I need a benchmark. Can anyone post a close-up picture of the focus test patter with blue only? I have a NEC PG9xtra

-Greg
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post #2 of 36 Old 01-16-2008, 07:56 PM
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The blue is defocused. There is an option you can turn the defocus option off but you will lose gray scale tracking.
For best result turn the defocus off then focus it best you can then enable the defocus.

It is all about quality...that is the picture

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post #3 of 36 Old 01-16-2008, 09:54 PM
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blue focus is irrelevant...as long as optical is spot on, electrical will have no effect on the image. In fact, if you want to get the sweetest possible NEC pic, then defocus blue as far as it can be defocused via the focus pot....this will level grey scale even more. Colors will be terrific!

Don't evaluate projectors with test patters!!! Evaluate them with video!
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post #4 of 36 Old 01-16-2008, 11:55 PM
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I worked real hard once at getting my blue focused as good as the red and green. I was able to get it real close, but that required walking up to the screen to see the blue focus pattern good enough.

All I do now is get it good though the binoculars and make sure that any flare is equal all the way around the dots.

Deron.

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post #5 of 36 Old 01-16-2008, 11:58 PM
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benareeno

How do you use video to focus?

So it helps to defocus the blue even more?

Deron.

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post #6 of 36 Old 01-17-2008, 01:58 AM
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blue can be focussed just as well as red and green.

Of course you'll want to defocus it to get proper greyscale tracking.

Kai
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post #7 of 36 Old 01-17-2008, 07:39 AM
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blue can be got as good as others, I prefer all defocusing off as everyone knows, but you will need the ability of a scaler to flat the greyscale out which will in turn makes things a bit harder on your blue tube

I have did comparisons with and without the blue defocus and I just can't stand the later, it is instantly visible on movie credits for example

-Gary
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post #8 of 36 Old 01-17-2008, 07:43 AM
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Defocusing in Nec is done very conservatively. It is NOTover done.
If you do manually put a grid up and line it up with red to make sure it is the same width or just barely wider.

It is all about quality...that is the picture

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post #9 of 36 Old 01-17-2008, 08:36 AM
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of course I use a pattern for focus...but I don't futz over blue focus. And then I don't spend hours looking at it...and once the credits roll, I really don't care.

Blue defocus is a must...and far more defocus than is currently done is a must.
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post #10 of 36 Old 01-17-2008, 09:01 AM
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Quote:
Originally Posted by Gary Murrell View Post

blue can be got as good as others, I prefer all defocusing off as everyone knows, but you will need the ability of a scaler to flat the greyscale out which will in turn makes things a bit harder on your blue tube

If you prefer the blue sharper, that's fine. But as Gary says, you have to use a carefully-tuned gamma-like adjustment on the blue or you'll pay for it with a bad color-temp response. You can't compensate for blue defocus just by boosting the signal in the source.

The reason for electrically (not optically) defocusing the blue is that the blue phosphor maxes out, output-wise, before red and green. Let's pretend the red and green have a linear response curve, so that an input signal of X results in an output of Y*X lumens. (It's actually not linear, but it makes the explanation simpler if we pretend.) Blue has a linear (X in produces Y*X out) response curve too, but only until it gets into the overdriven part of the response curve. Which, depending on your contrast levels & other things, may be at IRE 80, IRE 40, or somewhere else. After you get to that critical point where the blue starts to saturate, the response curve is no longer linear. It flattens out. See Fig. A in the attached pic for an approximation.

If you set your grayscale by measuring your color temp at, say, IRE 30 & IRE 80, you adjust your blue bias to match the R/G levels at IRE 30, and your blue gain to match the R/G levels at IRE 80. But since the blue response isn't linear, that means your blue levels are TOO HIGH in the region between IRE 30 & 80. This is what causes the "blue hump," which causes high color temps in the midrange and low color temps at high & low IREs. See Fig. B. This hump causes midranges (e.g. skintones) to look cold and blueish, while shadow details (and to a lesser extent bright areas) look reddish.

