Help me to understand Flickering...

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balnazzar

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Perhaps you can help me to better understand monitor flickering, and clarifying difference between low framerate flickering and PWM flickering.

It has been said that a good test for flickering could be to look at the sceen through a video camera (or cellphone).

Now, I own two CCFL monitors: Dell U2311H and Eizo L568. Both support 60Hz at native resolution.

It has been said flickering is a nonissue on CCFL models, due to higher light persistency.
Now I looked at my monitors through the cellphone and through a pocket camera: both displays exhibits strong vertical bands, although of different shape and speed.

Rising U2311H to 75Hz at non-native resolution, band move quickly, but does not disappear.
Both units cause eyestrain when used for long time.

Now summarizing:
1) If the cellphone test is reliable, even CCFL units suffer from flickering.:rolleyes:
2) Augmenting refresh rate does not eliminate problem.
3) I don't understand how much flickering is due to PWM and how much is due to low refresh rate. :confused:

I'd like to hear your opinions. Thanks.
 
LCDs don't flicker from the refresh rate, but the PWM might be tied to a multiple of the refresh rate. I think the Dell U2311H does 180 Hz PWM at 60 Hz, so it might do 225 Hz PWM at 75 Hz.
 
The waveform of the flicker also plays a role.
CCFL flicker is a "softer" flicker than LED flicker.
Even incandescent bulbs flicker (but very, very weakly, e.g. small 5% variances in brightness in high-speed video, as the bulb barely cools off during the AC crossings through the 0 volts line)

Also, PWM flicker can create a side effect of rough-looking motion artifacts, as you track moving objects on-screen:

(LED PWM, during Brightness=0% setting, from LCD Motion Artifacts 101)

CCFL flicker can do this too, but in a much softer/fainter way (less harsh looking than LED PWM):

(Flicker motion artifact from Samsung 245BW 180Hz CCFL. Triple copy of image per refresh, due to 3 flickers per refresh; but CCFL flicker is a softer waveform so the artifact is less harsh than LED PWM.)

Some people's eyes are bothered by the flicker itself, while others are bothered by the harshness of the motion artifacts, while others are bothered by the extreme brightness of LED monitors. Sometimes all the above plays a role simultaneously (PWM flicker, harsh-looking motion, monitor still too bright even at 20% brightness). No wonder some people get eyestrain.

Note: CRT and LightBoost has no PWM artifact, because at one flicker per refresh, you don't get multiple copies of images causing the harsh multiple-edge effect, as your eyes track a moving object across the screen.
 
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It is also possible for the flicker rate of the monitor to interplay in a most unpleasant way with the flicker rates of the lights in the room, especially if you are so unlucky as to have older-style fluorescent tube lights that operate at 60Hz or 120Hz (those could make headaches all by themselves, never mind whatever you're doing under them). Newer fluorescent tube lights use electronic ballasts that operate at thousands of Hz. See the wikipedia entry for fluorescent bulb flicker problems.

Even a poorly placed fan, be it a ceiling fan or just a floor fan in front of a light, can cause flickering light that can match up with a monitor's flicker rate in an unpleasant way.
 
evilsofa said:
Even a poorly placed fan, be it a ceiling fan or just a floor fan in front of a light, can cause flickering light that can match up with a monitor's flicker rate in an unpleasant way.
Click to expand...

You know how you feel when you look at the sun for 2 seconds and then you close your eyes? Now that is how people are affected by this PWM flickering, starting to see the white on the screen wherever they look after a while of using the monitor.

Read that and try it yourself so you can see how some of us feel when using a PWM flickering monitor, that our beloved monitor manufacturers don't care about. And the worse about it is that they take advantage of the fact that at higher frequencies the effect is less noticeable, and they just do it high enough that is not immediately obvious...

How to detect PWM flickering?
Simple: Make a big black filled rectangle in paint and draw a white vertical line from top to bottom, 2 pixels thickness. After that start waving the camera(phone camera works too) so that you see the line being made longer in the preview of the camera. Now you will see one thing or another. If you see a big white rectangle, without any space between lines, then you are perfectly safe.
If you see more lines instead of the one that you started with, and you could identify those as trails of the initial one, means that the monitor has a moment of "darkness" which is what results from trying to reduce costs of adjusting monitor brightness, also known as PWM Flickering.

