Solving the display black screen mystery - RGB color values

Discussion in 'Hardware Components and Aftermarket Upgrades' started by TrantaLocked, Feb 5, 2020.

  1. TrantaLocked

    TrantaLocked Notebook Deity

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    I did a laptop display upgrade recently. All of the context on previous discussion here. The laptop is an HP 15-bs234wm upgraded to the BOE NV156FHM-N4B display. I have identified the exact conditions that cause the display to black screen:

    For display cut-out to occur: greater than about 60% of the screen must be covered by a color or combination of colors which have an RGB value for which the difference between the highest and lowest sub value is greater than ~220. Examples that WILL cause black screen: (0-0-255), (5-5-255), (0-15-250). Examples that WILL NOT cause black screen: (30-30-255), (15-15-235), (0-0-220), (255-255-255), (0-0-0). Even if there are say, 20 different zones each with different colors on screen, as long as each one has a differential RGB value between lowest and highest greater than 220, and they cover more than 60% of the screen in total surface area, the display will black screen. There are small variations in the exact RGB difference threshold for black screen. For example 0-230-230 causes black screen but 0-0-230 doesn't.

    If the color, such as (0-0-255) pure blue, is on screen but only covering a small part of the screen and the rest of the colors on screen are less extreme, there is no problem and the display works perfectly. I created an ICC profile with the windows calibrator in which all colors sliders are lowered by a number of notches depending on how much stability I want, with 6 being reasonable and 25 for most stability. Assuming each notch represents 1 single value in the 256 color range, this essentially means I am only seeing colors up to 230-230-230 in RGB value, even though everything appears the same as before when brightness adjusted. I do not experience black screens at any time with the ICC profile installed.

    I have looked at the datasheets for these displays and have noticed some small differences in voltage tables and timings. The original screen had a luminance range of 187-220, new screen 255-230. If anyone wants to have a look at the datasheets, they are attached. The B156XTN07.0 HW0A is my original screen, NV156FHM-N4B is the new screen.

    What I want to solve is if this is a problem with my system or the display and what the problem specifically is. Thanks for helping.

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    My current best guess is that either the eDP cable or the firmware for the eDP output are not completely compatible with the display.

    If it's the cable, it would be because of the lack of individual power delivery wires inside the stock eDP cable that HP uses. For the LED power wires that go to pins 26, 27, 28 and 29, HP's cable uses only two wires, each one soldered to two pins simultaneously, instead of using individual wires for each of the four pins. This could mean that the display may usually work because it normally doesn't need different voltages or amps over these wires, but if it asks for different power from pin 26 compared to pin 27, it cannot get it because one wire in the cable is used for both pins 26 and 27 at the same time. This will cause immediate failure of the display.

    Another hypothesis is that the firmware controlling the eDP output isn't setup to send signals or power to the display in the way the display wants it. Whether because it is a resolution or contrast ratio upgrade, or voltages or timings are slightly different. And it probably cannot be fixed.

    That the display still works 99% of the time makes for a strange case where while it isn't fully compatible I am absolutely keeping because I love its performance for the price I paid (about $120 shipped from a USA seller on ebay). I have a replacement cable and might have a technician solder new wires in it to get the power pins their own wires, or maybe try to find someone who can identify a compatibility issue in firmware. It's difficult to find someone who would want to look at it.

    If you're upgrading a similar HP laptop and follow the things I've said in my other posts you will have a good experience with this display. Either live with the rare website or video frame causing a black screen, or create an ICC profile that at the least lowers colors by 6 notches each. Full stability requires lowering by 25, but 6 should be enough to prevent most black screens and at that level the loss of luminance range is very subtle. I've been blocking problem elements on websites with an adblocker and this allows me to go without using the ICC profile trick without much of an issue.

    There are plenty of reports of the upgrade to this display going perfectly well in other laptop models, so it depends on your system and cable. I have seen one report by someone on the MSI Russia forum having artifact issues. With display upgrades it is hit or miss and you may have to change your display cable, especially if your stock cable only has one data lane (which you can check yourself by pulling back the cover on the display side connector of the cable and seeing if pins 3,4 and 6,7 are connected).
     

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    Last edited: Apr 21, 2020
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