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Old 05-29-2012, 10:39 PM   #1
avp2306
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Default No-PWM Monitor Recommendation

I'm looking for a monitor that does not make use of PWM (Pulse Width Modulation)

For those not familiar with the concept here's a good article that describes it: http://www.tftcentral.co.uk/articles...modulation.htm

In short its a technique LCD monitors use to control brightness (both LED & CCFL), basically controls the frequency at which the backlight operates.

The reason I decided to look for a non PWM monitor is eye strain. I posted on here a couple of days ago and received lots of advice on how to reduce eye strain (lighting, flux, etc) - thanks to all who responded. In addition I decided to switch my monitor as I believe its PWM that irritates my eyes.

Unfortunately monitor manufacturers do not yet publish PWM frequencies in spec sheets, and very few sites include this in their reviews during testing. I really hope AnandTech starts doing this soon, maybe a quick round-up on PWM/flicker tests of major models.

Has anyone come across any monitors without PWM (continuous lighting) or really high PWM frequencies?

So far I've only found reports of HP ZR2740w being "flicker free".
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Old 05-29-2012, 10:57 PM   #2
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No PWM you won't find, at least not at any reasonable price. It's plain unreasonable for CCFL, and would take added time and cost for LEDs (run the LEDs for a few days to let Vf settle*, calibrate*, and use much costlier variable power supply technology, v. various simple and cheap unregulated, resistor-set, and one-chip CC PWM PSUs).

IMO, all monitors aught to have a white sheet with such basic panel and backlight info. Most power LEDs are quite happy up to 100kHz, IIRC, and if the fast switching itself is a problem, that can be smoothed out. From the link: "LED backlights have been reported running from 90Hz up to 420Hz" WTF. I hate white LED backlights because of the color, but 420Hz? No wonder they look so bad. I'd have expected 1-2kHz to be minimums. Anything in the hundreds is likely to cause the same kind of strain as watching a film projector, IMO.

* note: not worth it by an au for white LEDs
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Old 06-09-2012, 12:41 AM   #3
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No PWM you won't find, at least not at any reasonable price.
Well, compared to the 50% cheaper Catleap and such, I suppose the HP is expensive, but for someone like me who is sensitive to PWM flicker, there is no alternative but to pony up.

The HP ZR2740w has a constant control backlight, according to prad.de, the first site on the net to discuss PWM flicker:

http://translate.google.com/translat...26prmd%3Dimvns

I'm about to replace my current panel with the HP because of headaches.
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Old 06-09-2012, 01:35 AM   #4
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So in this respect, does that mean that CCFL-backlit LCDs are superior to LED-backlit LCDs? I don't remember ever reading about PWM control for CCFLs.
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Old 06-09-2012, 04:19 AM   #5
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So in this respect, does that mean that CCFL-backlit LCDs are superior to LED-backlit LCDs? I don't remember ever reading about PWM control for CCFLs.
Not strictly, but more that they're by and large not bothering to Do It Right™.
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Old 06-09-2012, 04:13 PM   #6
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So perhaps the issue is not necessarily the use of PWM, but rather poorly implemented PWM?

The article says CCFL is usually better: "Flicker from LED backlights is typically much more visible than for CCFL backlights at the same duty cycle because the LED's are able to switch on and off much faster, and do not continue to "glow" after the power is cut off."

I think the flicker of household fluorescent lighting is worse than the flicker of monitor backlights. Have you banished all fluorescent lights from your house?

Maybe I missed it in the article, but I didn't see where they explained what was used when PWM was not used? They claim the macbook does not use PWM, but how in the world can you adjust the brightness of an LED or a CCFL if you are not using PWM? Certainly they don't use incandescent bulbs that get brighter/darker with voltage, and it just seems wasteful to turn the LED/CCFL on full-blast then darken it with contrast/color settings, so I don't see how you can avoid PWM any other way?
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Old 06-09-2012, 05:48 PM   #7
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So perhaps the issue is not necessarily the use of PWM, but rather poorly implemented PWM?
An LED backlight at 40kHz would be imperceptible, and should be too high for the control circuit to create audible whine.