If you just boost the blue (equally across the IRE range) in your scaler, you're doing the exact same thing as boosting the blue gain in your CRT -- you're increasing the input signal. That is still going to drive the blue into the nonlinear range. You're back to Fig. B and a blue hump. **IF** you can tweak a gain transfer function that perfectly compensates for the blue saturation, and produces the proper linear response, then you should be able to get correct color temps without defocusing the blue. Be aware, though, that this gain transfer function is contrast-dependent; change the contrast and you change the IRE level where the blue saturates, so your gain transfer function is no longer correct. And, as Gary said, this approach drives your blue really hard and will shorten its life.

The usual solution to this is to prevent the blue from saturating in the first place. One way to do that is to lower the gain or contrast so blue never gets into the overdriven range -- but unless you've got a screen with huge gain, that will result in such a dim picture that you probably wouldn't want to watch it.

The other way is to defocus the blue. The blue saturates because it can only handle a certain level of electrons hitting the phosphor -- call it Bsat -- before it goes into saturation. Bsat is a count of the # of electrons hitting the phosphor per second, PER AREA. With a sharply-focused blue beam, you hit that Bsat level very quickly in a small focused dot. But if you defocus it slightly, so the electrons spread out over a larger area, you've got more phosphor contributing to the light output. You can get more lumens out before you hit the saturation area, so your blue response stays more linear across the IRE range, and you avoid the blue hump.

Since your eye can't focus on blue very well, most people won't notice the defocused blue, at least most of the time. It will show up most in test patterns, and in occasional things like the movie credits Gary mentioned. If that drives you insane, then keep the blue sharp. But realize you WILL NOT get proper grayscale if you do that, unless you do some very careful magic with your scaler.

Gary
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post #11 of 36 Old 01-17-2008, 09:33 AM
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I think Gary has access to blue gamma via its VP50.
Defocussing should be done conservatively and keep the contrast level at moderate level so it can keep up. Too much defocus does add some softness to the picture to my eyes in certain scence. I am running my defocus with nec's default and with contrast up to 60 it trackes the scale well.

It is all about quality...that is the picture

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post #12 of 36 Old 01-17-2008, 03:03 PM
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the gain to proper color temp far exceeds the lack of sharpness(which I don't see anyhow). That is to say, if you set proper greyscale and defocus blue...your pic will be far nicer than a sharply focused blue with bad color temp.
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post #13 of 36 Old 01-17-2008, 06:44 PM - Thread Starter
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There are alot of lessons here. Does someone have a picture of what I should shoot for?

-Greg
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post #14 of 36 Old 01-17-2008, 07:03 PM
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Saturation is saturation - I don't understand how you can tweak a gamma curve around it (although the Marquee apparently tries it).

I've posted a 1080i 72hz 1:1 pic of Red on an Xtra many times. Green was as sharp, but didn't photograph as well. Blue is blurry, as it should be.

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post #15 of 36 Old 01-17-2008, 07:15 PM - Thread Starter
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can't do #3. Which is why i need reference point. I am trying to get to good enough.

-Greg
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post #16 of 36 Old 01-17-2008, 07:21 PM - Thread Starter
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yes.
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post #17 of 36 Old 01-17-2008, 07:27 PM
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Binoculars don't work. It's almost impossible to hold them still while trying to adjust something.

A video camera and a TV work much better.

Blue electronic focus is dead easy - it's the point where blue goes dim.

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post #18 of 36 Old 01-17-2008, 09:45 PM
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Mark's point about "blue going dim" illustrates the blue saturation perfectly. Assuming your contrast is set high, above the saturation point, then blue is focused electrically when it gets dim. Defocus a bit and it gets brighter -- which is exactly what you want to avoid saturation.
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Saturation is saturation - I don't understand how you can tweak a gamma curve around it

I don't believe the saturation point is a hard ceiling. I.e. the blue doesn't get to a certain point and then stop getting any brighter regardless of input level. I believe it rolls off like in the picture I drew. If that's the case, and you apply an inverse gain function, you should be able to straighten out the rolled-off curve, assuming your blue has enough gain to drive it that hard. But you might rapidly fry the blue in the process.
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post #19 of 36 Old 01-19-2008, 07:12 PM - Thread Starter
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I have attached these pictures. Does blue look correct? Should Green and Red be more infocus.

