CurtPalme.com Home Theater Forum

[mp20748]

[Nashou66]

[wkosmann]

[Nashou66]

[Sparky015]

I couldn't make it that soon up to NY. I just have way to much work at home that needs to be done by Christmas Eve (My father-in-law graciously volunteered my house for the family Christmas Eve party ). I can definitely make the Febuary meet though.

[macgyver655]

[mp20748]

OK, I'm warming it up.....

I've also sent Craig a PM on this.


I'll be bak!

[mp20748]

I found out if I put the camera in the Aperture mode, it'll allow me to capture the differences between regular contrast (were Craig set it) to 100.

Craig will have to comment on this, because the camera is not showing how bright it really gets.

They are all 1080P untouched:

[macgyver655]

Its going to take me a while to write this up so if others want to comment in the mean time, go ahead.

Nice pics by the way.

[Robert A. Hill]

Nash,
What are you going to do with the Ampro? Unlike the Marquees, no mods are necessary with Ampros. They shipped optimized from the factory:-)

[Nashou66]

[macgyver655]

Enjoy the read........



I'm going to try to explain why I believe that manipulating the video signal can affect high resolution, high contrast blooming. In order to do this we need to understand just how the electron beam is manipulated in a CRT tube. I'm going to try to explain this without using technical jargon and may change some technical words into simpler ones to help everyone understand.

When it comes to the electron beam there are 3 things involved with the flow of electrons. Speed, intensity (pressure) and volume.

Now the cathode is what emits the electrons and it must be heated to do so. The heater circuit’s job is to only heat the cathode and that’s it. Now the volume or amount of electrons the cathode emits is determined by the voltage applied to it. The more voltage, the more electrons. When the picture is black there is no voltage so there are no electrons. If the picture requires an area of the phosper to be half lit then half of full voltage is applied and if full phosphor needs to be lit then full voltage is applied. As you can see it’s the volume of electrons that determines how bright a certain area of phosper will get, not the speed. This is what contrast is and this pertains only to contrast.

Now when the electrons leave the cathode they pass through 4 grids. G1 first, then G2, G3 and G4.
Alright guys, stop looking at me funny and let me explain. Now the electrons don’t shoot out of the cathode, they are drawn to the tube face by an electronic charge. There are 2 grids that are used for this process. G2 and G4. These are used to control the speed of the beam, or electron flow as they draw the electrons from the cathode. Now G4 is actually the high voltage grid and this is a constant voltage. The G2 grid however has a variable voltage so when you are adjusting the G2, you are adjusting the speed of the beam. This is where if you increase the speed of the electrons so fast that the focus can’t keep them all together, many of the electrons just spray out and hit the entire phosper face randomly. Now there's not a lot of electrons in the spray so the phosper only glows dimly but you also still have the beam to light the proper areas of the image. This is whats called washout because it washes out the true image.

So as we can now see the G2 and G4 grid are responsible for getting the electrons to the tube face but we still need to control the shape of the electron beam to get a nice fine and stable beam to light precise areas of the tube face. G2 and G4 have no control over the beam shape.What controls the beam shape is G1 and G3.

As the electrons leave the cathode they have no shape so this is the job of the G1 grid. The grid forces all the electrons together with such force that they actually cross over each other before they enter the G2 grid. So G1 is responsible for the first stage of beam shaping. This is also where astigmatism is done if the tube is so equipped but for this explanation were not going to discuss this.

The final shaping takes place in the G3 grid which is actually the focus grid. This grid also forces the electrons into a tighter group to make a finer beam, like an extra fine point pen. Now the G3 grid is only used internally for ES (electro static) focus. Many projectors use external EM (electro mechanically) focusing and if so equipped don’t use the internal G3 grid. EM focus has better control over forcing the electrons into a tighter group to make a finer beam. This is why when you defocus the beam the G3 grid is not forcing all the electrons tightly and the main beam is surrounded by stray electrons which don’t have a lot of volume and when they hit the phosper, that area doesn't glow as brightly as the main center beam.

After the beam leaves the G3 grid it enters the deflection area which guides the beam to draw the image on the tube face. For this discussion were not going to go into the deflection operation.

So we have discussed G2 which controls beam speed which sets black level cutoff, to prevent electron spray. We've discussed electron volume which controls contrast, or phosper glow intensity. So what about brightness. Brightness is controlled by beam intensity or pressure. This is achieved by modulating (adjusting) the potential difference (Voltage difference between two points) between the cathode and G1.

So now that we know how this works, what does that have to do with blooming and how it can be controlled.

Lets say that at a low resolution it will take x amount of electrons to draw an image. Now if you increase the resolution of the same image it will take more electrons to draw that image because your drawing more lines. And a high resolution will take a lot of electrons for a lot more lines. So what that means is that at higher resolutions, the grids that shape the beam have a lot more electrons to force together. And as we also learned it takes more electrons to produce a higher contrast. So we have more electrons at higher resolutions along with more electrons for higher contrast. Now if the beam shapers cannot force all these electrons into a tight group the stray electrons are going to hit the surrounding phosper and since there are so many of them, now the surrounding phosper will glow strongly and this is your blooming affect. And the blooming will be more significant in higher contrast areas of the image.

