LG Plasma Panel tweaks for better blacks. - Page 16

Last night I tried adjusting set_down alone to see if it was contributing to the black level, and it was. Most of the improvement is from set_up, but set_down also plays a role.

As an experiment, I moved it beyond the 140us setting recommended by the manual and the screen got very dark, and then eventually pixel misfires occurred. At the point where the misfires just start the MLL is so low I can not measure it. Too low for my probe, just says 0.000. However the TV is not stable there as bright scenes lead to pixels that get stuck on as some have noticed.

Looking at the waveform, I can see that the maximum I can go before the shape changes is 150us. Adjusting beyond that starts to lift the negative ramp bottom corner off the floor. After moving is about 50V off the floor misfires start to occur. I decided to set it somewhere in between 150 and very slightly off the floor and I measure MLL at .005 now. I may keep it there. Still evaluating. I am guessing that the 140us figure is specified to make sure that the waveform never lifts off the floor even as the set ages or when cold.

So here is what I see now as a function of set_down pulse.

140us MLL = 0.009
150us MLL = 0.007
160us and 25V off floor MLL = 0.005

All of these are for 60HZ input btw. The MLL for 24HZ input is more than double these figures unfortunately and there appears no way to go below about 0.016 without misfires.

So with tweaking and the right equipment, this tv can get a measured 0.005-0.007 MLL black level. What is the measurement of the VT30 again? Doesn't this make the tv extremely close to the VT30 now?

I am curious about this Rob, but I don't know enough to feel confident closing up the TV and putting back on the wall if I mess with Va/Vs. (I'm already considering drilling holes in the back for easy adjustment of set_up/Down in the future)

I did a quick (slight) turn of Va, Vs, and VZB while watching the screen and did not observe any difference, though the voltage changed quite a lot. I put them back at their design values.

The thing I am most curious about now, is why I can not get my set_up pulse to be 280V high even with maximum closkwise turning of the pot. As I mentioned, my TV was originally showing 350V here, and I could get it down to about 285-290 minimum.

My question now, is what power supply voltage effects the maximum positive excursion of the waveform? If I reduce this, then perhaps I will be able to achieve the specified 280V pulse height without maxing out the pot.

Does anyone on the thread know the answer to this?

Not sure, but I can tell you that .007 looks extremely black compared to what I was used to looking at, so I am very satisfied.

Also, I am running my white point presently at a very bright 50 FtL, so my contrast ratio is over 7000:1 after calibration to 6500K. It is really an amazing experience to see the picture look like this. I really like this set ;-)

A sub $1,500 PZ950 competing with a $2499+ VT30 with these tweaks is unbelievably awesome. Just wish I knew someone close by with a scope. Tweaking by eye is a pain, but still works pretty well.

It looks like this product on Amazon can turn your PC into an oscilloscope for 189.39 Never heard of these before, but looks interesting.

http://www.amazon.com/100MS-Based-Di...4572655&sr=8-1

Can't be sure without a full waveform but from the description the negative excursion sounds like the amplitute below ground (GND) of the set-down period on the Y electrode. If you have a scope you should be able to see the waveform peak drop or rise relative to GND. Just guessing.

Thanks xrox. Do you know what power supply voltage effects the maximum *positive* excursion of the waveform? Trying to figure out why I can't achieve the target 280V for the set_up pulse in my case. I am coming in too high.

The set_up (rising slope) and set_down (falling slope) both produce a discharge that we see as black level. The pots may only control the slopes. If you want more excursion latitude the Vs may provide it because it sets the excursion point at which the set_up slope begins. See the waveforms on post #277. However, note that the Vs (voltage sustain) also is the applied voltage for firing the pixel during sustain period so it may mess up pixel firing as well.

Possibly Vs (see post #277) but I'm not sure.

Thanks xrox. The diagrams in post 277 are very helpful in understanding this a bit better. Looking at this figure, what I am trying to figure out is how to reduced the maximum peak of the ramp-up region shown in the reset period. I can not get mine down to the specified 280V even with set_up fully clockwise. I am guessing that this may be a design issue and is the reason most people are finding set_up fully clockwise is the best setting to give the improved MLL.

As you can see in these diagrams, the peak of the scan electrode (Y) waveform is much higher than the supply voltages Vs and Va (202 and 60 respectively in my set), so they must be generated by some form of step up circuit on the video board itself.

It is not labelled as such on those plots but the Vs sets the starting point of the ramp up and ramp down (as well as being the sustain voltage). If you draw a straight line from the top of the SUS waveform it will intersect with the beginning of the ramp up and ramp down.

That's it Rob! I think you are right. I see from the block diagram in the manual that VSC is derived on the Y-SUS board from the Vs and Va input voltages and, as you quoted, the description does say VSC controls the amplitude of the complex waveform whereas set_up and set_down control the upper and lower ramps respectively. It sounds like I may want to try reducing VSC a little which should make the overall waveform p-p smaller, so that I can achieve the correct ramp-up height in the waveform.