Quote:
I think the flicker of household fluorescent lighting is worse than the flicker of monitor backlights. Have you banished all fluorescent lights from your house?
Have you been using any such lights made in the last two decades? >20kHz is typical, these days. There's no way you can see flicker in anything from the big lighting brands. Color temp, whiteness (CRI), and output are still important issues, though (my bed lamp, bathroom light, and stove light are staying incan as long as I can buy them).

Quote:
Maybe I missed it in the article, but I didn't see where they explained what was used when PWM was not used?
Constant current. There is no need to ever turn the LED off. PWM saves on cost, and provides color stability for white LEDs.
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Old 06-09-2012, 10:58 PM   #8
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Constant current. There is no need to ever turn the LED off. PWM saves on cost, and provides color stability for white LEDs.
So it looks like the LED will respond to varying current to adjust brightness?

I guess that means limiting choices to LED backlighting only, instead of CCFL?

Even though the article suggests that CCFL might have some advantages for less apparent flicker, as far as looking nicer at a given PWM rate compared to LED, you could just avoid flicker altogether by going with LED, so long as the LED uses constant current? And CCFL cannot use constant current?
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Old 06-11-2012, 11:56 PM   #9
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Read the prad.de article I linked to and your questions will be answered. It's better than tftcentral's derivative article.
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Old 06-14-2012, 01:00 AM   #10
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What I'd like to know is if any other external monitors use constant control. I'm curious about the Korean Catleap/Shimian/etc monitors, although I'm not a fan of glossy so I'm not seriously considering those.

It's annoying to be limited to just one choice in monitors. I'd be satisfied with 1080p.

What would really be nice is an A-MVA panel with constant control. Outstanding contrast plus no flicker...
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Old 06-24-2012, 09:12 AM   #11
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Found this thread on Google when searching for "no PWM monitors". Sadly, it seems the only one on the market is the HP ZR2740w, which isn't suitable for me for two reasons (too big - I want 24" at most, and has too big input lag, so not really suitable for gaming).

The reason I'm looking is because my brand new LED-backlit LCD (Dell U2312HM) is giving me serious eyestrain / pain after looking at it for just a few minutes. No such problems with my 6-year-old CCFL LCD. I'm not sure whether it's due to backlight flicker or something else (like, the light tone of the LED which is harsher than CCFL?). U2312HM definitely uses PWM, but I don't notice the strobe effect with the "hand test" - maybe I'm just doing it wrong.

And of course it's too damn bright at 100 Brightness unless I'm just watching movies from far away. Have to set it to 0 for reading / writing text, which I do a lot.
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Old 07-14-2012, 02:17 PM   #12
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Quote:
Originally Posted by ButtMagician View Post
Found this thread on Google when searching for "no PWM monitors". Sadly, it seems the only one on the market is the HP ZR2740w, which isn't suitable for me for two reasons (too big - I want 24" at most, and has too big input lag, so not really suitable for gaming).

The reason I'm looking is because my brand new LED-backlit LCD (Dell U2312HM) is giving me serious eyestrain / pain after looking at it for just a few minutes. No such problems with my 6-year-old CCFL LCD. I'm not sure whether it's due to backlight flicker or something else (like, the light tone of the LED which is harsher than CCFL?). U2312HM definitely uses PWM, but I don't notice the strobe effect with the "hand test" - maybe I'm just doing it wrong.

And of course it's too damn bright at 100 Brightness unless I'm just watching movies from far away. Have to set it to 0 for reading / writing text, which I do a lot.
DELL seems to be the worst offender when it comes to PWM, I bought U2412 which I had to return for the same exact reason, I would instantly get intense headaches just after 5 min of usage
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Old 07-15-2012, 03:47 AM   #13
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Constant current. There is no need to ever turn the LED off. PWM saves on cost, and provides color stability for white LEDs.
What I've read leads me to believe that LEDs tend to shift tint as their driving current changes. Unless you're willing to pay for some fancy scheme in the monitor to alleviate this (or recalibrate your monitor every time you change brightness), you're just going to switch one problem for another.

