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Wannabe Geek


Topic # 56546 16-Jan-2010 10:10
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I've always been told that LCD's do not flicker, but every time I walk into any showroom with a wall of LCD TV's, and I see them flickering away at what seems to be 50Hz. 60Hz on a monitor is another story, but when I set my LCD monitor (LP2475w) to 50Hz, it's definitely flickering.

None of the foreigners in any major tech forums seem to be able to detect this, but to me 50Hz LCD flicker is just as obvious as the flicker of a 60Hz CRT.

Is it something to do with NZ's mains power?

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  Reply # 290497 16-Jan-2010 11:14
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is there a reason you're purposely setting such a low refresh rate? The native resolution of that monitor is 1920x1200 @ 60Hz.

Unless you're playing back 50Hz video there is no reason at all to be using 50Hz.






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  Reply # 290513 16-Jan-2010 12:43
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sbiddle: is there a reason you're purposely setting such a low refresh rate? The native resolution of that monitor is 1920x1200 @ 60Hz.

Unless you're playing back 50Hz video there is no reason at all to be using 50Hz.





Yes, there is a reason: I have an upscaling DVD player and Freeview HD box connected, and they both output 50Hz signals.

That's beside the point though. Why do LCD's flicker at 50Hz?

 
 
 
 


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  Reply # 290520 16-Jan-2010 13:12
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Using fluorescent light bulbs? Change to incandescent. It is not the LCD flickering, it is an optical interference between the flickering LCD and the flickering light bulb. Incandescent bulbs glow, they do not flicker (unless the element is burnt through and just making contact with spring force so it flickers every time you bump it, another topic).

All video displays flicker, it is just too fast to see. CRTs are better since the phosphor glows, but there for at least 30 years CRTs used a fast phosphor and you can still see the flicker in some circumstances. If you move your head in the same direction as the picture is scanned then it is as if it is flickering at a slower rate which you can pick up (i.e. the whole relativity thing).

Some people just can see the flicker. Me for example at 36 years can hear with an almost flat frequency response up to about 16kHz while most people my age can not hear much above 12kHz. Around 18kHz I can still identify sounds (with hearing tests performed at work using proper medical equipment). And my eye sight is perfect, I can read the finest line on test charts. Perhaps your eyes simply have an unusually fast response which means you can see the flickering. I've always had to set my displays to at least 70Hz or I'd see flickering under some conditions.

Regarding lights, keep in mind the LCD also has a backlight most likely fluorescent which run can also have an interference pattern with your fluorescent lights.

It is not necessarily 50Hz flickering. It can be 2 fluorescent light sources running with say 150.00kHz and 150.05kHz inverters, then you will get a 50Hz interference pattern (i.e. the difference between the switching frequencies). Actually I would not be surprised if this is the issue, because there are not many frequencies available for inverters to run at. They all have to comply with EMC and EMI regulations which limits what you can do.

Fluorescent lights age, and their inverters age. There are components to filter out 50Hz (or actually 100Hz from rectified mains, but also 50Hz). If these fail, you get modulation of the backlight which will be perceived as flicker.




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  Reply # 290522 16-Jan-2010 13:17
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I've never ever noticed flicker on any LCD TV or monitor at 50Hz, I certainly don't walk into a store and see flickering on TV's. If you have flourescent lighting it can be an issue with some people who have sensitive eyes.








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  Reply # 290535 16-Jan-2010 14:58
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Niel: Using fluorescent light bulbs? Change to incandescent. It is not the LCD flickering, it is an optical interference between the flickering LCD and the flickering light bulb. Incandescent bulbs glow, they do not flicker (unless the element is burnt through and just making contact with spring force so it flickers every time you bump it, another topic).

All video displays flicker, it is just too fast to see. CRTs are better since the phosphor glows, but there for at least 30 years CRTs used a fast phosphor and you can still see the flicker in some circumstances. If you move your head in the same direction as the picture is scanned then it is as if it is flickering at a slower rate which you can pick up (i.e. the whole relativity thing).

Some people just can see the flicker. Me for example at 36 years can hear with an almost flat frequency response up to about 16kHz while most people my age can not hear much above 12kHz. Around 18kHz I can still identify sounds (with hearing tests performed at work using proper medical equipment). And my eye sight is perfect, I can read the finest line on test charts. Perhaps your eyes simply have an unusually fast response which means you can see the flickering. I've always had to set my displays to at least 70Hz or I'd see flickering under some conditions.

Regarding lights, keep in mind the LCD also has a backlight most likely fluorescent which run can also have an interference pattern with your fluorescent lights.

It is not necessarily 50Hz flickering. It can be 2 fluorescent light sources running with say 150.00kHz and 150.05kHz inverters, then you will get a 50Hz interference pattern (i.e. the difference between the switching frequencies). Actually I would not be surprised if this is the issue, because there are not many frequencies available for inverters to run at. They all have to comply with EMC and EMI regulations which limits what you can do.

Fluorescent lights age, and their inverters age. There are components to filter out 50Hz (or actually 100Hz from rectified mains, but also 50Hz). If these fail, you get modulation of the backlight which will be perceived as flicker.


