By flicker-free I mean displays that do not flash the background light to simulate lower brightness, but lower that light by lowering a DC current instead.

What are some laptops currently with a flicker-free display?

Is it worth it?

I haven't seen any laptops that do this. I would think flickering the backlighting would actually increase power consumption and shorten the lifespan of the backlight (might not apply to LEDs though). My old Gateway laptop lowers the actual lighting, but that is using CCFL backlighting.

Signals turning on and off increase power consumption only if between transitions the consumption is much lower than during transitions. Not true with LEDs, when they are on they consume.

I tested with oscilloscope the following models:

Thinkpad T440p, T440 are using DC current control from 100%-50%.
Below 50% they switch to PWM

Thinkpad T440s is using DC current control from 0%-100%

Thinkpad T540 - PWM only.


However I am not sure if this is true for all the machines from the same series or it depends on the product line and batch in the factory.

psy__, did you test Thinkpad T440s with what type of display (HD+, FHD, touch or normal)? And how did you exactly test this laptops with oscilloscope? With some type of diode, or meausuring current to display?

Hi,

I tested t440s with FullHD touch display.

The test is done with diode: BPW34

Mount it in some material which will allow you to place it on the screen without light coming from outside, then just plug it directly in the oscilloscope.

You will have to play a while in the beginning with known source of PWM so you can find the oscilloscope settings which will display you beautiful impulses .

Hi,

Regarding the laptop which I bought, I finally got DELL Latitude E7440 fullHD, which has no PWM, but I have another issue with it.
Since I am working under Linux only, there is a problem with the linux driver for the trackpoint/touchpad and it sporadically jumps and presses buttons from time to time, which can make you crazy. There are some very weak efforts to fix the driver, but still no fix.

Thank you very much for your answer. No-PWM is essential for me, so thx for giving me antoher laptop which I can consider to buy I also work on Linux only, so it's a pitty that touchpad doesn't work properly, but maybe in future release of Linux drivers they fix it...

But there is a question if these laptops with resolutions lower thant FHD also have no PWM...

Hi

I dont know for the lower resolution. That is why I bought oscilloscope and tested the laptops which I liked. You will actually want to test the exact unit you intend to buy.

It's trivial to flash bulbs/leds fast enough to fool the human eye. In my experience(TVs & Monitors over decades), all flicker comes from low refresh rates. To keep within the GPU & max horizontal scan rates some reduction in scaling might be needed.

Some people can be irritated by refresh under 65 Hz. I have never heard of issues with refresh above that - until perhaps now. What refresh rate have you set? Adding something like

VertRefresh 65-90

into xorg.conf.d/ somewhere should clear issues. Mind you, if you have this sensitivity and use a crappy GPU, too bad. Interlacing the picture also helps fool the eyes that there is no flicker when you might spot it.

TV worked here at 50Hz and nobody complained.

The refresh rate you mention is completely different story. This is the number of times which the picture itself is refreshed. The flickering problem is actually triggered by flickering backlight which is different flickering from the other.
On the old CRT monitors you have no separate backlight and then matrix on top of it. There is only one layer which does both. You usually have no problems, because the phosphor has very big momentum and the picture does not go do zero brightness.

Actually the leds are not similar to any technology used before.
What are the main problems with leds:
1. The amplitude of the brightness is at least 2.5 bigger than with CCFL (the previous backlight technology for monitors ). What happens when you lower the brightness is that you fool the eye to open the pupils, because it sees less brightness, but in the same time you actually receive very powerful impulses from the leds (because they are with much higher amplitude) and the go from 0 to 100% brightness and for some people it have very negative impact.

2. The leds have almost no momentum when they go from ON to OFF state, which basically mean that you "see" picture goes off completely then back to 100% (100% of the leds brightness. The monitor brightness setting has nothing to do with this). While with the CCFL you have momentum, which actually protects your eye because the backlight does not go to 0 brightness.


One workaround is to put the brightness to 100% and try to lower the brightness using the tools coming with the video drivers.
What is the difference that way? When you put the brightness to 100% you actually pushing the backlight to 100%, and then with the video driver tools lowering the brightnes usually does not control the backlight but the way the matrix (which is on top of the backlight) transmits the light which is generated by the backlight.

For Linux you can play with the xcalib utility which will probably do what the video drivers do under windows.

Phosphor has/had a slightly longer persistence, and a slower decay. I am aware of backlights, and the difference from CRTs. It is untrue to say all leds have no persistence.

The theory behind both Pal & Secam relies on the significant 'averaging out' ability of the human eye. Birds cannot be fooled so easily. I wish you well in your quest for the perfect backlight; because if you are convinced you are being upset by something, that will become a self fulfilling prophecy.

If as you suggest, backlight power is held at average instead of being switched, the circuit design would have to be much different (and slower). Semiconductors would be 'half on' and dissipating power internally. Ouch!

I think the manufacturers are starting to take notice, because I see more and more monitors which do not use PWM but control the backlight by controlling current through the backlight diodes. I know this because it is easily seen with the oscilloscope. Of course in this case, you have much smaller amplitude between min and max brightness, which actually does not bother me, if the backlight is correctly sized for brightness. For monitor you dont need high brightness, you need the right brightness, which is very far from the MAX brightness which most of the backlights can do. And when you have the right brightness with PWM, it kills you (This is not valid for all people, some people probably have no problems but this is questionable)


Regarding the effects from PWM I know a lot of people which actually do not know why they have a headache when work on computer. Personally for me, If I work more than 30 minutes on such monitor, I start seeing artifacts, especially when i look aside. I am seeing big black spots which are constantly moving.

After one day on such monitor I got really red eyes with headache and had 3 days (after I throw the monitor) of discomfort and pain in the eyes.

Had that, too. I also have a 'scope. How fast do they pulse the backlights? That will probably eliminate them as a source of your problems.

I found eyestrain a problem also. I didn't wait until I saw black spots! What I found is that nobody knew how to set up a tv or monitor for comfortable viewing. With tvs, I developed the following procedure myself as a TV techie.
1. Turn down all brightness, contrast, & colour.
2. Turn up the brightness for best picture, then a little extra (slightly too bright)
3. Turn up contrast for best picture, then down a little (a bit grey)
4. Bring in colour to suit. Everything is pretty good
Although it is messier to do with monitors, it works well every time I tried it.

There is a known problem from too much contrast which is this: The eye expects rounded edges to shapes, which is the way natural optics present them. High contrast and high definition present square edges to shapes, which troubles the focussing mechanism of the eye. As a result the eye constantly refocusses, and muscles tense up.

Two other known problems exist.
1. Focal distance: The eye is not well able to cope with the short set distance of focus that a monitor presents. You need to vary focus; short distances tense the eyes, and become impossible as the male grows past 40 because his eyeballs shorten.
2. Blink rate is supressed by looking at a monitor and it is necessary to consciously blink to compensate.

I also find fonts generally too small. In X you can fix that; In the console, I use terminus 28 point fonts (redhat did them). The font sizes go to 32, but anything over 28 freaks the console and results are poor. My rc.local has the line
and that lets me live in consoles with 1600x900 resolution. The only console I haven't figured is the white on black one that comes with most distros.

HTH

The backlight flickering frequency is usually 200Hz, the highest I have seen was 1kHz which is probably not that bad, but I prefer the flat line

My problems completely disappear when I work on non PWM led, which basically proves my theory that in my case the PWM is the problem.

I also had problem with the high contrast and played with xcalib, but after one day I got used to the contrast.

However while I am at work I use docking station, connected to CCFL monitor at max brightness and lowered contrast + lenovo KB (old style keys) with trackpoint.