Instead of constant current, I'd settle for high PWM frequency and accept the slightly increased power usage that entails. Of course, there's always going to be the one genetic freak of nature that claims that even PWM in the MHz range is noticeable.
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Old 07-15-2012, 11:39 AM   #14
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What I've read leads me to believe that LEDs tend to shift tint as their driving current changes.
White LEDs do this. The response of the phosphors is not linear (a white LED is a blue LED painted with phosphors that emit slower wavelengths), and they are made and sold based on rated current. Color LEDs do not have this issue, though they only give off a narrow range of frequencies, so you need at least 3 different sets.
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Old 07-15-2012, 08:06 PM   #15
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Hmmm... So does the HP ZR2740w have a color LED backlight?
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Old 07-16-2012, 12:43 PM   #16
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So it looks like the LED will respond to varying current to adjust brightness?
The problem with this is that LEDs vary in brightness and color unpredictably if current is adjusted. Even "matched" LEDs from the same batch can show wildly unpredictable and varied color and brightness shifts when operated away from their test current. PWM avoids this problem. Even going to 3 LED systems doesn't solve this, as individual LEDs will show different response to current control, leading to the same uneven brightness and color as with white LEDs.

CCFL lamps run on AC, so they have to have an inverter. Adding PWM to an inverter is trivial, so CCFL can easily have PWM incorporated. However, CCFLs work best at 20-40 kHz, which means that any PWM is totally imperceptible.

Most LED backlight manufacturers have cheaped out and use low-cost low frequency PWM drivers, rather than pay the extra $.05 for a 20 kHz PWM system.
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Old 08-18-2012, 11:10 AM   #17
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I have devided the monitors that have been found to have a safe PWM value into 3 groups according to how likely they are to have no PWM or a safe frequency: 1)The safest group 2) Middle group 3) Dangerous group

The safest group:

1) EIZO EV2736W

2) EIZO EV2436W

3) EIZO EV2416W

4) EIZO EV2336W

5) EIZO EV2316W

All of these EIZOs have this description on eizo.com:

"Flicker-Free Images

Due to the way brightness is controlled on LED backlights, a small number of people perceive flicker on their screen which causes eye fatigue. The FlexScan EV series utilizes a hybrid solution to regulate brightness and make flicker unperceivable without any drawbacks like compromising color stability."

http://www.eizo.com/global/products/...36w/index.html

On forums you get the information that they only use no PWM if brightness is set to above 20%:

"For over one year i was locking for a PWM-free 24" monitor. I think the new Eizo ist the first LED-24"-monitor with hybrid dimming (i.e. above ~ 20 % the dimming is flicker-free).

Since two weeks i have the Eizo EV2436. Here my short Impression:

The best thing: It is PWM- and flicker-free to a brightness of 18 %! Below this mark heavy pwm-flicker starts. For my eyes brightness of 20 % is o.k. especially when the contrast is reduced a little bit. It is important to switch off all "EcoView-Options" otherwise the flicker starts in a dark room."

http://hardforum.com/showthread.php?t=1732166

6) BenQ GW2760HS

All I know about this monitor comes from the official site:

http://www.benq.com/product/monitor/gw2760hs/

Middle group:

1) HP ZR2740W

prad.de found their LCD to have to PWM.

A quote from https://discussions.apple.com/thread...t=825&tstart=0

"My current HP zr2740w is like a book. It does not cause any fatigue. "

2) DGM IPS-2701WPH

tftcentral.co.uk. found their version of that monitor to have no PWM.

3) Shimian QH270 -IPSMS

prad.de. measured their monitor to have no PWM.

4) Dell U2713HM

tftcentral.co.uk. found that their version of the model had no PWM.