So the task is to try to force almost all of these large volumes of electrons into as tight a group as possible. So we know we have 2 places to accomplish this. G1 and G3 (we will use EM external focus here since it applies). So we can try improving the focus coils to better force the electrons together. However we are limited in the end.

Now G3 is also limited by the shape of the beam as it comes into G3. Meaning if there are a lot of loose electrons as the beam come into G3 then it has to work harder to force all them together. But if the beam was fairly tight when it comes into G3 then it may be able to finish grouping the final strays. So we now look at the only area left to try and shape the beam to a nice tight group. G1. If we can get G1 to function in a way that it can force this large volume of electrons into a really tight beam here, then G3 can just finish up the remainder.

So how can we get G1 to perform better. We give it as clean of a signal that we possibly can so it can perform with the greatest efficiency that it can.

So where does G1 get its signal from?

The video signal......................................

I left out other things involved in the overall function of the CRT as they are not relevant to this discussion.

And…..now I have a headache…………

[1031]

[quote="macgyver655"]

Now the cathode is what emits the electrons and it must be heated to do so. The heater circuit’s job is to only heat the cathode and that’s it. Now the volume or amount of electrons the cathode emits is determined by the voltage applied to it. The more voltage, the more electrons. When the picture is black there is no voltage so there are no electrons. If the picture requires an area of the phosper to be half lit then half of full voltage is applied and if full phosphor needs to be lit then full voltage is applied. As you can see it’s the volume of electrons that determines how bright a certain area of phosper will get, not the speed. This is what contrast is and this pertains only to contrast.

quote]

Are you sure?. Beam current forms between two potentials -> HV and voltage at cathode, greater the difference, greater is beamcurrent (more light). Normally cathode is drived inverted (less light->higher voltage and vice versa)
Example HV=34Kv - Vcathode=150v=33,850KV
HV=34Kv - Vcathode=30v=33.970Kv (bigger potential difference here)
OK this was simplified example, off course G1/G2 plays big role here also..
Blooming happens also when HV-regulation fails under high beamcurrents (HV drops a little)

[antorsae]

[CIR Engineering]

Hey guys, I just skimmed this thread so I don't know all the details on what everyone is saying but...

I was at Mike's place this past Sunday. We played with his Marquee and indeed his projector can run extreme contrast levels. Typically on a Marquee if the magnets are done really well the green will still bloom at or below level 60 with contrast. On Mike's projector there is very little blooming on any tube including green all the way up at 100% output. Mike's drive was also set to 96 in the gray scale menu, so it's not like the contrast was set high and the drive was too low either.

We looked at the SMTPE 1:1 pixel patter at 1080p 60Hz at the center of the screen. This pattern makes it easy to see blooming, because when the tube blooms you start to see that the 1:1 gets distorted and then disappears into a smear of bloomy mess. Mike's projector only blooms slightly at 100 contrast (less than a typical Marquee with contrast at 60).

We forgot to measure light output at 100 contrast so I don't know how many flt's it is, but I will say that it looked like the projector was going to burn a hole through Mike's screen. At contrast level 75 I calibrated the gray scale and the image was perfectly focused and very bright. The projector also tracked gray very nicely at this high output level with only a dE hump up to 11 at the 20 ire point. All other dE's were below 4 across the sweep.

These VNB's would be really good to use with LUG tubes in a 9500 Marquee because typically the LUG tubes are much dimmer and bloom out much earlier than stock Marquee tubes. I have a client local with a Madrigal 9500 Ultra in which I put a green LUG and Barco 909 magnet. Before I installed the 909 magnet (LUG with Marquee stock magnetics), the LUG bloomed if I set contrast above level 53. With the 909 magnet I can raise the contrast to 60 and it starts to bloom there. Next I am going to try Mike's new VNB's on this Madrigal because I want to get more light output for this client... I'll post back with results on this when I get it done... could be a while though. But this will be an LUG with Mike's VNB in a 9500.

Mike's also mentioned that he is probably going to try LUG's in his projector next so that should be interesting.

craigr

[macgyver655]

I'll respond to questions later today. If anyone else has questions, fire away.

[Nashou66]

Craig i think you mean Mike VIM not VNB. Mike was talking about his VIM. Now I'm getting confused

Athanasios

[mp20748]

[antorsae]

Hi Mike,

Do my neck boards (Mikrons) exhibit the anti-blooming behavior?

rgs - Andres

[Sparky015]

Yes, I only have a MP VIM, my VNBs are stock (I only updated the two 22uF electrolytics with 105C 160V versions).

Paul