Bright area flickering partially demystified! - AVS Forum
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post #1 of 26 Old 12-12-2009, 07:59 PM - Thread Starter
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(First time poster here, please be nice. )

Like apparently 14% of people, I can notice flickering that most other people cannot.

Specifically, after just getting a Panasonic 50" G15, I noticed that bright areas were flickering badly. After some googling, I found ample evidence of other people noticing the flicker, some saying across all brands, some saying Panasonics are the worst, etc. Specifically I noticed that not only I see bright areas flicker, but I see them as "yellowish" when I move my eyes quickly, with a "side of blue aura" I couldn't quite explain. (My LCD screen leaves no trail when I move my eyes, weird.) I also noticed that the flicker seemed to interfere heavily with some 24fps motion, and almost felt "countable", so I couldn't believe it was 600Hz, and even 60Hz seemed too high. I guessed 20-30Hz. (Turns out I was spot-on! See below.)

Tonight I finally had a "lightbulb" moment (pardon the cheap pun) which I think might be of interest to others wondering about this. I tried to take high-speed photos of the screen displaying a bright scene and of course got inconsistent results, but not enough to figure out what's going on. I got yellow and blue takes though, which made me think I was onto something. Then I remembered that the high-def Handycam I have laying around has a 120fps burst mode, which I honestly never thought I'd find useful... until tonight!

I get consistently 2 frames of one kind, 2 frames of another, which means that A) the difference between the two screenshots below is not due to interference between the plasma screen and my camera (it would change at every frame, or the pattern would change) and B) that my G15 displays 1/60th of a second a very very blue image, 1/60th a very yellowish image, back and forth, meaning a 30Hz overall cycle. I'm sure they have a good technical reason for this, and they assumed that consumers wouldn't see the 30Hz flicker. Even in total darkness, let me tell you I see it pretty darn well, as 30Hz is way too slow to be acceptable, enough to shop for an LCD thanks to my store's generous return policy. (Very sad, I love how lifelike the picture is! Have you seen the daytime Lost island shots on such a plasma? WOW!) It also explains why I see it interfering with 24fps motion, 30Hz being relatively close.

Attached are same area crops of the two frames. What we consciously see is the average of the two.

Bottom line is: it's tough as a layman to understand why this is apparently necessary. Power saving, maybe? Surely though, the 600Hz displaying engine doesn't seem all that exciting when you realize it's being used to display significantly different images each 1/60th of a second... I'm surprised so few people see it, given how slow it is.

I also don't think the set is faulty, but just to be sure I'll go to the store tomorrow with my 120fps camera to see if all their plasmas do it, or just Panasonics, or none at all (which would then point to mine being defective - I'd be surprised).


(Note: yes I disabled all the digital processing I could find in the Picture menu, and the 24p option isn't clickable so I don't think it's related to that. It happens with all input sources I could try: HDMI from PS3, HDMI from computer, analog component from PS2, it's even clearly visible in the TV's menus on all inputs. What I filmed is displayed by the G15 itself, does not come from my sources. The two frames you see above were filmed in total darkness, so there's no intereference from other light sources either.)

Update:

Having been to the store knowing what to look for, I could easily differenciate plasma screens from LCDs, even from 40 feet away. (The salesman was impressed. ) It turns out that all plasma panels do this, so it has nothing to do with Panasonic per se. I'm attaching two frames taken in-store where we clearly see a plasma screen (non-Panasonic) on the left and an LCD panel (Sony) on the right.

So it appears that all plasma displays absolutely must spend half their time displaying mostly blue, and half mostly yellow.

(And the headaches are now gone with my replacement CF-backlit LCD panel. I also must admit that modern LCDs have somewhat competitive black levels. Cloudy but decent. Forget about viewing angles though, ugh... 45 degrees from dead center at the very most.)
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post #2 of 26 Old 02-20-2010, 05:26 PM
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Well this is a great post. I thought I was seeing things because my girlfriend didnt notice anything at all. And I also get very uncomfortable when seeing this flicker all the time.

I really hope Sears will take my Samsung plasma back without charging me the restocking fee.
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post #3 of 26 Old 02-20-2010, 06:13 PM
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Panasonic 2009 plasmas are a wreck. Huge problems across the board. A once great copmpany seems to be going down the tubes
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post #4 of 26 Old 02-20-2010, 06:39 PM
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VPhantom,

Plasma displays must compile a full image with a series of sequential partial images called subfieds. Using a high speed camera essentially captures portions on the subfields (not the full image) and it does this in a regular repeating fashion as the camera has its own sample rate.