OK, I'm satisfied that flourescent lighting may cause flicker in showroom LCD's but what about my own unit?

My flickering has NOTHING to do with my interior lighting. Firstly, I only use incandescents and secondly, just to be 100% sure, I tested with no ambient light source other than sunlight, and I could still see the LCD flickering at 50Hz.

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  Reply # 290566 16-Jan-2010 17:48
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Try setting the backlight to maximum (i.e. brightness to maximum). Also check the monitor's menu to make sure you have set the monitor to 50Hz (mains supply).




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  Reply # 290567 16-Jan-2010 17:50
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I think you simply have very sensitive eyes. You're seeing flicker that most people can't.

Lots of people who have epilipsy suffer from photosensitivity where a fit can be triggered from watching both CRT and LCD TV's. People who do suffer can see flicker on TV's that the majority of the population don't see. I know somebody who thought 100Hz LCD's were the best thing ever because they could actually watch them without suffering any visual disturbances or seisures. If they watched a CRT or regular LCD they could see flicker.

My only suggestion would be to look at an upscaling TV that does 100Hz rather than a monitor that only does 50Hz for video and see it it solves your issue.

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  Reply # 290568 16-Jan-2010 17:55
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I've found an instance on the internet where a flicker was visible when using HDMI while components leads are also plugged in. Try unplugging everything and using only one signal source.




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  Reply # 290573 16-Jan-2010 18:09
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Niel: Try setting the backlight to maximum (i.e. brightness to maximum). Also check the monitor's menu to make sure you have set the monitor to 50Hz (mains supply).


You know, that's actually fixed it. Well, when I say 'fixed', having to have the brightness at 90-100% all the time on a monitior that is already too bright at 0% is totally unacceptable. I dread to think what kind of half arsed modulation system HP have used as backlight control.

With the majority of users in 60Hz NTSC countries, it's not surprising such a fault would go undetected.

I'm going to RMA this worthless monitor (for the fifth time now), just to be sure, but every unit I've had so far has exhibited 50Hz flicker.

(just as a side note, I'm not an epilept or anything, and although I'm on the sensitive side when it comes to PC displays [ie. I don't buy any of that "The human eye can't see more than 25 frames per second" crap], I can tell you if you notice 60Hz flicker on a CRT, you would be able to see this 50Hz flicker on my monitor. Even my brother can see it, and he's the type who doesn't even notice when AntiAliasing is enabled/Disabled)

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  Reply # 290601 16-Jan-2010 20:15
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The backlight of all LCDs are controlled by PWM. It is the only way to do it cheaply, and the only way to do it to comply with energy efficiency requirements. You are seeing an artefact of the PWM. (My TV gets a buzz over the speakers when the backlight is less than about 80%.)

The high backlight brightness is required to get a high contrast ratio. You sound like a perfectionist when it comes to video so you should enjoy a high contrast ratio. But if it is too bright for you, use your video card settings to turn it down instead of the monitor's controls so you can run the backlight at full brightness. It will also produce the purest colour (because the light runs at rated power).

My HD TV does not allow turning down the backlight when driven by HDMI. This is to ensure a high quality picture when using a high quality connection. LCDs have a brightness control only because CRTs had one and people expect to have it.




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  Reply # 290633 16-Jan-2010 22:54
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Niel: The backlight of all LCDs are controlled by PWM. It is the only way to do it cheaply, and the only way to do it to comply with energy efficiency requirements. You are seeing an artefact of the PWM. (My TV gets a buzz over the speakers when the backlight is less than about 80%.)

The high backlight brightness is required to get a high contrast ratio. You sound like a perfectionist when it comes to video so you should enjoy a high contrast ratio. But if it is too bright for you, use your video card settings to turn it down instead of the monitor's controls so you can run the backlight at full brightness. It will also produce the purest colour (because the light runs at rated power).

My HD TV does not allow turning down the backlight when driven by HDMI. This is to ensure a high quality picture when using a high quality connection. LCDs have a brightness control only because CRTs had one and people expect to have it.


I've never heard that reasoning before, but it sounds plausible. The one fatal flaw however, is raw black levels. In LCD's they are already piss poor, and to be honest I could barely handle them with the backlight at 0% - I needed a 100 watt incandescent behind the monitor to bathe the room enough light to even begin to give the illusion of half decent blacks. They are just so pitifully weak that I'm constantly cursing myself for wasting a grand on this LCD, and lusting after my brothers 2070SB... you get the idea.

I'll try it with the brightness up high, but I doubt I'll be able to stomach it for long...

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  Reply # 290662 17-Jan-2010 09:31
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I am an electronic engineer, I know how the electronics work. Most computer guys only know about modules.

Get a Samsung LED screen and sell the HP on TM at a profit. LED screens have no backlight.




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Wannabe Geek


  Reply # 290688 17-Jan-2010 12:59
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Niel: I am an electronic engineer, I know how the electronics work. Most computer guys only know about modules.

Get a Samsung LED screen and sell the HP on TM at a profit. LED screens have no backlight.


At a profit? Yeah right. I spent $1200 on this POS last year.

Aside from the Dreamcolor monitor, I don't know of any LED backlit displays that don't use TN panels...

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