I really hope I explained this clearly enough, so that I can help as many people as possible to detect this problem, and hopefully this cheat will be removed fro the manufacturers book forever.
 
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I really cannot unerstand why such an important issue is so much underestimated both by users and by manfacturers...

I propose to place a sticky thread with an upgraded list of flicker (PWM) free monitors...
 
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Also I noted that coupling my monitor with an halogen light (versus ordinary fluorescent powersaving bulbs), headaches and eyestrains diminish noticeably...
 
It seems that we have a situation lately where Dell is selling the same monitor with or without PWM Flickering. I am about to buy one if is WITHOUT. But in all seriousness, is it or not??? You do realize that I can't trust anyone, as no one actually is bothered to make a movie to prove that the Dell they have is PWM free or not.
 
What model you are planning to buy? For example, u2413 and the new 21:9 u2913 are pwm-free, for sure.
 
balnazzar said:
What model you are planning to buy? For example, u2413 and the new 21:9 u2913 are pwm-free, for sure.
Click to expand...

Do we have to pay premium for flicker free? For example, I don't want 8 bit colors or letter box wide screens. Any idea if there is any in the 200-250$ price range, 1080p and 23-24 inch?
 
balnazzar said:
What model you are planning to buy? For example, u2413 and the new 21:9 u2913 are pwm-free, for sure.
Click to expand...

I want to buy a monitor with these specifications:

- glossy
- either 23 inch 1920x1080, 24 inch 1920x1080, 24 inch 1920x1200, 27 inch 2560x1440
(27 inch 1920x1080 is not an option).
- no PWM flickering
- IPS

Now those 4 specifications are already hard to find in one single monitor, not to talk about the fact that I would like it to be input lag free, low motion blur, no backlight bleeding, and no other defects.

Unfortunately Dell U2413 has ag coating (is it light or is the same). Is it flicker free or not? Proof?
S2440L is glossy and nice but is PVA and has flickering?
S2340L is glossy and has flickering? According to TFTCentral it doesn't.
S2740L is only 1920x1080, is glossy and has flickering? According to TFTCentral it doesn't.

So anyone care to post a video proving it has flickering or not? Cause if it doesn't, I'm in for a S2340L.
 
Mark Rejhon said:
Some people's eyes are bothered by the flicker itself, while others are bothered by the harshness of the motion artifacts, while others are bothered by the extreme brightness of LED monitors. Sometimes all the above plays a role simultaneously (PWM flicker, harsh-looking motion, monitor still too bright even at 20% brightness). No wonder some people get eyestrain.

Note: CRT and LightBoost has no PWM artifact, because at one flicker per refresh, you don't get multiple copies of images causing the harsh multiple-edge effect, as your eyes track a moving object across the screen.
Click to expand...

very accurate.. that is why i am on CRT.. lcd is just too harsh.
 
cerbul said:
I want to buy a monitor with these specifications:

- glossy
- either 23 inch 1920x1080, 24 inch 1920x1080, 24 inch 1920x1200, 27 inch 2560x1440
(27 inch 1920x1080 is not an option).
- no PWM flickering
- IPS

Now those 4 specifications are already hard to find in one single monitor, not to talk about the fact that I would like it to be input lag free, low motion blur, no backlight bleeding, and no other defects.

Unfortunately Dell U2413 has ag coating (is it light or is the same). Is it flicker free or not? Proof?
S2440L is glossy and nice but is PVA and has flickering?
S2340L is glossy and has flickering? According to TFTCentral it doesn't.
S2740L is only 1920x1080, is glossy and has flickering? According to TFTCentral it doesn't.

So anyone care to post a video proving it has flickering or not? Cause if it doesn't, I'm in for a S2340L.
Click to expand...

u2413 has light AG, and should be pwm-free al last over certain values of brightness.
Source: reviews from usual guys like tftcentral and prad.
 
According to Dell support in USA, the S2340L doesn't have PWM.
I would like anyone to confirm/prove wrong. Solid proofs please.
 
Mark Rejhon said:
The waveform of the flicker also plays a role.
Click to expand...
Hello, Mark. I've been meaning to ask you about flicker and LED LCD-TVs. From what I've seen all of them flickers badly when dimming. But then I dont think I have ever looked closely at sets with local dimming nor scanning backlights. Whats your thought on those? They will still use PWM ?
 
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