The flicker you describe is duty cycle flicker and it happens due to the Plasma display compiling an image with "weighted" subfields.

The blue and yellow you see when moving your eyes quickly is due to the blue phosphors responding over 1000 times faster than the red and green phosphors causing a temporal color seperation effect.

Cheers
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post #5 of 26 Old 02-21-2010, 12:09 PM
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I wonder if people with only great vision see this occuring with a plasma?
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post #6 of 26 Old 02-21-2010, 12:26 PM
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Quote:
Originally Posted by rongo View Post

I wonder if people with only great vision see this occuring with a plasma?

Hard to say. I notice this bright scene flicker bigtime & I don't think I have very good vision. I was having a hard time initially trying to decide if I could live with it. And I decided that the great picture overall was worth the annoyance. I find I need to set the Contrast lower on this set (50G10) to minimize this problem or its too much.

For me it's not the whole screen that flickers but only a section of it like the sky in a scene that's in the upper part of the screen for example. I also can't believe most people don't experience this. But I don't see the blue yellow stuff the OP saw thank goodness.
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post #7 of 26 Old 02-21-2010, 04:56 PM
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Quote:
Originally Posted by rongo View Post

I wonder if people with only great vision see this occuring with a plasma?

I know that I dont have great vision and I see the flicker big time.
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post #8 of 26 Old 02-21-2010, 05:08 PM
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i've seen the flicker on our panny plasma with mostly white images on the screen, it doesn't really bother me though and its only really visible if i'm close to the TV at seating position its much less apparent

it appears to also be sort of like a pseudo blanking method of sorts or at least perhaps has that effect and that is one of the reasons why motion appears much better to many people on plasma than it is on LCD.

LCD's unless they employ a scanning backlight have what is called sample and hold blur because they always display a static image on the screen until told to change and if you want to know about it I'm sure xrox has explained it in great detail on these forums before

If you're a gamer or interested in using an LCD TV as a primary monitor take a look at my thread on Input Lag
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post #9 of 26 Old 02-21-2010, 05:39 PM
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I think some of this is inherent to the phosphor technology, but my recollection from the Kuro threads is that there's something called "DSE" (dirty screen effect) and that it seemed to have something to do with the AR coating not being applied 100% perfectly since some have it and some don't and some have it to a degree. Granted that is for a different company, but I'd expect there may be similarities.
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post #10 of 26 Old 02-21-2010, 08:31 PM
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i have recently noticed this on my plasma a panasonic tc-p42s1. at first i would only see it on the top 2 inch of the screen going horizontal and on a white screen, now i can see it on more then half the screen. it also makes a slight change from bright white to a darker white which is really annoying. i contacted panasonic about this because it just seem to start happening and they are sending a tech out this week. i also have the black level rise and my ir seems to be getting worse. the first few months i had my plasma they had to replace my A-board as the tv turned off by itself and would not power back on. i personally think this all has to do to the increase of voltage that panasonic has stated happens as the panel ages. i say this because the first few hundred hours the pic was fantastic,black were black and i saw no problems in the whites flickering or anything else. now since the a-board replaced and a few months has past the black levels have risen, IR has gotten worse and now the whites seems to flicker and surge from bright white to a darker white and then normal over and over. i really think this all ties into the voltage increase and it happening over a very short time. i am probably wrong but it all seems fishy this all happens after the balck level rise/voltage increase.
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post #11 of 26 Old 08-04-2012, 07:28 PM
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Same issue with my 55" GT50 purchased just yesterday. Thanks for this thread - I now know that I'm not going nuts. This should be pinned..
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post #12 of 26 Old 10-11-2012, 12:03 PM
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I wonder if there are any plasmas that have less noticeable bright area flickering..
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post #13 of 26 Old 10-14-2012, 11:26 AM - Thread Starter
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Quote:
Originally Posted by thebigdaddye View Post

the first few hundred hours the pic was fantastic,black were black and i saw no problems in the whites flickering or anything else. now since the a-board replaced and a few months has past the black levels have risen, IR has gotten worse and now the whites seems to flicker and surge from bright white to a darker white and then normal over and over. i really think this all ties into the voltage increase

Just to clarify, in my case the flickering was clearly visible on the brand new unit, no break-in done. I can also see it on all plasmas on demo in stores, though of course those probably are quite broken in. smile.gif

I guess the only way to get blacks, rich colour and no flickering will be the day when we get affordable Super AMOLED TV's in the 40-60 inch size range. Best I saw was some weird $30,000 24-inch OLED monitor, so I guess we're still a solid 5+ years before that happens...