Dangerous group:

1) HP ZR2440W

The LCD HP ZR2440W is measured to have a PWM frequency of 430 Hz. Experience shows that it can be high enough to not cause problems.

This monitor has been tried by a US user and it reduced the monitor symptoms greatly for him, but a user from Asia he couldn't use it.

2) HP L2035

A measurement with an oscilloscope on youtube showed that that particular version of the LCD had no PWM.

But a user bought it and he had the same pain he always had with PWM monitors. Later he found a truly PWM-free LCD with which he had no major symptoms.

3) BenQ RL2450HT

The PWM frequency was measured to be 360 Hz.

This LCD was tried by a person and he says that it solved 80% of his symptoms. This is evidence that 360 Hz is no longer perceived by us subconsciously.

4) Acer S243HLAbmii

The measured PWM frequency was 510 Hz by prad.de.

5) Samsung S27B970D

This LCD seems to have changed its PWM properties. tftcentral.co.uk measured it to have no PWM brightness control, while prad.de found it to have no PWM only at full brightness, but 180 Hz PWM at a lower brightness. It is a reminder that it is a gamble with these LCD because the manufacturer can change the PWM frequencies of the monitors at any time.

A person has bought the display and says he can only look at it when it is set to 100% brightness with no PWM.

Keep in mind that there is a probability that the PWM values of all of the LCDs, except possibly the EIZOs and the BenQ, may be different in different part of the world and there is no guarantee that the manufacturers won't change them over time.

Last edited by wassja; 03-19-2013 at 08:16 PM. Reason: New relevant information was added
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Old 08-18-2012, 04:27 PM   #18
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Well, I guess pro monitors directed towards graphic professionals should be very good on this respect.
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Old 08-20-2012, 02:49 PM   #19
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Some of those monitors are older and have CCFL backlights (HP L2035, for example). I think I recall reading somewhere that all CCFL LCDs use PWM. But I guess that might be wrong after all.
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Old 08-24-2012, 09:24 AM   #20
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No, some CCFL LCDs use analog dimming. Here are some more LCDs without PWM:

9) Dell U2713HM

The Dell U2713HM is an LED LCD with no PWM brightness control. It was found to have no PWM by tftcentral.co.uk

10) DGM IPS-2701WPH

It uses no PWM brightness control. It was found to have no PWM by tftcentral.co.uk

11) Samsung S27B970D

It uses no PWM brightness control. It was found to have no PWM by tftcentral.co.uk

Last edited by allisolm; 03-19-2013 at 08:56 PM.
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Old 08-24-2012, 09:31 AM   #21
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Hmmm... So does the HP ZR2740w have a color LED backlight?
If you mean RGBLED, then the answer to your question is no.
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Old 08-24-2012, 04:54 PM   #22
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What could be the reason behind the fact that it's only 27" LED backlit LCDs which use constant dimming, and not smaller ones?

prad.de confirmed no PWM on Dell U2713HM:

http://www.prad.de/board/monitore/te...linie-prad-de/

It's nice to have a choice, hope this trend continues (but I'd still like a smaller monitor ).

Last edited by ButtMagician; 08-24-2012 at 06:08 PM.
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Old 09-04-2012, 05:24 PM   #23
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Found the Shimian QH270-IPSMS LED without PWM on prad.de.

Last edited by allisolm; 03-19-2013 at 09:06 PM.
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Old 09-15-2012, 06:32 AM   #24
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For anybody interested, here's my site: http://vasyafromukraine.webs.com/

Last edited by allisolm; 03-19-2013 at 09:07 PM.
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Old 09-15-2012, 09:39 AM   #25
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For anybody interested, here's my site: http://vasyafromukraine.webs.com/
Read your website and its good. What do you mean when you wrote about setting the camera shutter speed as a flicker detector??

"If you see flicker on at least on of them at minimal brightness, then your phone is capable of being a flicker detector. This test is not 100% foolproof. If you had a camera, on which you could set the shutter speed, then, when the right speed is selected, it would be foolproof."
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