I am using my 4MP Canon Optura 600 Camcorder
LL
LL
LL
LL
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post #20 of 36 Old 01-19-2008, 07:17 PM
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All three colors should be better unless the camera is doing this. Doug

1) Where is contrast set at?
2) Are the focus pots on the deflection optimized for screen center?
3) Mechanical astig done followed with electronic?
4) Lenses refocused?

Blue is flared to the right on astig but not by much
Looks almost like a tad to low of HV or lenses are fighting each other but not common on the 144-145's
Could be camera close up doing it
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post #21 of 36 Old 01-19-2008, 08:04 PM
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Mark has it right on blue electric focus, go over it many times, over and under until you find the dimmest spot or in my case where I use no blue defocus use a 1:1 horizontal pattern (1920x1080p) and get the sharpest most defined lines, this requires perfect blue optical focus though

I have always found red to be the sharpest on any PJ I have owned, usually followed by green then blue, but blue is 100% equal to green on my 1352lc and it makes for one heck of a pic

-Gary
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post #22 of 36 Old 01-19-2008, 08:28 PM
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I agree. For HD you will need good focus for all three. There is a reason NEC default is conservative for blue defocus.

It is all about quality...that is the picture

JVC & NEC 8" CRT with 106" wide Stewart screen. All NHT speakers driven by Pioneer Elite AVR and bluray

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post #23 of 36 Old 01-21-2008, 08:02 PM
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Quote:
Originally Posted by benareeno View Post

blue focus is irrelevant...as long as optical is spot on, electrical will have no effect on the image. In fact, if you want to get the sweetest possible NEC pic, then defocus blue as far as it can be defocused via the focus pot....this will level grey scale even more. Colors will be terrific!

Don't evaluate projectors with test patters!!! Evaluate them with video!

Ben, which way do you defocus the blue? + or - ?

I don't know what to set my tracking at
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post #24 of 36 Old 01-21-2008, 08:06 PM
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Quote:
Originally Posted by CaspianM View Post

I agree. For HD you will need good focus for all three. There is a reason NEC default is conservative for blue defocus.

What is the NEC default?
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post #25 of 36 Old 01-21-2008, 08:07 PM
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Hey Ken,

long time no talk!

Just make it bigger...not sure if it matters which way you twist. As long as the H's get bigger and blurrier, you're on your way to better blue output.

Are you still running an XGLC?

Ben
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post #26 of 36 Old 01-21-2008, 09:01 PM
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Kinda curious what resolution you were running when you took those screen shots ?

Your focus is not good, on all colors, have you done electronic astig correction ? Have you done RGB focus after astig ? Have you been screwing around with your mechanical astig/flare rings ??

NEC's should be able to quite easily focus to the point where you can see each tiny element that makes up the "H" of the focus pattern...

As a benchmark for defocussing your blue, after you've fixed the focus of your green and red so you can see each piece of the the "H", defocus your blue enough so that all the pieces blend together, but not so much that the outline is a lot bigger than the red and green "H"s...
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post #27 of 36 Old 01-22-2008, 06:41 AM
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Are you zooming with your camera by chance and causing those H patterns to appear out of focus? If you are standing physically closer to the screen you need to use the macro option on your camera and pull zoom all the way back in (no zoom). If the camera is not the problem then you have some focus work to do. Get optical focus the best you can for each color. Start with the rear section of the lens to get center focus good. Lock it down. Move to the front section of the lens for corner focus. This will be more difficult to judge but just get it to where the H's don't flare in any direction. Lock that down. Move back to the rear lens section to do center focus once again. Lock that down and move to the next lens and then the next one. Green, then red then blue.
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post #28 of 36 Old 01-22-2008, 07:31 AM
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Quote:
Originally Posted by YONEXSP View Post

What is the NEC default?

I was referring to Nec's preset defocus. It is under the option menu I think.

It is all about quality...that is the picture

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post #29 of 36 Old 01-22-2008, 11:51 AM
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it's a switch on the deflection board...right beside the blue focus pot.

Ben
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post #30 of 36 Old 01-22-2008, 01:33 PM
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We need to see shots off a 1:1 black/white line test pattern.

But at 1920 x 1080 at 48hz (96i actually) I cannot see dots inside the H's on the internal focus pattern.

Anyway, my XG setup is terrible at the moment...so don't feel too bad if you are not happy with yours!

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