(It reminds me of the early plasma days, when a 40" unit would set you back $25K. It truly is just a matter of time before we get SciFi-ish cardboard-thin flexible OLED TV's that weigh just a couple of pounds and consume a handful of Watts per square foot... My guess is around year 2020.)
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post #14 of 26 Old 10-14-2012, 06:28 PM
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I notice 2 types of flicker with my 4 or so year old panasonic TC-P54S1

1 seems to stem from the phosphors or the subfield drive or something you see the screen look like a wave goes accross it at a regular rate. I do not consider this to be a problem. It is a tradeoff for not seeing lossy ghosting images from LCDs.

The other seems to be more of a problem. As other say it only takes place on bright white screens. But I have found because I have my HTPC hooked up that when surfing web pages which are mostly white with black text that if I move the page up or down I can find locations where a more annoying type of flicker starts. I can move up the page and make it go away and then back down a little and make it return. It is odd because I do not consider the average content of the page to be changing its all just white with black text. I used to think it might be dynamic contrast ratio or something. Not sure though and I am bothered that newer plasmas still seem to exhibit this problem. Since 3D does not appear to mean 120hz I will keep holding out on an upgrade.
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post #15 of 26 Old 10-15-2012, 04:22 PM
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Quote:
Originally Posted by VPhantom View Post

525x525px-LL-3ee98dac_vbattach160653.jpeg

I noticed this when I was using a Canon 60D to record image retention on an ST50. I must have spent an hour trying to get rid of it, and changing the shutter speed would make it worse in the video. I eventually had to slow the shutter speed as much as possible.

I do notice the flicker on bright white without the camera, but I assumed it had something to do with a combination of motion smoother and 24p at 48Hz.
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post #16 of 26 Old 10-16-2012, 06:31 AM - Thread Starter
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Originally Posted by Childs View Post

I do notice the flicker on bright white without the camera, but I assumed it had something to do with a combination of motion smoother and 24p at 48Hz.

Unfortunately it's because the plasmas all flicker strongly at a steady 30Hz (regardless of claimed refresh rate - mine said 400Hz or 600Hz I believe, suuure), so motion looks absolutely horrible with 24fps content.

Not only that, but with my Panasonic G15 I had plenty of banding and temporal dithering clearly visible from several feet away. From up close it looked like a 256-color bitmap which really surprised me given the claims of 6144-level gradation per color. I'm really hoping for OLED to achieve what plasma never could. smile.gif LCD, for now, seems to have the edge. No flickering, banding, etc. on my Sony Bravia Z. If only it had the blacks of a brand new plasma...
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post #17 of 26 Old 10-16-2012, 06:43 AM
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Quote:
Originally Posted by PhoenixLev View Post

I wonder if there are any plasmas that have less noticeable bright area flickering..

While I don't think I could live with the flicker on a 50" plasma II don't find flicker noticeable on 42" plasmas. It's a pity they've stopped selling them nearly everywhere.

Some LED blacklit LEDs do have flicker because they strobe the backlight. That feature should be possible to turn off.
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post #18 of 26 Old 10-16-2012, 08:08 AM
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Re-read post #4 smile.gif

All PDPs flicker at 60Hz (not 30Hz) because they all use weighted subfields.

All PDPs produce yellow and blue color separation due to differeing phosphor decay rates.
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post #19 of 26 Old 10-16-2012, 10:37 AM
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Originally Posted by VPhantom View Post

LCD, for now, seems to have the edge. No flickering, banding, etc. on my Sony Bravia Z. If only it had the blacks of a brand new plasma...

Yeah, I think I should have went this route. Uneven backlight and possibly dead pixels or flickering, IR, and/or uneven panel. Cant win.
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post #20 of 26 Old 10-16-2012, 11:36 AM - Thread Starter
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Quote:
Originally Posted by xrox View Post

Re-read post #4 smile.gif
All PDPs flicker at 60Hz (not 30Hz) because they all use weighted subfields.
All PDPs produce yellow and blue color separation due to differeing phosphor decay rates.

What I'm describing in my original post was quantitatively measured at 30Hz, not 60Hz. I doubt I would've felt the need to return the TV if it would've been 60Hz. Basically for 1/60th of a second the whites are blue, and for the next 1/60th of a second they are yellow, in a continuous cycle and in equal durations each and with a strong sudden change between each, nothing smooth or uneven (say biased towards blue) about it.

