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Thread: Incorrect detection of screen size?

  1. #1
    Join Date
    Jun 2021
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    Incorrect detection of screen size?

    Sorry, I don't know what to input in the (Advanced) Search field for this. I tried booting up 21.04 iso of Ubuntu and Kubuntu.

    In System Settings - display settings - Ubuntu detects my 4k tv as 36"

    Kubuntu displays the brand and model name '50' is in the model name so I initially thought, 'hey, Kubuntu is displaying the correct info, Ubuntu isn't'

    But, I think this was not accurate, ultimately.

    I ran the xrandr and hwinfo commands.

    In one of the lines was '800mm x 450mm' - does that mean it detected it as a 32" screen? I am confused. I couldn't find any good programs/commands that output more detailed info of the TV display hardware. Yes, the resolution and refresh rates were output so plenty of info on that. But, I was looking for an output that displayed the dimensions of the TV. The TV brand and model was displayed but I wanted output of the screen size - e.g. '50"' or even 49.x would be acceptable.

    I think the measurements in mm was the detection/display of the screen dimensions? But, they are incorrect/inaccurate.

    Is that something to be concerned with when I install? Or is the programs just not detecting it correctly? Am I missing something? This is with live media - but, I don't expect it to be a different result with an actual install written to disk. Why would there be?

    Any ideas? When I researched this, I mostly found people asking why 'the resolution' was detected, inaccurately/incorrectly. That's not my problem or issue. Maybe it's inconsequential?

    My hardware: TCL 50" 4K TV, Dell Optiplex 3020 sff, using Intel igpu HD 4600... xrandr and hwinfo provided info on the TV brand/model, intel igpu info and the rest of the hardware. Just no mention of the screen size unless that number above '800mm vs 450mm' is what it detected? I couldn't find any other measurements after entering those CLI commands.
    Last edited by linuux; July 11th, 2021 at 03:18 PM.

  2. #2
    Join Date
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    Kubuntu 20.04 Focal Fossa

    Re: Incorrect detection of screen size?

    The computer only knows what the display tells it; your computer doesn't get out a tape measure. The mechanism for displays to advertise their capabilities is called EDID. Your display is lying in its EDID. Either because they've refused the EDID from a different model, or they're trying to fudge automatic DPI calculations, or something of that nature.

  3. #3
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    Re: Incorrect detection of screen size?

    Quote Originally Posted by CatKiller View Post
    The computer only knows what the display tells it; your computer doesn't get out a tape measure. The mechanism for displays to advertise their capabilities is called EDID. Your display is lying in its EDID. Either because they've refused the EDID from a different model, or they're trying to fudge automatic DPI calculations, or something of that nature.
    Yes, I'm aware of that - that the programs are just 'fetching' what the tv programming 'provides them.' I thought it might be something like that. Just wanted to confirm.

    I did read about the EDID program but I didn't think the output was any better than hdinfo. Perhaps, I am incorrect about that then.

    Anyway, I guess it is of little consequence but does this mean that all/most TV manufacturers are 'lying' or providing inaccurate EDID info? I don't see what reason the manufacturer would provide incorrect screen size.

  4. #4
    Join Date
    Mar 2010
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    Squidbilly-Land
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    Ubuntu

    Re: Incorrect detection of screen size?

    The TV sends whatever it wants to the GPU. If it lies, then you'll see those lies. This only works for digital connections as far as I know - so dvi-i or hdmi. I don't have DP, so don't KNOW what that does, but I'd expect DP would show the same as HDMI shows. VGA communicates only enough to prevent the GPU from over-driving the monitor and killing it. That was a concern in the early 1990s.

    Code:
    sudo get-edid | parse-edid
    Shows what the GPU is told. If there are any converters or KVMs between the GPU and the monitor, expect those to be shown. Si?
    For example,
    Code:
    Section "Monitor"
            Identifier "DELL U2412M"
            ModelName "DELL U2412M"
            VendorName "DEL"
            # Monitor Manufactured week 10 of 2013
            # EDID version 1.3
            # Analog Display
            Option "SyncOnGreen" "true"
            DisplaySize 520 320
            Gamma 2.20
            Option "DPMS" "true"
            Horizsync 30-83
            VertRefresh 50-61
    but going through a powered DVI-I -to- VGA converter to the same monitor (KVM switch), I see:
    Code:
    Section "Monitor"
            Identifier "\�"
            ModelName "\�"
            VendorName "AGO"
            # Monitor Manufactured week 45 of 2013
            # EDID version 1.3
            # Digital Display
            DisplaySize 300 230
            Gamma 1.97
            Option "DPMS" "true"
    The 2nd answer is about the converter device, not the actual monitor.
    The monitor does say "Dell" on the front and I think it is a 24 inch 1920x1200p @ 60hz model. https://www.dell.com/is/business/p/dell-u2412m/pd It has been a good monitor. Replaced a Gateway 24inch that died 1 day. I've been looking at 4K options the last few months, but decided to wait for better quality to get cheaper based on some feedback from about 15 friends.

  5. #5
    Join Date
    Aug 2021
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    2

    Re: Incorrect detection of screen size?

    Only way for computer to calculate it's display size is if it knew it's pixel density measurement called ppi(pixel per inch).
    It could then divide its horizontal and vertical resolution in physical pixels with ppi to get its size in inches.

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