However, the figures xrox posted show VSC active during the address period, so it may have other effects too. The nice thing is, with a meter and a scope I can always put it back to where it was if I don't like it.

What's the logic behind the increase in MLL when going from 60hz to 24 hz? (in Laymans terms )

It's interesting as Samsung does that too, but Panasonic does not.

When a 24 fps input is detected the TV shows each frame 3 or 4 times (depending on model) so the result is that we are watching either 72 or 96 "frames" displayed each second, which is higher than 60.

For some reason, showing these faster/briefer frames causes the panel to be a little brighter in it's blacks. It's as if the faster rate does not give the panel enough time to relax between frames or something. I really have no idea what the root cause is.

Hope someone else can chime in.

Increasing the refresh rate to 72Hz increases the number of reset pulses per second which in turn increases the black level. Also since there is less time available the reset may need to be strengthened to accomplish the same goal in a shorter time period.

Edit: A more accurate description would be that when the refresh rate is increased the addressing time is shortened and thus the reset must be strengthened to increase priming particles and speed up discharge response.

Interesting. All of my prior measurements were with a 24hz signal. I guess that explains why my measurements are higher than others.

Yeah, 24 Hz operation is really disappointing. After seeing just how dark I can get with 60 Hz, I decided to set my blu-ray player to output 60 rather than 24.

I experimented more last night, and it turns out that VSC does not significantly effect the waveform amplitude. In fact, it hardly changed it at all. I reset all voltages to their design values except set_down, which I decided to leave set at the maximum ramp width before it starts lifting off the voltage floor (150us). This gives me 0.007 FtL for 60fps inputs and 0.017 FtL for 24fps inputs. Very happy.

The TV is back on the wall. Hopefully it will stay there...

That was a lot of fun.

I'm wondering about 50Hz material. Is that shown in 100Hz, to avoid flicker? And does that mean even worse black-levels than 24p/72Hz?

Nice! So happy to hear your results. I think Set_Up is really all most people will have to touch, though in my case, Set_Down did give a little extra dip.

Isn't that strange that LG left these pots so far from where they should be? almost completely counter clockwise? I don't understand why the other voltages are right on, but set_up is so far from optimal, even when measured with a scope.

I think we can say 100% for certain now, that for optimal black levels on LG Plasmas, this tweak MUST be performed. The difference in black levels is astounding, like rob said. Rpaul has the measurements to prove it.

After the tweak, would you say that LG can now compete subjectively for the #1 #2 best plasma of 2011? Price wise, I believe so. Almost $500 less than the Samsung at the same size, and includes 2 3d glasses.

I am excited about the excellent blacks now, and yes, I think it is one of the best in this regard, but there is a lot more to PQ than just blacks. In many regards the LG still shines, but there is one area in particular that this TV stinks at, and that is near-blacks. Level 16 is pure black (MLL), but levels 17, 18, and even 19 look terrible on this set due to extremely coarse dither. There is no way around this, and it is not effected by the recent adjustments at all, jst a property of the design. I still think it is among the best though overall now that the blacks are so good. I am very please with this set.

You are absolutely right! In some movies, like "Jonah Hex" the PQ is whole crap, cause of that dithernoise down there in the greyscale.

I hope, LG fixes this for next years models.

Anyway, I got deeper with my german PK350 (aka US PK550) than before and that is without the MLL getting lighter again after a few days.

a) Before I turned the set on, I made sure, that I had already found the optimum settings for all pots besides Vy-, Set_Dn and Set_Up

b) I set Vy-, Set_Dn and Set_Up full counterclockwise

c) I turn the TV on and let itself warm up with a fullscreen movie for about three to four hours

d) I turn the TV off and remove the power cord.

e) I removed the LVDS-cable from the Y-Board

f) Turned the TV on again and let it warm up for half an hour

g) Turned Set_Dn to the exact location I knew it won't result in any noise

h) Turned Set_Up the same amount than before clockwise, but after that went futher clockwise (slowly) until I reached a point, where there were some little clouds of missfired pixels began to arise

i) I turned Vy- some very little amount - the pixelmissfires disappeared

j) I turned the TV off and removed the power cord again

k) I connected the LVDS-cable again with the Y-Board

l) Turned the TV back on


tadaaaaaaa


Okay, now the TV's blacks are better (still 24p) than before, I'm more clockwise with Set_Up still without any pixelmissfires, but you have to let the TV warm up itself for at least half an hour, before pixelmissfires won't happen anymore.

Maybe the LVDS-cable removemnt helps some people who experienced a blacklevel degradtion a few days after theri pot tuning.

The idea is, that the control board isn't aware of any changes you do to the Set_Up and Set_Dn pots while the LVDS-cable isn't connected.