Thanks for your reminder, I read up and I (now) understand the general concept of plasma driver subfields.

What I don't understand is why they apparently do their 600Hz refreshes in "bunches": 10 combined red+green updates, then 10 blue updates, yielding the perceived 1/60th of a second blue tinge, 1/60th yellow tinge, thus the 30Hz cycle I observed.

If they were interleaving them through time we wouldn't notice any flicker. (i.e. if they can't update red green and blue simultaneously, at least update them sequentially: 1/600th red+green, 1/600th blue, etc. The perceived flicker would be at 300Hz and that's beyond all human eyes I ever heard of. wink.gif )

I assume there's an unavoidable technical limitation preventing such partial refreshing, but bottom line these 10-in-a-row are still why I can't watch current plasma screens without getting a headache. smile.gif
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post #21 of 26 Old 10-16-2012, 11:42 AM - Thread Starter
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Originally Posted by Childs View Post

Yeah, I think I should have went this route. Uneven backlight and possibly dead pixels or flickering, IR, and/or uneven panel. Cant win.

I've been extremely lucky with my LCD panels so far. Viewsonic and Dell monitors, no dead pixels. The Sony Bravia Z TV also no dead pixels, though the backlight is brighter/cloudier than I'd like. (A bias light around/behind the TV helps a lot with that.) Saturation also doesn't match a plasma. Never heard of flickering problems though; that must be for those new LED-backlighted panels, but I assume the well-designed ones are fed proper DC for a stable output.

(Maybe I'm expecting too much of this industry, though. I sure hate my LED Christmas lights... Apparently $30 for the 100-light strand isn't enough to include the $0.25 capacitor it'd need to attenuate the flickering. tongue.gif )
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post #22 of 26 Old 10-16-2012, 11:53 AM
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Originally Posted by VPhantom View Post

Never heard of flickering problems though; that must be for those new LED-backlighted panels, but I assume the well-designed ones are fed proper DC for a stable output.
(Maybe I'm expecting too much of this industry, though. I sure hate my LED Christmas lights... Apparently $30 for the 100-light strand isn't enough to include the $0.25 capacitor it'd need to attenuate the flickering. tongue.gif )

In regards to flickering, IR, and uneven panels, I was referring to plasmas. When I first started buying HDTVs I bought LCDs, and dealt with a lot of issues then. Its like it the tech for either LCD or plasmas never matures.
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post #23 of 26 Old 10-16-2012, 02:03 PM
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Quote:
Originally Posted by VPhantom View Post

What I'm describing in my original post was quantitatively measured at 30Hz, not 60Hz. I doubt I would've felt the need to return the TV if it would've been 60Hz. Basically for 1/60th of a second the whites are blue, and for the next 1/60th of a second they are yellow, in a continuous cycle and in equal durations each and with a strong sudden change between each, nothing smooth or uneven (say biased towards blue) about it.
The TV is compiling frames at 60Hz. Your camera is capturing frames at 120Hz (or whatever). Overlap the two and you get a repeating pattern where the same two portions of a compiled frame is captured. This explains the result you are getting. In other words, the TV is not producing a yellow white followed by a blue white. You are just capturing a yellow portion of one frame followed by a bluish portion of another. This is made possible by the phosphor decay differences between blue and green mentioned earlier.

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What I don't understand is why they apparently do their 600Hz refreshes in "bunches": 10 combined red+green updates, then 10 blue updates, yielding the perceived 1/60th of a second blue tinge, 1/60th yellow tinge, thus the 30Hz cycle I observed.
They don't. Each frame is compiled using 10 sequential subframes of red, green, blue. But the response of the blue is faster than the green creating some color seperation in time.
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Originally Posted by VPhantom View Post

If they were interleaving them through time we wouldn't notice any flicker. (i.e. if they can't update red green and blue simultaneously, at least update them sequentially: 1/600th red+green, 1/600th blue, etc. The perceived flicker would be at 300Hz and that's beyond all human eyes I ever heard of. wink.gif )
I assume there's an unavoidable technical limitation preventing such partial refreshing, but bottom line these 10-in-a-row are still why I can't watch current plasma screens without getting a headache. smile.gif
The flicker is due to weighted subfields.

Each subfield (eg - 10 per frame) has a different brightness (i.e different weight). And they are displayed from brightest to darkest. This creates a bright initial part of a frame and a dark ending part of a frame. Same goes for the next frame. At 60 frames a second this creates 60 bright pulses and 60 dark pulses (i.e. - 60Hz flicker)
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post #24 of 26 Old 10-17-2012, 11:10 AM - Thread Starter
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Originally Posted by xrox View Post

Each subfield (eg - 10 per frame) has a different brightness (i.e different weight). And they are displayed from brightest to darkest. This creates a bright initial part of a frame and a dark ending part of a frame.

Thank you xrox! Now that was the missing piece for me! cool.gif That makes a lot more sense. (Although now of course the question becomes, why can't they drive the pulses in random order of brightness over time, to reduce the perceivable flicker.)
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Same goes for the next frame. At 60 frames a second this creates 60 bright pulses and 60 dark pulses (i.e. - 60Hz flicker)

That, I still have a problem with. I've seen white 24p action scenes where the flicker was "fighting" the action making it unwatchable (it's the correct technical term in French, my native language, describing when two frequencies are close enough to one another that a much slower "beat" becomes apparent). If you've ever seen material de-interlaced with the wrong field order, it almost looked as bad as that. Anyway that's what I subjectively used to confirm my apparent 30Hz find with the camera (which I concede, could've been an effect of a fast shutter speed and the inconvenient 120Hz being a round multiple of what we're analyzing). I have a hard time imagining that the effect would've been this dramatic if the flicker was at 60Hz instead of 30Hz. For that matter I don't recall having had issues with NTSC CRT's and those refreshed at 60Hz as well, but I guess that's not apples to apples... smile.gif

I wish I had a 1000fps camera handy to really look at the whole subfield update sequence in real-life detail...
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post #25 of 26 Old 10-19-2012, 07:49 AM
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Quote:
Originally Posted by VPhantom View Post

Thank you xrox! Now that was the missing piece for me! cool.gif That makes a lot more sense. (Although now of course the question becomes, why can't they drive the pulses in random order of brightness over time, to reduce the perceivable flicker.)
They can but choose not to. There are a few patents and technical papers on this very idea. IIRC the reason they choose not to is that the vast majority of persons do not percieve PDP flicker so why bother.
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That, I still have a problem with. I've seen white 24p action scenes where the flicker was "fighting" the action making it unwatchable (it's the correct technical term in French, my native language, describing when two frequencies are close enough to one another that a much slower "beat" becomes apparent). If you've ever seen material de-interlaced with the wrong field order, it almost looked as bad as that. Anyway that's what I subjectively used to confirm my apparent 30Hz find with the camera (which I concede, could've been an effect of a fast shutter speed and the inconvenient 120Hz being a round multiple of what we're analyzing). I have a hard time imagining that the effect would've been this dramatic if the flicker was at 60Hz instead of 30Hz. For that matter I don't recall having had issues with NTSC CRT's and those refreshed at 60Hz as well, but I guess that's not apples to apples... smile.gif
I wish I had a 1000fps camera handy to really look at the whole subfield update sequence in real-life detail...
We know the waveform and make up of a PDP frame displaying white. We know that cameras integrate over a shutter open time period. So we can draw it out on paper. Are the following parameters correct?

- PDP displaying white and operating at 60Hz refresh

- Camera capturing at 120fps

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post #26 of 26 Old 10-29-2012, 11:59 AM - Thread Starter
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Quote:
Originally Posted by xrox View Post

We know that cameras integrate over a shutter open time period. So we can draw it out on paper. Are the following parameters correct?
- PDP displaying white and operating at 60Hz refresh
- Camera capturing at 120fps

Unfortunately we don't have any idea about my specific camera's shutter time. Because of the low light condition I wouldn't be surprised if it was close to the full 1/120 second but anything down to 1/500 seems plausible and would of course greatly affect the result. I don't think it's worth going any further at this point. smile.gif I fully accept the concept that its shutter speed (120fps) was problematic being an exact multiple of both 60 and 600.

It's just the time-wasting geek in me who would've liked to fool around with a true high-speed camera in the thousands of frames per second to witness the effect hands-on.

Disappointing about the manufacturers' choice to ignore the minority which I'm a part of. I miss the black and saturation of the darn thing. (Although I do NOT miss the temporal dithering all over the place. Nice stable 10-bit image on LCD now.) Meanwhile I'm saving up for an eventual Super Duper AMOLED rollable 100" paper-thin monitor in 2020. biggrin.gif
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